Neodymium magnet force calculator – full engineering analysis
Pull force, shear, temperature and safety in one place
Where the data comes from: the physical model (Maxwell equations for the magnetic circuit) was calibrated against 127 real products from our warehouse – discs, blocks and rings with measured pull force. The median deviation from measurement is below 0.1%, and the maximum deviation does not exceed 2%.
magnet parameters
shape:
grade properties
Calculation result
Real holding force depends on more than just the magnet's dimensions:
- material grade – N52 is roughly 60% stronger than N35 at the same volume,
- closing the circuit with steel – a magnetic pot can be 2–3 times stronger than a bare magnet,
- contact surface area and shape proportions (Pc coefficient),
- working temperature – above the grade limit the loss of strength is irreversible.
The quoted pull force is measured perpendicular, on perfectly smooth steel at least 10 mm thick, with zero air gap. A layer of paint, thin sheet metal or pulling at an angle is enough to drop the real force to a fraction of the catalogue value.
A practical rule: if the load acts parallel to the surface (the magnet must not slide), assume 20% of the catalogue value. If it acts perpendicular but the surface is not perfect – assume 50%. For applications where mounting failure would be dangerous, use at least a threefold margin.
The effect is greatest for flat magnets, which have the worst magnetic circuit in their bare form. A typical gain is 2–3 times the force of a bare magnet of the same diameter.
There is a flip side: a pot works in one direction only – from the working face. At the back and sides the field is much weaker, which can be an advantage (less risk of accidental attraction) or a drawback, depending on the application.
A purely theoretical model always deviates from measurement, so we calibrated it against 127 products from our warehouse database – disc, block and ring magnets with known, measured pull force. After calibration the median error is 0.09%, and nine out of ten results fall within 0.5% of the measured value.
Pull force: tick the steel pot option in the calculator – you will see not only the catalogue value, but also the real holding force in four scenarios, from clean steel to pulling at an angle. On a rusty, silted object you can realistically count on 30–50% of the catalogue value.
Coating: standard nickel (NiCuNi) corrodes in water, especially salt water. For regular fishing choose an epoxy coating or a magnet in a stainless steel housing.
Rope: the calculator gives the minimum strength with a threefold margin. That margin is not excessive – jerking during retrieval generates a dynamic force many times greater than the static pull.
The market standard is the N38–N45 range, which offers the best strength-to-price ratio. Higher grades (N48 and above) are used where space is tight but force must be high – in miniature electronics, sensors or couplings.
Letters after the number denote temperature resistance: no letter means max. 80°C, M up to 100°C, H up to 120°C, SH up to 150°C, UH up to 180°C, EH up to 200°C. Higher thermal resistance always involves a trade-off – at the same N class, a higher-coercivity material has slightly lower remanence.
How to read the calculator results
A magnet's catalogue force is measured under ideal conditions: smooth, clean steel at least 10 mm thick, zero air gap and force acting perpendicular to the surface. In practice each of these conditions tends to be violated – which is exactly why the calculator shows a set of tables rather than a single number.The distance table answers what happens with an air gap: a 1 mm layer of paint can halve the holding force. The steel thickness table shows the saturation effect – a magnet placed on thin car body panel will not reach its catalogue force, because the sheet cannot carry the full flux. The shear table is critical for mountings: force parallel to the surface is only 10–50% of the perpendicular force, depending on the finish.
The B-H curve chart and the Pc coefficient are tools for design engineers. An operating point sitting low on the curve means a magnet of these proportions is prone to self-demagnetization – at elevated temperature it will lose strength permanently. Flat discs and thin blocks are the most exposed here.
How to match a magnet to a load
Four steps that separate the catalogue figure from the load a magnet will actually hold in a finished assembly.
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1
Determine the direction of the load
Force perpendicular to the surface is the catalogue value. Under a shear load, when the magnet slides along the plate, about 30% of that value remains. Vertical wall mounting is designed from the shear table.
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2
Apply a safety factor
For static workshop mounting, divide the result by 2. Under dynamic loads, vibration or overhead mounting the factor rises to 3–4.
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3
Check the substrate
Plate thinner than 10 mm will not carry the whole flux and reduces the holding force. Austenitic stainless steel is practically non-magnetic. Paint, rust and roughness act as an air gap.
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4
Test on the actual part
Calculation narrows the choice but does not replace measurement. A production spread of remanence of about ±5% translates into roughly ±10% of force – more than the uncertainty of the model itself.
How we calculate the pull force of a neodymium magnet
The calculator does not read values from a table; it computes them from an analytical model. The induction on the magnet axis follows from the potential of a uniformly magnetised body, and the pull force from the Maxwell equation for magnetic stress at the magnet–steel boundary. The only fitted parameter is the calibration constant k_cal = 1.4837, obtained by least-squares regression over 127 catalogue items; the median deviation after calibration is 0.09%.
Catalogue value versus real holding force
The quoted pull force applies to ideal conditions: an S235 structural steel plate at least 10 mm thick, a smooth, clean and degreased surface, zero air gap and a perpendicular load. Every departure from those conditions lowers the result. Under a shear load, when the magnet slides along the plate, about 30% of the catalogue value remains. That is why vertical wall mounting is designed from the shear table, never from the perpendicular force.
Permeance coefficient and shape efficiency
The permeance coefficient describes how strongly the shape demagnetises itself. For a ⌀38×3.5 mm magnet it is 0.145 – a very flat body that uses 43% of the energy potential of the material and works close to the knee of the demagnetisation curve. The same material in a ⌀10×10 mm proportion reaches 99%. Flat magnets are best stored stuck to steel: closing the magnetic circuit nearly doubles the permeance coefficient and clearly widens the margin against permanent demagnetisation.
Surface induction versus the remanence of the grade
Surface induction is measured on the axis, in air, without a steel plate. For a ⌀38×3.5 mm magnet in grade N38 it is 1123 G. It should not be confused with the remanence Br of the grade, which for N38 falls between 12.2 and 12.6 kG – the induction at the surface is always noticeably lower, because part of the flux closes through the side wall rather than the pole.
Production spread decides the margin
The industry works with a remanence tolerance of about ±5%. Because force scales with the square of induction, this translates into roughly ±10% of force – more than the effect of dimensional tolerance and many times more than the uncertainty of the model itself. The required safety margin is therefore set by the spread of the material, not by the accuracy of the calculation.