Resistance Calculator

Resistance, in ohms (Ω), is how strongly something opposes the flow of electric current. You can find it from two electrical measurements using Ohm's law and the power equations, or from the physical make-up of a conductor: its material, length and thickness. This calculator does both and, for circuit work, suggests a nearby E24 preferred resistance. Check actual part availability, tolerance and power rating separately.

How to use this tool

1. Choose how you want to find the resistance.
2. For the electrical methods, enter the two values you know: voltage in volts, current in amps or power in watts.
3. For a wire, pick the material, then enter the length in metres and either the diameter in mm or the cross-section area in mm².
4. Read the resistance in ohms, with a k/m prefix, plus the conductance and the extra values for your method.

Formula

R = V / I R = V² / P R = P / I² R = ρ × L / A

V is voltage, I current, P power, ρ (rho) the resistivity of the material in Ω·m, L the length in metres and A the cross-section area in m². A round wire of diameter d has A = π × d² / 4. Conductance is 1/R, measured in siemens (S).

Example

A lamp draws 0.5 A from a 12 V supply, so R = 12 ÷ 0.5 = 24 Ω and it uses 12 × 0.5 = 6 W; 24 Ω happens to be an E24 standard value. For a wire, 10 m of 1 mm diameter copper has A = 0.7854 mm², giving R = 1.68 × 10⁻⁸ × 10 ÷ (0.7854 × 10⁻⁶) ≈ 0.2139 Ω.