Slide a compass near a power line and the needle twitches — current makes magnetism, and the field circling a straight wire follows Ampere's law: field strength equals the permeability constant times current, divided by 2π times distance. In symbols: B = μ₀I / (2πr), with μ₀ = 4π × 10⁻⁷. Double the current, double the field; double the distance, halve it.

MRI machines are this formula industrialized: superconducting coils carry enormous currents to forge fields 60,000 times Earth's, aligning hydrogen nuclei in your body for imaging. On a smaller scale, every electric motor spins because current-carrying wires push against magnetic fields, and every clamp meter reads current by sensing the field around the wire — no circuit cutting required.

This calculator evaluates the field formula's core multiplication. Type μ₀ times your current (μ₀ × I) in the Variable A box — for 100 amps that is about 0.0001257 — and the reciprocal factor 1/(2πr) in the Variable B box for your distance in meters. The Result box gives the field in teslas. At 10 cm from a 100-amp cable the field is about 200 microteslas — four times Earth's field, plenty to swing a compass needle.

With the pre-filled defaults — Variable A 10, Variable B 5 — the Magnetic Field Calculator returns Result 50.0000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.