Ohm's Law Calculator
Voltage, current, resistance, power — know any two, instantly fill in the other two.
What it does: Use Ohm's law and the power formulas to find the other two from any two electrical quantities.
When to use it: When analyzing a circuit, estimating current, or choosing a resistor power rating.
MEANS The two values you entered determine the whole circuit; the four values below are mutually consistent.
Common cases pre-computed: browse Ohm's law presets →
No history yet. Each calculation is automatically saved to this device.
Read the guide: LED current-limiting resistor: theory & sizing
How to use the Ohm's law calculator
Enter two, get two.
- 01
Enter any two values
Voltage V, current I, resistance R, power P — fill in whichever two you know and leave the rest blank. Supports notation like
1k,20mA. - 02
Click Calculate
The tool fills in the remaining two values automatically using Ohm's law and the power formulas.
- 03
Read the result
All four values are shown at once. Click "See how it's calculated" to see which formula was used.
Common questions, answered in 3 minutes
What is Ohm's law?
V = I × R: voltage equals current times resistance. It is the most fundamental relationship in circuit analysis and the starting point of nearly every calculation.
Why does entering three values give an error?
Ohm's law uniquely determines the other two values from two knowns. Entering three over-constrains it (they may contradict each other), so only two are allowed.
How does power fit in?
P = V × I, which can also be written P = I²R or P = V²/R. This tool automatically picks the corresponding formula based on the two values you give.
Can resistance be 0?
No. R=0 is a short circuit, current is theoretically infinite, which is physically meaningless, so the tool gives an error.
Standards and sources referenced by this tool
| Item | Value / Formula | Source |
|---|---|---|
| Ohm's law | V = I × R | Ohm's law (G. Ohm, 1827) |
| Power | P = V × I = I²R = V²/R | Joule / Watt law |
Pure formula calculation, no external API.