Shopping for magnets shouldn’t feel like guessing.
But that’s what happens when you only look at pull force and price.
The truth is, five key factors determine whether a magnet will actually work in your application.
This guide breaks down each factor so you can make smarter buying decisions.
Table of Contents
Magnet Types (The Materials)
The material determines everything else about your magnet.
You can’t change a magnet’s heat tolerance or corrosion resistance after you buy it.
Those traits are locked into the material from day one.
Here are the four main types you’ll encounter.
Neodymium (NdFeB): The Powerhouse
Neodymium magnets are the strongest permanent magnets you can buy.
They pack serious pull force into a small size.
A neodymium disc the size of a quarter can hold over 20 pounds against a steel surface.
Visual identification:
Look for a shiny metallic coating, usually silver (nickel-plated) or gold (gold-plated). Some have a dark zinc coating or colored epoxy. The surface looks smooth and reflective like chrome. These magnets feel heavy for their size and have sharp edges that can chip.
How to identify them by specs:
Look for grade codes like N35, N42, N52. The number indicates strength. Higher numbers mean stronger magnets. You’ll also see these called “rare earth” or “NdFeB” magnets.
Why you should care:
If you need maximum strength in minimum space, neodymium wins.
But there’s a catch.
- These magnets corrode easily and lose strength above 80°C (176°F).
- Standard grades start weakening around 60-80°C.
- High-temp versions cost more but handle up to 200°C.
Samarium Cobalt (SmCo): The Survivor
Samarium cobalt magnets handle extreme conditions.
They resist heat up to 300°C and don’t rust.
Drop one in saltwater and it comes out fine.
Visual identification:
Look for a dark charcoal gray or black color with a matte, non-reflective surface. The texture looks ceramic or stone-like, not metallic. These magnets feel surprisingly light compared to metal magnets of the same size. If you tap them together, they make a dull clacking sound instead of a metallic ping.
Identify them by specs:
Look for codes like Y30, C5, C8. The “C” stands for ceramic.
Why you should care:
If your application involves high heat, corrosive environments, or both, samarium cobalt is your answer.
The trade-off is price and brittleness. These magnets chip and crack more easily than neodymium when dropped or impacted.
Ferrite (Ceramic): The Workhorse
Ferrite magnets are the budget option.
They’re made from iron oxide and deliver about 10% of neodymium’s strength. But they cost pennies and never rust.
Visual identification:
Look for a dark charcoal gray or black color with a matte, non-reflective surface. The texture looks ceramic or stone-like, not metallic. These magnets feel surprisingly light compared to metal magnets of the same size. If you tap them together, they make a dull clacking sound instead of a metallic ping.
How to identify them by specs:
Look for codes like Y30, C5, C8. The “C” stands for ceramic.
Why you should care: When you need rust-proof magnets on a tight budget, ferrite delivers. They work well for low-force applications like refrigerator magnets or sensor triggers. They also create deep magnetic fields, making them useful for holding through thick materials.
The weakness is pull force. A ferrite magnet needs to be 5-10 times larger than neodymium to match the same holding power.
Alnico: The Specialist
Alnico magnets handle temperatures that melt other magnets.
They stay magnetic above 500°C (932°F). Some grades work up to 600°C.
Visual identification:
These have a distinctive metallic appearance, either bright silver or reddish-copper color depending on the alloy.
The surface looks like polished metal or cast aluminum.
They feel solid and heavy.
Unlike ceramic magnets, Alnico magnets can be machined with standard metal tools and produce metal shavings when cut.
How to identify them by specs:
Look for codes like Alnico 5 or Alnico 8. These are actual metal alloys, not ceramic compounds.
Why you should care:
Alnico solves one specific problem: extreme heat.
If your magnet sits inside an engine, furnace, or high-temp industrial process, Alnico is often the only option.
The downside is low coercivity.
These magnets demagnetize easily when exposed to opposing magnetic fields or physical shock.
