Magnetic Field of a Wire Calculator

B = μ₀·I / (2π·r), where μ₀ is the permeability of free space, I is current, and r is the perpendicular distance from the wire.

The permeability of free space: 4π × 10⁻⁷ T·m/A. This was exact by definition in the pre-2019 SI; since the 2019 redefinition it is a measured quantity, μ₀ = 1.25663706212(19) × 10⁻⁶ T·m/A, differing from 4π × 10⁻⁷ only at the ~2 × 10⁻¹⁰ level. The 4π × 10⁻⁷ value remains an excellent approximation for this calculation.

No - this formula is for a single straight wire. Coils, solenoids, and toroids have their own formulas (B = μ₀nI for a long solenoid, for example).

Because B = μ₀I/(2πr) falls off as 1/r, distance matters a lot. A transmission line carrying 400 A produces about 8 µT at 10 m - comparable to Earth's ~50 µT field - but under 1 µT at 100 m, since the field drops tenfold for every tenfold rise in distance. As a rule of thumb, once you are several tens of meters from a typical line the field is well below everyday background levels. Real power lines run several conductors carrying opposing 50/60 Hz currents that partly cancel, so actual values are often lower than this single-wire estimate.