Straight Wire Magnetic Field Calculator

An electric current in a straight conductor creates a magnetic field that circles the wire. For a wire that is effectively infinite compared with the observation distance, symmetry makes the field magnitude depend only on current, distance, and magnetic permeability. Ampere's law gives B = μ₀μrI / (2πr). In free space, relative permeability μr is approximately one, and the expression simplifies numerically to B = 2 × 10⁻⁷ I/r when current is in amperes and distance is in metres. The field grows directly with current: doubling current doubles magnetic flux density at every fixed distance. It decreases inversely with radial distance: doubling the distance halves the field. Its direction is tangential to an imaginary circle centred on the wire. Apply the right-hand grip rule by pointing your thumb with conventional current; the curl of your fingers shows the magnetic field direction. Reversing current reverses direction but leaves the reported magnitude unchanged if current magnitude is used. Relative permeability represents how a linear surrounding medium changes the field compared with vacuum. Air is usually treated as μr = 1 for everyday calculations. Some materials can have larger or field-dependent permeability, but this simple model assumes a uniform, linear, isotropic medium. Values for ferromagnetic materials should be used cautiously because permeability can depend on field strength, prior magnetization, temperature, and geometry. The calculator multiplies the vacuum result by the entered relative permeability and reports both teslas and microteslas for convenient comparison. The infinite-wire approximation is best when the straight conductor extends many times farther than the observation distance in both directions. Near a wire end, bend, return conductor, or finite segment, the Biot-Savart law with actual geometry gives a better result. The model also treats the conductor as thin, so distance is measured from its central axis and should normally exceed its radius. Nearby currents contribute their own vector fields and are not included. Use SI units consistently: convert centimetres to metres before entering distance. This calculator is useful for electromagnetism exercises, initial busbar estimates, sensor placement studies, and checking the order of magnitude around laboratory conductors while keeping its geometric assumptions in mind.