Magnet Size Selector

selector — requirement in, part out

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.

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

• 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.

related tools

Once you have a candidate size, the pull force calculator gives you the full force-versus-gap picture for that exact geometry. If temperature is the binding constraint, the temperature derating calculator shows where each grade gives out. For anything outside stocked sizes, custom manufacturing starts at the RFQ.