Decision Point: Matching Material to Application
Start with your environment.
- High heat? Choose samarium cobalt or Alnico.
- Corrosive conditions? Rule out neodymium unless you add protective coating.
- Tight budget? Consider ferrite if you can accept lower strength.
Then check your space constraints.
- Small footprint with high force? Neodymium is hard to beat.
- Plenty of room? Ferrite saves money.
The material you choose locks in your magnet’s ceiling for strength, heat resistance, and durability.
Pick wrong here and no amount of size or grade adjustment will fix it.
Magnetic Performance (The Strength)
Performance specs tell you how strong a magnet is and how it behaves under stress.
But these numbers on a datasheet don’t always match real-world results.
Understanding what each spec actually measures helps you predict how a magnet will perform in your application.
Br (Remanence): The Internal Magnetic Muscle
Remanence measures the magnet’s internal magnetic strength.
Think of it as the raw power stored inside the material. It’s measured in Tesla (T) or Gauss (G). Higher Br means the material is magnetically stronger.
How to read it:
You’ll see values like 1.2T or 12,000G for strong neodymium magnets. Ferrite magnets show around 0.4T or 4,000G.
Why you should care:
Br sets the upper limit for everything else. A magnet with higher Br can produce stronger pull forces and surface fields. But Br alone doesn’t tell you the whole story.
A small magnet with high Br might still produce less pull force than a larger magnet with lower Br.
Hci (Intrinsic Coercivity): Resistance to Demagnetization
Coercivity measures how hard it is to erase a magnet’s magnetism.
When opposing magnetic fields hit your magnet, Hci determines whether it keeps its strength or weakens. It’s measured in kA/m or kOe.
How to read it:
Standard neodymium shows around 900-1,000 kA/m. High-coercivity grades reach 1,500+ kA/m. Alnico magnets drop below 100 kA/m.
Why you should care:
If your magnet faces opposing fields, impacts, or high temperatures, you need high Hci.
- Motors and generators create opposing fields constantly.
Low Hci magnets lose strength over time in these applications.
High-temperature environments also require high Hci because heat weakens a magnet’s resistance to demagnetization.
BHmax (Maximum Energy Product): The Grade Number
BHmax combines strength and efficiency into one number.
This is what manufacturers use to assign grades like N52, N42, or C8. It’s measured in kJ/m³ or MGOe (megagauss-oersteds).
How to read it:
The grade number tells you BHmax directly.
- N52 means 52 MGOe.
- N35 means 35 MGOe.
- For ferrite, C8 means approximately 3.5 MGOe.
Why you should care:
BHmax tells you the power density of the material. Higher grades pack more magnetic energy into the same volume. An N52 magnet delivers more pull force than an N35 magnet of identical size. But higher grades cost more and often have lower temperature resistance.
The trade-off:
N52 magnets lose strength around 60°C.
N35 magnets handle 80°C.
If you need both strength and heat resistance, you’ll pick something like N42SH (high strength, high temperature) and pay a premium.
Pull Force: The Real-World Metric
Pull force measures how much weight a magnet holds against a steel surface.
This is the number most buyers care about because it’s practical. You’ll see values like 10 lbs, 50 lbs, or 200 lbs.
How to read it:
Pull force specs assume perfect conditions. The magnet sits flat against thick, smooth steel with no air gap. Real applications rarely achieve this.
Why you should care:
Pull force drops dramatically with air gaps. A coat of paint creates a 0.1mm gap that cuts pull force by 20-30%. Thin steel (less than 3mm) reduces pull force by 50% or more. Sliding force (shear) is only 10-20% of direct pull force.
The caveat:
Don’t trust pull force specs alone. If your magnet holds through a plastic case, across a painted surface, or pulls on thin sheet metal, expect 30-70% less force than the datasheet claims.
Surface Gauss (The Sensor Metric)
Surface Gauss measures magnetic field density at the magnet’s surface.
Hall effect sensors and reed switches trigger based on field strength, not pull force.
