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Estimate the straight pull force of a permanent magnet against a flat steel plate. The calculation is based on magnetic pressure, magnet geometry, material Br, pole-face area, air gap and practical correction factors for steel and contact condition.
For a magnetic field crossing an air gap normally into a steel surface, the magnetic pressure is B² / (2μ0). Pull force is this pressure multiplied by effective pole area. The difficult part is estimating B in the real contact gap.
| Step | Formula / Method | Engineering Notes |
|---|---|---|
| Open Field | Bopen is estimated from Br, shape, thickness and air gap | This is the magnet centerline field in air and is not enough by itself for pull force. |
| Steel Return Path | Bgap = Bopen × Ksteel, limited by Br and steel saturation | Thick low-carbon steel increases effective gap flux compared with a free magnet in air. |
| Ideal Force | Fideal = Bgap² × A / (2 × μ0) | A is pole face area in m². μ0 = 4π × 10⁻⁷ H/m. |
| Practical Force | F = Fideal × Kcontact | Kcontact accounts for coating, roughness, paint, tilt, imperfect steel and non-ideal testing. |
| Unit Conversion | 1 kgf = 9.80665 N, 1 lbf = 4.44822 N | Catalog pull force is normally straight pull against thick flat low-carbon steel. |
There is no universal formula that can replace physical testing for every magnet and steel condition. Many commercial calculators are calibrated by measured catalog data. This page uses a transparent engineering estimate so users can see the assumptions.
Values below are approximate reference values for quick calculation only. Use datasheet values or measured magnetic data for final engineering decisions.
| Material | Grade | Approx. Br (T) | Approx. Br (G) | Common Use |
|---|---|---|---|---|
| NdFeB | N35 | 1.17 | 11700 | General-purpose high-strength magnets |
| NdFeB | N42 | 1.29 | 12900 | Higher strength standard grade |
| NdFeB | N45 | 1.33 | 13300 | High-performance general use |
| NdFeB | N52 | 1.45 | 14500 | Maximum energy grade selection |
| SmCo | SmCo 1:5 | 0.85 | 8500 | High-temperature and corrosion-resistant applications |
| SmCo | SmCo 2:17 | 1.00 | 10000 | High-temperature motor and sensor applications |
| SmCo | SmCo 2:17 Premium | 1.10 | 11000 | Premium SmCo magnetic performance |
| Ferrite | Y25 / C5 | 0.35 | 3500 | Cost-sensitive permanent magnet applications |
| Ferrite | Y30 / C8 | 0.38 | 3800 | Common ceramic/ferrite magnet grade |
| Ferrite | Y35 | 0.40 | 4000 | Higher Br ferrite magnet selection |