The tabs above switch to a series and parallel resistor calculator and a voltage divider calculator.
Ohm's law formulas
| To find | From V and I | From V and R | From I and R | From P and V | From P and I | From P and R |
|---|---|---|---|---|---|---|
| Voltage V | — | — | I × R | — | P ÷ I | √(P × R) |
| Current I | — | V ÷ R | — | P ÷ V | — | √(P ÷ R) |
| Resistance R | V ÷ I | — | — | V² ÷ P | P ÷ I² | — |
| Power P | V × I | V² ÷ R | I² × R | — | — | — |
Unit prefixes
| Prefix | Factor | Example |
|---|---|---|
| µ (micro) | 0.000001 | 250 µA = 0.00025 A |
| m (milli) | 0.001 | 20 mA = 0.02 A |
| k (kilo) | 1,000 | 4.7 kΩ = 4,700 Ω |
| M (mega) | 1,000,000 | 1 MΩ = 1,000,000 Ω |
Standard resistor values (E24, 5%)
1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1, and the same values × 10, × 100, × 1k and so on. The voltage divider tab suggests the nearest E24 value from the IEC 60063 series.
OhmCalc assumes ideal components and DC (or resistive AC) circuits. For AC loads with a power factor, real power is V × I × PF. Mains voltage can kill: these tools are for learning and low-voltage design.
Frequently asked questions
What is Ohm's law?
V = I × R: the voltage across a resistor equals the current through it times its resistance. Rearranged, I = V ÷ R and R = V ÷ I.
How do I calculate power?
P = V × I, or P = I² × R, or P = V² ÷ R. All give watts when volts, amps and ohms are used.
Does it work for AC?
For resistive loads such as heaters and incandescent lamps, yes. Motors and electronics have a power factor below 1, so real watts are lower than volts × amps.
Is OhmCalc free?
Yes, with no sign-up or ads.
Are my values saved?
Yes, in this browser only.