How to use this calculator
- Select magnet grade. Pick a representative material grade or enter a custom Br value from the datasheet.
- Enter pole geometry. Choose disc/cylinder or block/rectangle, then enter the pole dimensions and thickness in the magnetization direction.
- Set gap and derates. Include paint, coating, adhesive and roughness in the air gap, then derate circuit efficiency and target contact.
- Apply safety factor. Use the safe working load output for a first-pass static load, not the estimated peak pull force.
How it works
A permanent magnet does not have one universal pull force. The measured result depends on grade, shape, magnetization direction, gap, steel thickness, surface finish and the surrounding magnetic circuit. This calculator uses a conservative screening workflow: estimate the useful gap flux density, then convert magnetic pressure into pull force.
B_contact = B_r · k_c
B_gap = B_contact · t / (t + g)
The pull force then comes from magnetic pressure on the pole area:
F_pull = B_gap² · A · k_t / (2 · mu0)
The safe working load divides that estimated pull by the entered safety factor:
SWL = F_pull / SF
If you already know measured pole flux density for a powered fixture, use the electromagnet holding force calculator. For coil-driven actuators, start with the solenoid force calculator.
Worked example
Verified against the live calculator
Suppose a 20 mm diameter by 5 mm thick N42 disc magnet
is separated from a steel target by a 0.5 mm paint/coating gap.
Use 70% circuit efficiency, 60% target/contact factor
and a safety factor of 3.
The representative remanence is 1.32 T. Contact flux density is
1.32 x 0.70 = 0.924 T. The gap factor is
5 / (5 + 0.5) = 0.909, so the estimated gap flux density is about
0.840 T.
The pole area is 314 mm², magnetic pressure is about
0.281 MPa, and the target factor gives estimated pull force of
about 52.9 N. With safety factor 3, the safe
working load is about 17.6 N, equivalent to roughly
1.80 kg under static vertical loading.
Frequently asked questions
How do you estimate magnet pull force?
This calculator derates the magnet remanence to an estimated working gap flux density, then uses magnetic pressure: F = B_gap^2 * A * k_target / (2 * mu0). It is a screening estimate, not a replacement for measured pull-force data.
Why does a tiny air gap reduce magnet force so much?
Magnetic pull force scales with flux density squared. Paint, plating, adhesive, rough steel, coatings and small clearances reduce the working gap field, so the pull force can fall much faster than the gap looks by eye.
What should I use for target/contact factor?
Use lower factors for thin steel, rough or painted surfaces, curved targets, edge contact and unknown alloys. Use supplier tests or your own pull tests when the load matters.
Can this size a lifting magnet?
No. Use it only for early screening. Lifting and safety-critical holding need rated equipment, proof testing, standards compliance and a large margin for shear, vibration and surface condition.
Method & assumptions
- Grade values are representative Br values, not a substitute for the exact supplier datasheet.
- The magnet is assumed magnetized through its thickness and pulling normal to a flat steel target.
- The gap model is a simple thickness/(thickness + gap) screen, so it is most useful for comparing options and seeing gap sensitivity.
- Target/contact factor is a lumped derating for steel thickness, paint, rust, roughness, curvature, fringing and nonuniform contact.
- Sliding or shear loads are not equal to normal pull force. Use friction and mechanical retention for lateral loads.