Ohm's Law Calculator
Solve for voltage, current, resistance or power from any two known values — all 12 Ohm's law wheel formulas, unit choices from µA to kV and MΩ, worked steps and a resistor wattage check.
How it works
V = I × R = P ÷ I = √(P × R) · I = V ÷ R = P ÷ V = √(P ÷ R) · R = V ÷ I = V² ÷ P = P ÷ I² · P = V × I = V² ÷ R = I² × R
Ohm's law (V = I × R) links the voltage across a component, the current through it and its resistance. Combined with the power law P = V × I, any two of the four quantities fix the other two, which gives the twelve formulas of the Ohm's law wheel. Choose what you want to find, then which two values you know: the calculator solves the unknown, fills in the fourth quantity and shows each step with your numbers. Every field has its own unit menu (µV to kV, µA to kA, mΩ to MΩ, mW to MW, horsepower and BTU/h), and answers are scaled to a readable prefix such as mA or kΩ. Extras include the conductance, the energy used over a running time you choose, a suggested resistor wattage with a 2× safety margin, a table that checks all twelve wheel formulas against each other, and the straight-line current-versus-voltage graph that defines an ohmic resistor. The relations hold for DC and for the RMS values of purely resistive AC loads; motors and other reactive loads need impedance and power factor.
Worked example
A 2 A current through a 6 Ω resistor gives V = I × R = 2 × 6 = 12 V and P = V × I = 12 × 2 = 24 W, so the resistor needs a rating of at least 48 W (a 50 W part). Working backwards from power: a 60 W lamp on 120 V draws I = P ÷ V = 60 ÷ 120 = 0.5 A and has a hot resistance of R = V² ÷ P = 14,400 ÷ 60 = 240 Ω. For a 5 V supply across a 4.7 kΩ resistor, I = 5 ÷ 4,700 ≈ 1.064 mA and P = 5² ÷ 4,700 ≈ 5.32 mW, well within a 1/8 W part.
Frequently asked questions
Which two values do I need?
Any two of voltage, current, resistance and power. Pick what you want to find under “Solve for”, then choose which pair you know — for example voltage from power and resistance uses V = √(P × R). Only the two values you selected are used; the other fields are ignored.
How do I enter milliamps or kilohms?
Use the unit menu next to each field. You can type 20 and pick mA instead of entering 0.02 A, or 4.7 with kΩ instead of 4,700 Ω. Results are shown with a readable prefix automatically.
Why can resistance or current not be zero?
Several formulas divide by them. Zero resistance would mean infinite current at any voltage (a short circuit), and with zero current there is no way to tell the resistance from the voltage, so those combinations return a message instead of a number.
What wattage resistor should I use?
Work out the power the resistor dissipates (P = I² × R or V² ÷ R) and choose a part rated for at least twice that. The calculator suggests the next standard rating — 1/8, 1/4, 1/2, 1, 2, 3, 5, 10, 25, 50 or 100 W — with that margin.
Does Ohm's law always hold?
It holds for ohmic conductors, whose resistance stays constant. Diodes, LEDs, lamps (whose filament resistance rises as it heats) and semiconductors are non-ohmic, so the straight-line relationship only approximates them at one operating point.
Does it work for AC?
Yes for purely resistive loads such as heaters and incandescent lamps, using RMS voltage and current. Loads with coils or capacitors have reactance, so you need impedance instead of resistance and real power becomes V × I × power factor.
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Educational estimate. Check the formula and units for your exact case.