Magnet Size Selector

method — how it solves

The forward physics is identical to the pull force calculator, so the two tools always agree. The selector wraps four derating steps around that core and then searches.

step	treatment
1 — effective gap	Air gap and any surface layer (paint, powder coat, plating) are summed. A coating on the magnet and a coating on the target are two separate layers — enter the total.
2 — temperature	Br is derated linearly by the material temperature coefficient (−0.12 %/°C for NdFeB). Force scales with B², so a 5% Br loss is roughly a 10% force loss. The lowest grade suffix rated at least 20 °C above your input is selected automatically.
3 — steel saturation	The target plate must carry the flux radially without saturating. Required thickness is `t = B·R / (2·B_sat)` with B_sat ≈ 1.6 T for low-carbon steel. Thinner plate scales flux linearly and force quadratically.
4 — loading direction	Shear capacity is the normal force times the friction coefficient at the interface — typically 0.15–0.25 for clean plated steel, lower if oiled or painted.

With the derated force model in hand, the selector evaluates every stocked part against your requirement, keeps the ones that clear required force × safety factor, and ranks by magnet volume — smallest qualifying part first, since volume drives both price and package size. If nothing in stock clears it, the tool sweeps a ladder of standard geometries and reports the minimum disc and block that would.

what it can't see

This is a conservative analytical estimate. On a thick, flat, ground steel plate, measured pull typically runs 10–25% higher than the model predicts. Every real-world factor below moves in the other direction. Size from this tool, then verify with a physical sample before committing to production.

Surface flatness and finish. A rough, curved or machined-scaly surface creates an effective gap the model doesn't know about — often a 20–50% loss on its own.
Target alloy. The steel model assumes low-carbon mild steel. Austenitic stainless (304, 316) is essentially non-magnetic and will hold a small fraction of the predicted force, if any. 400-series stainless behaves closer to mild steel.
Edge effects. If the magnet overhangs the target or sits near an edge, flux escapes the circuit and force drops. The model assumes a target substantially larger than the magnet.
Adhesive and mounting. A bonded magnet is limited by the weaker of the magnetic force and the adhesive shear strength. See bonding & mounting magnets.
Demagnetization risk. Temperature grade selection here is based on maximum operating temperature only. A thin magnet in a large gap can sit below the knee of its B-H curve and lose flux irreversibly even within its temperature rating — that requires a load-line check, covered in magnets 201.
Dynamic loading. Shock, vibration and repeated impact all reduce usable capacity well below the static number.
Wide, thin geometries are under-rated. The model applies the on-axis centre field across the whole pole face. On a flat disc that centre value is the lowest field on the face, so the error grows as the magnet gets wider relative to its thickness. A 4:1 diameter-to-thickness disc can hold roughly double the figure shown. Thick, compact shapes (D/T near 1:1) track measured values closely. If the selector pushes you toward a large part on a thin-disc requirement, check that geometry directly in the pull force calculator and test a sample.
Grade suffix and Br. High-coercivity grades trade a little remanence for temperature headroom. The model uses the base grade Br for every suffix, which slightly over-states force for UH, EH and AH parts — a few percent, well inside the safety factor.

Lifting applications. Apply at minimum a 3:1 safety factor to any calculated or catalog rating, and qualify in the actual assembly. Our magnets are not sold or approved for overhead-lifting-below-the-hook use without customer qualification.

selector — requirement in, part out

method — how it solves

what it can't see

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