How to use this calculator
- Enter pole flux density. Use measured or estimated normal flux density at the pole face in tesla.
- Enter pole area. Use the effective contact width and length for each active pole face.
- Derate contact. Reduce the contact factor for air gaps, paint, roughness, thin steel and leakage.
- Set safety factor. Use a larger safety factor for lifting, vibration, sliding or uncertain surface conditions.
How it works
A magnetic field stores energy density that acts like pressure normal to a pole
face. For a flat pole with flux density B, the magnetic pressure is:
p_m = B² / (2 · mu0)
The ideal force is pressure times total pole area. This calculator applies the entered contact factor before reporting the derated holding force:
F_hold = p_m · A_total · k_c
The safe working load is the derated holding force divided by safety factor:
SWL = F_hold / SF
If you do not know pole flux density yet, start with the solenoid force calculator or solenoid magnetic field calculator to estimate a first-pass field from coil geometry.
Worked example
Verified against the live calculator
A magnetic fixture has two rectangular poles, each 40 mm by
50 mm. Measured pole flux density is 1.2 T. Contact
quality is estimated at 60%, and the safety factor is 3.
Total pole area is 2 x 40 x 50 = 4000 mm², or
0.004 m². Magnetic pressure is
1.2² / (2 x 4*pi*10^-7) = 572958 Pa. The ideal force would be about
2292 N, and the 60% contact factor derates it to about
1375 N.
With safety factor 3, the safe working load is about
458 N, equivalent to roughly 46.8 kg under static
vertical loading.
Frequently asked questions
How do you calculate electromagnet holding force?
For a flat pole face with known normal flux density, magnetic pressure is p = B^2/(2*mu0). Holding force is that pressure times total pole area, then derated by contact factor.
What is contact factor?
Contact factor is a practical derating for air gap, paint, rust, roughness, thin backing steel, curved parts, leakage and nonuniform pole contact. A perfect lab contact is not the same as a shop-floor fixture.
Why does the safe working load divide by safety factor?
The calculated holding force is an estimate. The safe working load applies your chosen design margin for uncertainty, vibration, off-axis load, surface condition and measurement error.
Can I use this for lifting people or overhead loads?
No. Use this as a screening estimate only. Overhead lifting needs rated equipment, proof testing, redundancy and applicable safety standards.
Method & assumptions
- Flux density is assumed normal to the pole area and roughly uniform over the entered contact area.
- Contact factor is the only derating for leakage, air gap, roughness, steel thickness, paint and nonuniform flux.
- The mass output is a static vertical equivalent from force divided by standard gravity.
- This is not a certified lifting calculation. Safety-critical workholding or lifting needs test data and rated equipment.