Ohm's Law Calculator
Know the voltage and resistance but need the current? Have a wattage rating and a supply voltage? Enter any two of the four electrical quantities — voltage, current, resistance, power — and the other two are computed instantly.
Solving from: Voltage + Resistance — edit any field to swap it in.
Voltage
12 V
entered
Current
2 A
computed
Resistance
6 Ω
entered
Power
24 W
computed
Assumes a resistive load (DC, or AC using RMS values with power factor 1). Real resistors also change with temperature — an incandescent bulb's filament has roughly 10× more resistance hot than cold, which is why they usually fail at switch-on.
How Ohm's Law Connects All Four Values
Two relationships generate everything: Ohm's law (V = I × R) and the power law (P = V × I). Combine them and any pair of values pins down the other two. A 12 V supply across a 6 Ω resistor pushes 12 ÷ 6 = 2 A of current and dissipates 12 × 2 = 24 W. Run it the other way: a 60 W bulb on a 120 V circuit draws 60 ÷ 120 = 0.5 A, which means its hot filament resistance is 120 ÷ 0.5 = 240 Ω.
The Ohm's law wheel — all 12 formulas
Voltage V = I × R V = P ÷ I V = √(P × R) Current I = V ÷ R I = P ÷ V I = √(P ÷ R) Resistance R = V ÷ I R = V² ÷ P R = P ÷ I² Power P = V × I P = I² × R P = V² ÷ R Example: V = 12 V, R = 6 Ω I = 12 ÷ 6 = 2 A P = 12² ÷ 6 = 24 W
Frequently Asked Questions
How do I find resistance if I only know watts and volts?
Use R = V² ÷ P. A 60 W bulb on a 120 V circuit has a hot resistance of 120² ÷ 60 = 240 Ω, and draws 60 ÷ 120 = 0.5 A. The calculator does this automatically — enter power and voltage and it solves the other two.
Why do I only enter two of the four values?
Voltage, current, resistance, and power are linked by just two independent equations — V = I × R and P = V × I — so any two values fully determine the other two. Entering a third would either be redundant or contradict the first two. The calculator tracks your two most recently edited fields and solves from those.
Does Ohm's law work for AC circuits?
Yes, for resistive loads like heaters, kettles, and incandescent bulbs — just use RMS values, which is what your multimeter and the 120 V on the outlet already are. For motors and electronics, reactance and power factor enter the picture, so real current can run 10–25% above the simple V ÷ R prediction.
Is Ohm's law exact for real components?
Only for 'ohmic' materials whose resistance stays constant. Many real parts aren't: an incandescent filament has roughly 10× more resistance hot than cold, and diodes and LEDs don't follow a straight V–I line at all. For wire, resistors, and heating elements at steady temperature, though, it's accurate to a fraction of a percent.