For motors and other inductive loads with a power factor below 1, the real current is higher than watts ÷ volts. Check the nameplate amps where available.
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 amps is 1500 watts?
12.5 A at 120 V, 6.25 A at 240 V and about 6.5 A at 230 V.
How many amps is 1000 watts?
8.33 A at 120 V and 4.35 A at 230 V.
How many watts can a 15 amp circuit handle?
1800 W at 120 V in total, and about 1440 W for a continuous load (80%).
Is OhmCalc free?
Yes, with no sign-up or ads.
Are my values saved?
Yes, in this browser only.