NdFeB magnet force calculator – full engineering analysis
Pull force, shear, temperature and safety in one place
Enter the dimensions and grade, and the calculator will work out the full set of parameters: pull force, shear force, surface induction, force loss versus temperature and safe distances from devices sensitive to magnetic fields. Results recalculate live – no clicking, no page reloads.
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%.
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:
mm
mm
mm
mm
mm
mm
Calculate holding force for a fishing magnet – the steel housing closes the magnetic circuit and multiplies the pull force.
grade properties
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 basis is a physical model derived from Maxwell's equations: we calculate the induction at the pole surface, correct it for the effect of closing the magnetic circuit with steel (the so-called steel factor, dependent on the magnet's proportions), and then convert it to force through the contact surface area.
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.
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.
Under laboratory conditions – yes. In practice – almost never.
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 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 letter N stands for neodymium (NdFeB alloy), and the number is the maximum energy product (BHmax) expressed in MGOe. The higher the number, the stronger the magnet at the same volume – N52 is roughly 60% stronger than N35.
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.
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.
The key is not force alone, but a combination of three things: pull force, anti-corrosion coating and rope strength.
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.
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.
A bare magnet loses a significant part of its flux "into the air" – the field closes around it instead of passing through the attracted object. A steel housing (pot magnet) acts as a concentrator: it guides the flux from the rear pole around the magnet and focuses it on the working face.
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.
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.
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.