For AC loads with a power factor below 1 (motors, some power supplies), real power is volts × amps × power factor, so the watts figure here is the maximum, also called volt-amps.
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
How many watts is 15 amps?
1800 W at 120 V and 3600 W at 240 V.
How many watts is 20 amps?
2400 W at 120 V and 4800 W at 240 V.
What is the difference between watts and VA?
VA is volts × amps. Watts is the real power, VA × power factor; they are equal for resistive loads.
Is OhmCalc free?
Yes, with no sign-up or ads.
Are my values saved?
Yes, in this browser only.