Engineering estimates for common magnet design questions. All calculations run locally in your browser — nothing is uploaded.
Units
Enter dimensions in the selected unit. Results stay metric (G, T, N, lbf).
Surface field (center, on axis)
Estimated flux density at the center of the magnet surface (or at a small gap X, e.g. the sensing distance of a Hall sensor). Radial = diametrically magnetized, measured at the pole on the curved surface.
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Analytical approximation assuming an ideal, uniformly magnetized magnet with no coating and no chamfer. Surface field is shape-dependent and not a material property — treat it as an estimate (±10–20%). The radial-magnetization result is an engineering approximation (demagnetizing-factor model).
Pull force (holding force against steel)
Maximum static holding force when the magnet face is in contact with a thick steel plate. Set the test gap z (magnet surface to steel) to see how fast the force and the field decay with distance — even 1 mm of paint, rust or clearance cuts the force dramatically.
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Assumes an infinitely thick, flat, unsaturated steel plate. The gap model uses a mirror-image approximation (permeance Pc reduced by the air path L+2z); at z = 0 it matches the standard closed-circuit estimate. Measured pull force varies ±10% between suppliers and test setups. Radial = pole (curved surface) in contact with steel.
Magnetic moment
Dipole moment — the most reliable "how much magnetism" figure when comparing magnets of different sizes. Uses the SDM field-shape model (nominal, uncoated dimensions).
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SDM model: m = Br[G]·V[mm³]·(Pc+1)/(Pc+1.05)/10¹³ (Wb·m), Pc = permeance coefficient along the magnetization direction. 1 Wb·m = 10⁸ μWb·cm. Verify with a Helmholtz coil measurement.
Permeance coefficient Pc
Pc describes the load line of a magnet in free space — geometry only. Higher Pc = higher operating point = better resistance to demagnetization and temperature. For radial magnetization the demagnetizing factor differs (approximation model).
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Rule of thumb: Pc > 2 — stable, good temperature tolerance; Pc 0.5–2 — typical, check the demagnetization curve at working temperature; Pc < 0.5 — easily demagnetized, avoid thin flat shapes in heat or opposing fields. Radial Pc uses the perpendicular demagnetizing factor N ≈ 0.5·(L/D)/(L/D+0.85).
Repulsion force between two magnets
Two identical magnets, same poles facing, separated by a gap z. Force drops steeply with distance (~1/z⁴). Works for cylinders (axial or radial) and blocks.
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Magnetic-charge model approximation (engineering estimate). At contact (z→0) the repulsion approaches the pull force between two magnets, which is roughly the pull against a steel plate for the same magnet.
All results are engineering estimates from closed-form approximations. Real magnets have coatings, chamfers, tolerances and finite steel geometries. Radial-magnetization results are approximations of a demagnetizing-factor model. Always validate critical designs by measurement (Helmholtz coil, fluxmeter, pull tester) or FEA.