Four quantities, two rules
Every simple circuit links four quantities: voltage (V, in volts), current (I, in amps), resistance (R, in ohms) and power (P, in watts). Ohm’s law, V = I × R, ties the first three together, and the power rule, P = V × I, adds the fourth. Knowing any two is enough to work out the rest.
This Ohm’s law calculator does it for all six pairs, and writes small and large answers with the usual prefixes, such as 22.7 mA or 4.7 kΩ, so the result reads the way it would on a component or a meter.
How to use it
- Choose the two values you know, such as voltage and resistance.
- Enter them in base units: volts, amps, ohms and watts. For 20 mA, type 0.02.
- Read the two missing values at the top, and all four together underneath.
- Open the working to see which formula was used.
The formulas
V = I × R; I = V ÷ R; R = V ÷ I
P = V × I = I² × R = V² ÷ R
I = √(P ÷ R); V = √(P × R)The power forms come from putting Ohm’s law into P = V × I. They are handy for checking that a resistor can handle the heat it will produce: a resistor’s power rating must be above the power the calculator shows.
Worked example: an LED resistor on 5 V
- Voltage 5 V across a 220 Ω resistor
- Current: I = V ÷ R = 5 ÷ 220 = 22.7273 mA
- Power: P = V² ÷ R = 25 ÷ 220 = 113.6364 mW
The resistor passes 22.7273 mA and turns 113.6364 mW into heat, so a common quarter watt (250 mW) resistor is enough. In a real LED circuit, the LED takes part of the voltage, so the resistor sees less than the full 5 V.
Prefixes at a glance
| Prefix | Symbol | Multiply by | Example |
|---|---|---|---|
| mega | M | 1,000,000 | 1 MΩ = 1,000,000 Ω |
| kilo | k | 1,000 | 4.7 kΩ = 4,700 Ω |
| milli | m | 0.001 | 20 mA = 0.02 A |
| micro | µ | 0.000001 | 50 µA = 0.00005 A |
Type values in base units and the calculator chooses the clearest prefix for each answer.
Checking a circuit safely
Ohm’s law works for resistors, heaters and plain wires, whose resistance stays nearly the same as the current changes. Lamps, LEDs, motors and electronics do not behave so simply, so treat answers for them as estimates. Always switch off the supply before measuring resistance with a meter.
Where it is used
- Sizing resistors for LEDs and sensors.
- Checking that a fuse or wire can carry a load. See the watts to amps converter.
- Working out the voltage lost along cables, with the voltage drop calculator.
- For dividers and resistor networks, see the resistor and voltage divider calculator.
Frequently asked questions
What is Ohm’s law?
Ohm’s law says that the voltage across a resistor equals the current through it times its resistance: V = I × R. Double the voltage across the same resistor and the current doubles. It holds for ordinary resistors and wires, though not for parts like LEDs and diodes.
How do I calculate current from voltage and resistance?
Divide the voltage by the resistance: I = V ÷ R. Five volts across 220 ohms gives about 0.0227 amps, or 22.7 milliamps. Choose Voltage and resistance above and the calculator also gives the power the resistor turns into heat.
How do I calculate power from current and resistance?
Square the current and multiply by the resistance: P = I² × R. Two milliamps through 4,700 ohms is 0.002 × 0.002 × 4,700 = 0.0188 watts, or 18.8 milliwatts. Choose Current and resistance above to get power and voltage together.
How do I pick a resistor’s power rating?
Work out the power the resistor will dissipate with P = V² ÷ R or P = I² × R, then choose a resistor rated comfortably above it, often twice as much for margin. If the calculator shows 113.6 mW, a 250 mW resistor gives a sensible safety margin.
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