What this calculator does
Magnetic field of a straight wire works out field strength at a distance from a long straight wire, from Ampère's law. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaB = μ₀I/(2πr), μ₀ = 1.25663706212×10⁻⁶ N/A²
The inputs explained
| Field | What to enter |
|---|---|
| Current (A) | A number, measured in A. Starts at 10. |
| Distance from the wire (m) | A number, measured in m. Starts at 0.05. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with current
Every other input is held at the calculator’s starting values while current varies. Select any row to load that scenario into the calculator.