Magnetic Field of Straight Wire Calculator

The magnetic field around a long, straight current-carrying wire is given by B = μ₀I / (2πd), where B is the magnetic field strength in tesla, μ₀ is the permeability of free space (4π × 10⁻⁷ T·m/A), I is the current in amperes, and d is the perpendicular distance from the wire in metres.

The magnetic field strength decreases inversely with distance. If you double the distance from the wire, the magnetic field strength is halved. This inverse relationship (B ∝ 1/d) means the field weakens rapidly as you move away from the wire.

The magnetic field forms concentric circles around the wire. You can determine the direction using the right-hand rule: point your right thumb in the direction of the current, and your curled fingers indicate the direction of the circular magnetic field lines around the wire.

The permeability of free space, μ₀, is a physical constant equal to 4π × 10⁻⁷ T·m/A (approximately 1.2566 × 10⁻⁶ T·m/A). It represents how easily a magnetic field can be established in a vacuum and is a fundamental constant in electromagnetism.

The magnetic field strength is directly proportional to the current flowing through the wire (B ∝ I). Doubling the current doubles the magnetic field at any given distance. This is why high-current power lines can produce significant magnetic fields even at several metres away.

Earth's magnetic field is approximately 25–65 microtesla (μT) at the surface. A wire carrying 10 A of current at a distance of 5 cm produces a field of about 40 μT — similar in magnitude to Earth's field. At larger distances or lower currents, the wire's field quickly becomes much weaker than Earth's.

Strictly speaking, the formula B = μ₀I / (2πd) applies to infinitely long straight wires. In practice, it gives accurate results for finite wires when the distance from the wire is much smaller than the wire's length. For short wires or points near the ends, a more complex integration using the Biot-Savart Law is required.

Magnetic field strength (B) is measured in tesla (T) in SI units. Because the tesla is a large unit, results are often expressed in millitesla (mT, 10⁻³ T), microtesla (μT, 10⁻⁶ T), or nanotesla (nT, 10⁻⁹ T). This calculator displays results in all three scales for convenience.