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Magnetic field of a current calculator

The magnetic flux density B produced by a current, for three classic geometries: a long straight wire (B = μ₀I/2πr), the centre of a single circular loop (B = μ₀I/2R) and inside a long solenoid (B = μ₀nI). Choose the geometry and enter the current; only that mode's fields matter.

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Need Magnetic field B (T), In microtesla (µT), In gauss (G)? The Magnetic field of a current calculator derives it from Geometry, Current I (A), Wire — distance r (m), Loop — radius R (m), Solenoid — turns per metre n in one step. For instance, with Geometry = Long straight wire, Current I (A) = 10, Wire — distance r (m) = 0.05, Loop — radius R (m) = 0.1 and Solenoid — turns per metre n = 1,000 it returns Magnetic field B (T) = 0, In microtesla (µT) = 40 and In gauss (G) = 0.4.

How to use it

  1. Enter your values: Geometry, Current I (A), Wire — distance r (m), Loop — radius R (m), Solenoid — turns per metre n.
  2. Read the result instantly: Magnetic field B (T), In microtesla (µT), In gauss (G).

Frequently asked questions

How does the Magnetic field of a current calculator work?

It takes Geometry, Current I (A), Wire — distance r (m), Loop — radius R (m) and Solenoid — turns per metre n and derives Magnetic field B (T), In microtesla (µT) and In gauss (G) from them. The calculation is live as you type, so the result updates on every change.

Which values does the calculator ask for?

5 values: Geometry, Current I (A), Wire — distance r (m), Loop — radius R (m) and Solenoid — turns per metre n. Nothing else is required — no account, no file upload.

What does a typical calculation look like?

With Geometry = Long straight wire, Current I (A) = 10, Wire — distance r (m) = 0.05, Loop — radius R (m) = 0.1 and Solenoid — turns per metre n = 1,000, the calculator returns Magnetic field B (T) = 0, In microtesla (µT) = 40 and In gauss (G) = 0.4. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

How much does the result change with different inputs?

It moves a lot. Using Geometry = Centre of a circular loop, Current I (A) = 20, Wire — distance r (m) = 0.1, Loop — radius R (m) = 0.2 and Solenoid — turns per metre n = 2,000 instead, Magnetic field B (T) goes from 0 to 0 — which is why it is worth testing a few scenarios rather than trusting a single figure.

Which “Geometry” option should I choose?

You can pick between « Long straight wire », « Centre of a circular loop » and « Inside a long solenoid ». Each one changes what the calculator works out, so switch and compare — the default is « Long straight wire ».

When would I actually use this?

Checking a homework answer, sizing something before building it, and getting an order of magnitude before committing to a design — a torque on a bolt, the force a spring returns, the frequency a circuit resonates at, how long light takes to arrive.

What is the most common mistake?

Feeding in a value in the wrong unit. Physics formulas assume SI throughout, so grams instead of kilograms or centimetres instead of metres shifts the answer by powers of ten without any warning.

Where do the figures come from, and how current are they?

Constants are the CODATA values and the formulas are the textbook ones. Most assume an idealised case — no air resistance, no friction, a point mass — and the tool says so where the simplification matters.

Further reading

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