Magnetic Permeability Calculator

Use this magnetic permeability calculator to compute a material's permeability, relative permeability, and susceptibility.

Permeability used1.2566 µH/mµ₀, the vacuum value. Since 2019 this is a MEASURED quantity, not exactly 4π×10⁻⁷

Field at that distance40 µT0.4 gauss, 40 µT

Against Earth's field0.8×Earth's field is about 50 µT

Field at twice the distance20 µThalf — a straight wire's field falls as 1/r, not as an inverse square, because the source is a line rather than a point

Force per metre on a parallel wire carrying the same current400 µN/mattractive if the currents run the same way — this is what used to define the ampere

Distance where the field equals Earth's40 mm

Force per metre in a 1 T external field10 N/mF = BIL sin θ, perpendicular — this is the motor force, and it is distinct from the mutual force between two wires above

Force on a 100 mm length in that field1 Nscale by your own field and length — the relation is linear in both

The formula

wire B = µ₀I ÷ 2πr; solenoid B = µ₀nI; L = µ₀N²A ÷ l

Line sources fall off more slowly

A long straight wire's field falls as 1/r, not as an inverse square. That is not an exception to anything — it follows from the source being a line rather than a point, and integrating the contributions along it. A point source in three dimensions gives 1/r²; a line gives 1/r; an infinite sheet gives a field that does not fall off at all.

Inside a long solenoid the field depends only on the turn density and the current — not on the radius, and not on the total number of turns. Two solenoids of very different diameters with the same turns per metre produce identical interior fields. Inductance is different: it goes as the square of the total turns and with the cross-sectional area, so a fat coil stores far more energy at the same field.

The formula assumes a long solenoid. Below a length-to-diameter ratio of about five the ends leak appreciably and the real field is lower than calculated.

An iron core multiplies the field by its relative permeability, which can reach several thousand — but only until the core saturates, typically around 1.5 to 2 tesla, beyond which further current buys almost nothing. Note that µ₀ has been a measured quantity since the 2019 SI redefinition; it is no longer exactly 4π×10⁻⁷.