Surface Gauss tells you if the field is strong enough to activate your sensor.
How to read it:
Values range from 1,000G (weak ferrite) to 6,000G+ (strong neodymium). Sensor datasheets specify the minimum field strength needed to trigger.
Why you should care:
A magnet might have great pull force but weak surface Gauss if it’s thick and magnetized through its length.
Sensor applications need high surface Gauss, which usually means thin magnets magnetized through thickness.
Distance matters.
Surface Gauss drops fast as you move away from the magnet.
- At 5mm distance, field strength typically falls to 20-30% of surface value.
- At 10mm, it drops to 5-10%.
Putting Performance Specs Together
Start with your requirement.
Need maximum holding force? Look at pull force but verify the conditions match your application.
Need sensor activation? Check surface Gauss and measure the gap distance.
Need stability in motors or generators? Prioritize high Hci.
Then check the grade.
Higher BHmax (N52 vs N35) means more performance in less space.
But verify the temperature rating matches your environment.
Performance specs work together.
High Br and BHmax mean nothing if low Hci lets the magnet demagnetize.
Strong pull force means nothing if your air gap cuts it in half.
Thermal Parameters (The Temperature)
Heat is a magnet’s worst enemy.
Every magnet has a temperature ceiling.
- Cross it and the magnet loses strength.
- Cross it by enough and the damage becomes permanent.
Understanding thermal limits prevents failures in heated environments like motors, engines, or outdoor applications in hot climates.
Tmax (Max Operating Temperature): The Reversible Threshold
Tmax marks the highest temperature a magnet can withstand without permanent damage.
Stay below this limit and the magnet recovers its strength when it cools down. Exceed it and you lose magnetic power forever.
How to read it:
Standard neodymium grades show 80°C (176°F).
High-temp versions reach 150-200°C (300-390°F).
Samarium cobalt handles 300°C (570°F).
Alnico survives 500°C+ (930°F+).
Why you should care:
Motors generate heat. Electronics generate heat. Sunlight on black surfaces can push temperatures past 70°C. If your magnet sits in any of these environments, standard neodymium will fail.
The grade suffix tells you the temperature rating. N35 handles 80°C. N35M handles 100°C. N35H handles 120°C. N35SH handles 150°C. Each step up costs more but buys you thermal headroom.
Check your actual operating temperature, not the ambient temperature. A motor running at 40°C ambient can hit 90°C inside the housing.
Tcurie (Curie Temperature): The Point of No Return
Curie temperature is where magnetism dies completely.
At this temperature, the magnetic domains inside the material collapse. The magnet becomes non-magnetic. Cooling it down won’t bring the magnetism back.
How to read it:
Neodymium Tcurie sits around 310-340°C (590-645°F). Samarium cobalt reaches 700-800°C (1,290-1,470°F). Ferrite exceeds 450°C (840°F). Alnico survives above 800°C (1,470°F).
Why you should care:
Tcurie creates a safety margin above Tmax.
If your magnet briefly spikes above its rated Tmax but stays below Tcurie, you lose some strength but the magnet still works.
Cross Tcurie and the magnet is dead.
Welding near magnets can hit Tcurie instantly.
Friction heat from high-speed rotation can do the same.
Keep magnets away from welding, soldering, or processes that generate extreme localized heat.
The “Reversible” Loss: Temperature Coefficient
Magnets naturally weaken as they heat up, even within safe operating limits.
This isn’t damage. It’s physics. As temperature rises, magnetic strength decreases. As temperature drops, strength returns.
How to read it:
Temperature coefficient shows up as a percentage per degree Celsius. Neodymium typically shows -0.11% to -0.13% per °C for Br. This means a magnet loses about 11-13% of its strength for every 100°C increase.
Why you should care:
A magnet rated for 20 lbs pull force at 20°C might only deliver 17 lbs at 50°C. That’s a 15% drop even though you’re well below Tmax. If your design operates at the edge of performance, temperature swings will push you over that edge.
