What this calculator does
This tool computes the magnetic field strength B at a perpendicular distance r from a long, straight conductor carrying a steady current I. The magnetic field forms concentric circles around the wire, and its magnitude follows directly from Ampère's law. This is a universal physics calculation — the result is given in SI units (teslas).
How to use it
Enter the current flowing through the wire in amperes (A) and the perpendicular distance from the center of the wire in meters (m). The calculator returns the field in teslas (T) and the more practical microteslas (µT). The formula assumes an infinitely long, thin wire and a point in vacuum or air.
The formula explained
The magnetic field is given by:
$$B = \frac{\mu_0 \, I}{2\pi \, r}$$
Here \(\mu_0 = 4\pi \times 10^{-7} \approx 1.25664 \times 10^{-6}\ \text{T}\cdot\text{m/A}\) is the permeability of free space. The field is directly proportional to the current and inversely proportional to the distance — doubling the current doubles the field, while doubling the distance halves it.
Worked example
A wire carries \(I = 10\ \text{A}\) and we measure at \(r = 0.05\ \text{m}\) (5 cm). Then $$B = \frac{1.25664 \times 10^{-6} \times 10}{2\pi \times 0.05} = \frac{1.25664 \times 10^{-5}}{0.31416} = 4.0 \times 10^{-5}\ \text{T} = 40\ \mu\text{T}.$$ That is roughly the strength of Earth's own magnetic field.
FAQ
What direction does the field point? The field circles the wire; use the right-hand rule — point your thumb along the current and your fingers curl in the field direction.
Does this work inside the wire? No. This formula applies outside the wire. Inside a uniform wire the field grows linearly with radius instead.
Why teslas and microteslas? Practical fields near wires are tiny fractions of a tesla, so microteslas (\(1\ \mu\text{T} = 10^{-6}\ \text{T}\)) are easier to read.
