Use this free Ohm's law calculator to find voltage, current, resistance and power. Choose which two you know, such as voltage and current or power and resistance, type them with their units (mV, mA, kΩ, kW and more), and you get the other two, plus the conductance and the energy used in an hour. The answer shows the formula and the working, and tables list the 12 Ohm's law formulas, the current through common resistors and the unit prefixes. The Ohm's law wheel and triangle show how the quantities fit together. It is free and instant, and your numbers stay in your browser.
What is Ohm's law?
Ohm's law links three basic electrical quantities: voltage (V, in volts), current (I, in amps) and resistance (R, in ohms). It says that the current through a resistor is proportional to the voltage across it, so the same resistor passes twice the current when you double the voltage. Add power (P, in watts), which is voltage times current, and you have the four quantities that describe most simple circuits.
V = I × R I = V ÷ R R = V ÷ IP = V × I = I² × R = V² ÷ RHow to calculate voltage, current and resistance
To find the voltage, multiply the current by the resistance: 2 A through 6 Ω is 2 × 6 = 12 V. To find the current, divide the voltage by the resistance: 12 V across 6 Ω is 12 ÷ 6 = 2 A. To find the resistance, divide the voltage by the current: 12 V and 2 A is 12 ÷ 2 = 6 Ω. The calculator does this for any two values you know and shows the working.
The power formulas
Power is voltage times current. Put V = I × R into it and you get P = I² × R, and put I = V ÷ R into it and you get P = V² ÷ R. With 12 V and 2 A the power is 24 W, and run for one hour that is 24 Wh of energy. A 100 W lamp on 230 V draws 100 ÷ 230 = 0.435 A, so its resistance is 230 ÷ 0.435 = 529 Ω.
The Ohm's law wheel
The wheel puts the four quantities in four slices and gives three formulas for each, twelve in all. Find the slice for what you want, and use the formula that matches the two values you know. For voltage that is I × R, P ÷ I or √(P × R). The table on this page lists all twelve, and the triangle is a shorter memory aid for the first three: put V on top and I and R side by side underneath, and cover the one you want.
Units and prefixes
Real circuits use small and large numbers, so you can type a value with a prefix: milliamps (mA), microamps (µA), kilo-ohms (kΩ) and megohms (MΩ). 5 mA through 10 kΩ gives 5 × 10 = 50 V and 250 mW. The calculator changes everything to volts, amps, ohms and watts, and shows the answers with the prefix that reads best.
How much current flows through a resistor?
Divide the voltage by the resistance. A 220 Ω resistor on 5 V passes 5 ÷ 220 = 22.73 mA and gives off 113.6 mW as heat. The table of current through common resistors lists 100 Ω to 10 kΩ at 3.3 V, 5 V, 9 V and 12 V.
When Ohm's law does not apply
Ohm's law holds for resistors and most wires at a steady temperature. It does not describe parts such as diodes, LEDs and transistors, where the current does not rise in proportion to the voltage. In an AC circuit it works with RMS values if the load is purely resistive, such as a heater. Motors, capacitors and coils also have reactance, which needs impedance rather than resistance. Always check a component's power rating, and take care with any mains electricity.
Calculators that go with this one
To work with powers and square roots, use the Scientific Calculator. To change a result such as 0.435 A into a fraction, see the Fraction to Decimal Calculator, and for a quick sum, the Simple Calculator.
