Ohm’s Law Calculator
Pick the two values you know, out of voltage, current, resistance and power, and get the other two. Units like mA, kΩ and kW are changed for you, and the steps show which formula to use and why.
Current and power
2 A · 24 W
At 12 V across 6 Ω, the current is 2 A and the power is 24 W.
Show me how
Why does this work?
All steps as text
Tip: press ← and → to move between steps, or click any step.
Ohm’s law: V = I × R
The voltage across a resistor equals the current through it times its resistance: V = I × R, with V in volts, I in amps and R in ohms (Ω). The same rule gives the other two: I = V ÷ R and R = V ÷ I. With 12 V across a 6 Ω resistor, the current is 12 ÷ 6 = 2 A.
To remember them, draw a triangle with V at the top and I and R along the bottom. Cover the one you want: V over R means divide, and I next to R means multiply.
Power: P = V × I
Power in watts is voltage times current, P = V × I. Putting Ohm’s law into it gives two more forms, so you can use whichever two values you have: P = I² × R and P = V² ÷ R. A 60 W bulb on 230 V draws 60 ÷ 230 ≈ 0.26 A.
The calculator works out the second answer from the two values you gave, not from the first answer, so a rounded number is never used again.
All twelve formulas
| To find | From V and I | From V and R | From I and R | With power |
|---|---|---|---|---|
| Voltage V | — | — | I × R | P ÷ I, or √(P × R) |
| Current I | — | V ÷ R | — | P ÷ V, or √(P ÷ R) |
| Resistance R | V ÷ I | — | — | V² ÷ P, or P ÷ I² |
| Power P | V × I | V² ÷ R | I² × R | — |
mA, kΩ and the other prefixes
Electronics uses prefixes to keep numbers short. Change them to plain volts, amps, ohms and watts before you use a formula:
- µ (micro) ÷ 1,000,000: 500 µA = 0.0005 A.
- m (milli) ÷ 1,000: 20 mA = 0.02 A, and 60 mW = 0.06 W.
- k (kilo) × 1,000: 4.7 kΩ = 4,700 Ω.
- M (mega) × 1,000,000: 1 MΩ = 1,000,000 Ω.
Example: a resistor for an LED
A red LED on a 5 V supply needs about 2 V across it and 20 mA through it. The resistor takes the rest: 5 − 2 = 3 V. So R = V ÷ I = 3 ÷ 0.02 = 150 Ω. Its power is 3 × 0.02 = 0.06 W, so a common ¼ W (0.25 W) resistor is plenty. If the exact value isn’t sold, use the next size up, which lets a little less current through.
Check your LED’s data sheet for its real voltage and current. This page is for learning and small circuits: mains electricity is dangerous, so leave that to a qualified electrician.
Questions
How much current does a 2,000 W kettle draw?
I = P ÷ V. On 230 V that is 2,000 ÷ 230 ≈ 8.7 A; on 120 V it is 2,000 ÷ 120 ≈ 16.7 A.
Does Ohm’s law work for AC?
Yes for things that only have resistance, like heaters and kettles, using the RMS voltage and current (the values on the label). With motors, capacitors or coils, the resistance becomes an impedance that depends on the frequency.
Why does a bulb’s resistance change?
A filament’s resistance rises as it heats up, so a cold bulb measures much lower than V ÷ I when it is lit. Ohm’s law still holds at each moment; R just isn’t fixed.
What does the Ω symbol mean?
Ω, the Greek letter omega, stands for ohms, the unit of resistance. One ohm lets one amp through when one volt is across it.
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