Applications with tight tolerances need compensation. Either oversize the magnet to account for hot-day performance or design for the worst-case temperature.
Cold Temperatures: The Forgotten Variable
Most people worry about heat. But extreme cold affects magnets too.
Neodymium actually gets slightly stronger in cold temperatures. Ferrite gets weaker. Some ferrite grades lose 30-50% of their strength at -40°C.
Why you should care:
If your product ships to cold climates or operates in freezers, test at low temperatures. A ferrite magnet that works fine at room temperature might fail completely in a Minnesota winter.
Neodymium handles cold well, making it the better choice for outdoor applications in northern climates or refrigeration equipment.
Thermal Cycling: Heat + Cold Cycles
Repeated heating and cooling stresses magnets mechanically.
Different materials expand and contract at different rates. If your magnet is glued or potted in place, thermal cycling can crack the coating, break the adhesive bond, or chip the magnet.
Why you should care:
Applications that cycle between hot and cold daily (like outdoor sensors or automotive components) need careful material selection.
Samarium cobalt handles thermal cycling better than neodymium because it has a lower expansion coefficient.
Coating selection matters too. Epoxy coatings flex better than nickel during thermal cycling. Nickel-plated neodymium can crack if the temperature swings are severe.
Putting Thermal Parameters Together
Start with your maximum operating temperature.
- Measure it in the actual installation, not just ambient air temperature.
- Add 20-30°C safety margin for local hot spots.
Then check the grade.
- If you’re pushing 100°C, standard N-grade won’t work.
- You need M, H, SH, or a different material entirely.
Consider your temperature range.
If you see both 60°C summers and -20°C winters, test performance at both extremes. What works at room temperature might fail in the field.
Heat kills magnets slowly or instantly depending on how far you exceed the limits. Know your numbers before the magnet goes into production.
Durability & Environment (The “Resistance”)
Magnets aren’t indestructible.
They rust. They chip. They crack. Understanding how magnets fail in different environments helps you choose the right material and coating for your application.
Corrosion Resistance: The Rust Problem
Not all magnets rust, but the strongest ones do.
Neodymium magnets corrode rapidly when exposed to moisture. The iron content reacts with water and oxygen, forming rust that flakes off and weakens the magnet. Left unprotected, a neodymium magnet can turn into orange powder in humid environments.
How to identify protection:
Neodymium magnets come with protective coatings. Nickel plating looks shiny silver. Zinc plating appears darker gray. Epoxy coating shows as black, red, or colored plastic. Gold plating looks, well, gold.
Why you should care:
Indoor applications with controlled humidity can use basic nickel plating. Outdoor applications need zinc or epoxy. Marine environments, chemical exposure, or high humidity demand epoxy coating or a different material entirely.
Coating thickness matters. Standard nickel is 10-20 microns thick. Triple-layer nickel (nickel-copper-nickel) provides better protection. Epoxy coatings run 20-30 microns and seal the magnet completely.
Coating limitations:
All coatings can chip or scratch. Once the coating breaks, corrosion starts. If your magnet experiences impacts, abrasion, or handling, expect coating damage over time.
The rust-proof alternatives:
Ferrite magnets never rust because they’re already oxidized ceramic. Samarium cobalt resists corrosion naturally and needs no coating. If rust is a deal-breaker, skip neodymium entirely.
Mechanical Durability: The Brittleness Factor
Magnets are ceramic materials, not structural metal.
They shatter like glass when dropped on hard surfaces. They chip when magnets slam together. They crack under impact loads.
Visual identification of damage:
Look for chips on edges and corners. Check for hairline cracks across the surface. Listen for rattling sounds that indicate internal fracturing. A damaged magnet might still hold but will lose strength over time as cracks propagate.
Why you should care:
Don’t use magnets as structural components. They can’t support shock loads or sudden impacts. A magnet rated for 100 lbs pull force will shatter if you drop a 5 lb weight on it.
