⚡ Ohm’s Law Calculator
Enter values for any two of the four fields — the rest are calculated automatically.
Enter any two known values among voltage (V), current (A), resistance (Ω), and power (W), and this tool automatically calculates the remaining two using Ohm’s law (V=IR) and the power formula (P=VI).
How to use
- Enter values for any two of the four fields (voltage, current, resistance, power).
- The other two fields are calculated automatically.
- If you enter more than two, the first two in order (voltage, current, resistance, power) are used.
How the calculation works
Enter any two of voltage (V), current (I), resistance (R) and power (P), and this tool calculates the other two. Ohm's law: V = I × R Power: P = V × I Combining the two gives further forms: P = I² × R = V² ÷ R I = √(P ÷ R), V = √(P × R) Of the values you enter, the first two in the order voltage → current → resistance → power are used. Units are volts (V), amperes (A), ohms (Ω) and watts (W).
Worked example
12 V and 4 Ω Current: 12 ÷ 4 = 3 A Power: 12 × 3 = 36 W 100 V and 1,000 W (a Japanese household hair dryer, say) Current: 1,000 ÷ 100 = 10 A Resistance: 100 ÷ 10 = 10 Ω A Japanese household outlet is rated at 15 A, so two 1,000 W appliances at once would exceed it.
Things to be aware of
- With AC appliances containing motors, the power factor means volts × amps may not equal the real power consumed (W).
- For an LED's series resistor, use (supply voltage − LED voltage) ÷ desired current.
- Electrical installation and high-voltage work must be done by qualified people.
FAQ
Can I use any combination of two values?
Yes. Voltage and current, voltage and resistance, current and resistance, voltage and power — any two of the four fields can be used.
What formulas does this use?
It combines Ohm’s law (V = I × R) with the power formula (P = V × I) to derive the remaining two values from any two known values.
What happens if I enter more than two values?
Only the first two values, in the order voltage, current, resistance, power, are used for the calculation — the rest are ignored. Note that inconsistent values are not flagged.