Current in a wire creates circular magnetic field lines around it. A 10 A cable produces 20 µT at 10 cm – about half of Earth’s field – which is why compasses misbehave near power cables.
How to use the Magnetic Field of a Straight Wire Calculator
- Current I
- Distance from the wire d
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the current and the distance from the wire.
Inputs explained
| Field | What to enter |
|---|---|
| Current I | Units: A, mA, µA, kA |
| Distance from the wire d | Units: cm, mm, m, in, ft |
What each result means
- Magnetic field B = 1.25663706212e-06×[Current I]÷(2×pi×[Distance from the wire d])
- Compared with Earth’s field (≈ 50 µT) = [Magnetic field B]÷50e-6×100
Example values
How the main result changes when Current I changes and everything else stays at the example values above:
| Current I (A) | Magnetic field B (µT) |
|---|---|
| 5 | 10 |
| 7.5 | 15 |
| 10 | 20 |
| 12.5 | 25 |
| 15 | 30 |
| 20 | 40 |
Formula
B = μ₀·I / (2π·d)
Worked example
Inputs — Current I: 10 A; Distance from the wire d: 10 cm.
Results — Magnetic field B = 20 µT; Compared with Earth's field (≈ 50 µT) = 40 %.
Key terms
- Tesla
- SI unit of magnetic flux density.
Frequently asked questions
Which way do the field lines go?
Circles around the wire – use the right-hand grip rule.
Why does a two-core cable produce little field?
The opposite currents cancel each other.
How does the field change with distance?
It falls as 1/d.
Is the formula valid near the wire's ends?
No – it assumes an infinitely long wire.