Design around brittleness. Add protective housings for magnets in high-impact applications. Use rubber bumpers or spacers to prevent magnet-to-magnet collisions. Never tighten mounting screws directly through a magnet—it will crack.
Material differences:
Samarium cobalt is more brittle than neodymium. Ferrite is slightly tougher but still chips easily. Alnico is the most durable because it’s a true metal alloy, but it’s still not structural steel.
Adhesive Compatibility: Making Glue Stick
Different coatings affect how well adhesives bond to magnets.
Nickel-plated surfaces bond well with most adhesives. Zinc plating works okay but not as strong. Epoxy coatings can be problematic—some adhesives won’t stick to epoxy-coated magnets at all.
How to verify compatibility:
Test your adhesive on the actual coating before committing to production. A cyanoacrylate (super glue) that works on nickel might fail on epoxy.
Why you should care:
Magnets falling off assemblies cause failures. If you’re bonding magnets into plastic housings, motor rotors, or consumer products, adhesive failure means warranty returns.
Surface preparation matters. Clean the magnet surface with isopropyl alcohol before bonding. Oils from handling reduce bond strength by 50% or more. Roughening the surface with sandpaper improves adhesion but damages the coating.
The mechanical solution:
When adhesive isn’t enough, use mechanical retention. Pockets, slots, or press-fits hold magnets more reliably than glue alone.
Chemical Exposure: Beyond Water
Some environments attack magnets chemically.
- Acids dissolve nickel coatings quickly.
- Alkaline cleaners damage zinc plating.
- Solvents can soften epoxy coatings.
- Salt spray accelerates corrosion even through intact coatings.
How to identify risk:
If your magnet contacts cleaning chemicals, industrial fluids, or operates near the ocean, check chemical compatibility. Standard nickel-plated neodymium won’t survive.
Why you should care:
Chemical exposure creates localized corrosion that spreads under the coating. A small pinhole becomes a rust spot, then a weak point, then complete failure.
The resistant options:
Samarium cobalt handles most chemicals well. Ferrite is chemically inert. Epoxy-coated neodymium provides better chemical protection than metal plating but still has limits.
UV Exposure: The Outdoor Wildcard
Direct sunlight degrades some coatings over time.
Epoxy coatings can yellow, chalk, or crack after years of UV exposure.
This creates pathways for moisture to reach the magnet.
Why you should care:
Outdoor applications need UV-stable coatings.
- Black epoxy typically handles UV better than colored epoxy.
- Metal coatings don’t degrade from UV but offer no protection if they crack from thermal cycling.
Putting Durability Parameters Together
Start with your environment.
- Indoor and dry? Basic nickel plating works.
- Outdoor or humid? Use zinc or epoxy.
- Marine, chemical, or extreme? Consider samarium cobalt or ferrite.
Then assess mechanical stress.
- High impact or vibration? Add protective housings.
- Magnets slamming together? Use spacers or damping materials.
Finally, check your assembly method.
- Gluing magnets? Test adhesive compatibility with the coating.
- Pressing magnets into pockets? Verify tolerances won’t crack the magnet.
Durability failures happen slowly. Corrosion creeps in. Coatings chip. Adhesives weaken. Design for the worst conditions your magnet will see, not the average conditions.
Physical Parameters (The “Geometry”)
Size and shape determine how a magnet actually performs in your design.
Two magnets made from identical material can behave completely differently based on their dimensions and magnetization direction. Getting the geometry wrong makes everything else irrelevant.
Dimensions & Tolerances: The Fit Problem
Magnets are manufactured with tolerances, not precision.
Standard sintered magnets (neodymium, samarium cobalt, ferrite) come with ±0.05mm to ±0.10mm tolerances. That means a magnet spec’d as 10mm diameter could measure anywhere from 9.90mm to 10.10mm.
