Ohm's law and power calculator
Fill in any two boxes. The other two work themselves out.
How it works
V = I × R P = V × I
P = V² ÷ R P = I² × R
P = V² ÷ R P = I² × R
100 W into 8 Ω takes √(100 × 8) ≈ 28.3 V RMS and about 3.5 A. The same 100 W into 4 Ω needs only 20 V but 5 A. Lower impedance means more current, which is what makes amps run hot.
Audio examples
| Situation | Known | Result |
|---|---|---|
| Amp output swing for 500 W into 4 Ω | 500 W, 4 Ω | 44.7 V, 11.2 A |
| Full 70V line carrying 150 W | 70.7 V, 150 W | 33.3 Ω, 2.1 A |
| Full 100V line carrying 600 W | 100 V, 600 W | 16.7 Ω, 6 A |
| Power supply for a 24 V device drawing 1.5 A | 24 V, 1.5 A | 36 W |
Field notes
- Speaker impedance isn't pure resistance. It changes with frequency, so these numbers are a good planning estimate rather than an exact measurement of what's happening on the cable.
- Peak vs RMS. A sine wave's peak voltage is 1.414 times its RMS value. An amp delivering 28.3 V RMS swings to about 40 V peak. That matters when you check what a speaker line or an input can take.
- Amp power ratings aren't mains draw. A class D amp making 1,000 W of output may draw much less from the wall at normal program levels and much more at clipping. Use the manufacturer's current-draw figures to size circuits, not this formula.
- For DC power and long low-voltage runs, remember the wire has resistance too. The voltage at the far end drops with current.