How to read tolerances:
Datasheets show tolerances as ±0.05mm or ±0.1mm. Some suppliers list tighter tolerances for premium pricing. Machined magnets can hit ±0.02mm but cost significantly more.
Why you should care:
If your magnet press-fits into a housing, tolerances matter. A 10mm magnet in a 10.05mm hole might rattle loose. The same magnet in a 9.95mm hole won’t fit at all.
Stack-up tolerances compound the problem. If you’re assembling multiple magnets in a row, each one adds its own tolerance error. Five magnets with ±0.1mm tolerance can create a 1mm total variation in stack height.
Design strategy:
Add clearance for worst-case dimensions. If the magnet is 10mm ±0.1mm, design your pocket for 10.2mm. Use retaining clips, adhesive, or mechanical stops instead of relying on tight press-fits.
As-cast vs. sintered:
Some ferrite magnets are “as-cast” with rougher tolerances (±0.2mm or worse). These work fine for non-critical applications but won’t fit precision assemblies.
Magnetization Direction (Orientation): The Direction Trap
Magnetization direction determines which face of the magnet is north and which is south.
Get this wrong and your magnet pulls from the wrong surface or doesn’t pull at all.
The three main orientations:
Through Thickness (Axial):
The most common orientation. In a disc or cylinder, north and south are on the flat circular faces. In a block, north and south are on the largest rectangular faces. This creates the strongest pull force on the flat surfaces.
Through Length (Diametral):
Less common but critical for specific applications. In a cylinder, north and south are on opposite curved sides, not the ends. In a block, north and south are on the narrow edges.
Radial:
Specialized orientation where the field radiates outward from a center point. Used in motor rotors and sensors. Requires special magnetizing fixtures and costs more.
How to identify orientation:
Suppliers mark this as “magnetized through thickness,” “axial magnetization,” or “diametral magnetization.”
Look for arrows on technical drawings showing the magnetization axis.
Why you should care:
A sensor magnet magnetized through length won’t trigger if you mount it through thickness. A holding magnet magnetized diametrally provides almost zero pull force on the flat ends.
This mistake is permanent.
- You can’t re-magnetize a magnet in a different direction without specialized equipment.
- Order the wrong orientation and you’re buying magnets twice.
Common trap:
Assuming all disc magnets are magnetized through thickness.
Some sensor applications use through-diameter magnetization.
Always verify before ordering.
Permeance Coefficient: The Shape Ratio
Permeance coefficient describes the magnet’s shape stability.
Long, thick magnets (like rods) are magnetically stable.
Short, thin magnets (like pancakes) are weak and unstable, even at the same grade.
How it works:
The ratio of magnet length to pole area determines stability. A magnet that’s 20mm tall and 10mm diameter has a better permeance coefficient than a magnet that’s 2mm tall and 20mm diameter, even though both use the same material.
Why you should care:
Thin magnets lose strength faster when exposed to opposing fields or high temperatures. A 1mm thick disc magnet rated N42 might perform like N35 in actual use because of poor shape factor.
The practical impact:
If you need consistent performance, avoid extremely thin magnets.
A 10mm diameter disc should be at least 3-5mm thick for stable performance.
Thinner discs work but deliver less than their grade suggests.
Design trade-off:
Thin magnets fit in tight spaces but sacrifice stability. If space allows, use a thicker magnet at a lower grade instead of a thin magnet at a higher grade. The thicker magnet often performs better.
Ring magnets have similar issues. Thin-walled rings with large inner diameters show poor permeance coefficients. Thick-walled rings perform better.
Final Thoughts
Don’t buy magnets based on pull force and price alone.
Start with your environment.
- High heat, moisture, or chemicals narrow your material choices immediately.
- Then define the strength you actually need, accounting for air gaps and real-world conditions.
- Finally, verify the size fits and the magnetization direction matches your design.
Getting one parameter wrong means the magnet fails, no matter how good the other specs look.
- When in doubt, consult with your supplier or an engineer before placing large orders.
- Test samples in your actual application before committing to production quantities.
