Reading amps off a motor nameplate or clamp meter and wondering what horsepower that represents? This calculator converts current and voltage into mechanical horsepower, accounting for efficiency and power factor — the reverse of our HP to amps calculator.
Amps to HP Formulas
3-φ: HP = (A × V × 1.732 × η × PF) ÷ 746
DC: HP = (A × V × η) ÷ 746
The result is the mechanical power at the shaft, which is why efficiency appears: a motor drawing 1,440 W of electricity at 85% efficiency delivers only ~1,224 W (1.64 HP) of shaft power. For code-compliant wire and breaker sizing, use the NEC tables on our HP to amps page rather than a calculation.
Common Amps → HP (Single-Phase, 85% η, 0.9 PF)
| Amps | @ 120 V | @ 230 V |
|---|---|---|
| 5 | 0.6 HP | 1.2 HP |
| 8 | 1.0 HP | 1.9 HP |
| 12 | 1.5 HP | 2.8 HP |
| 16 | 2.0 HP | 3.8 HP |
| 20 | 2.5 HP | 4.7 HP |
| 30 | 3.7 HP | 7.1 HP |
Worked Example
Frequently Asked Questions
At 120 V single-phase with typical efficiency, about 1.5 HP. At 230 V the same current supports roughly 2.8 HP. Voltage matters as much as amps.
Nameplate amps are at full rated load; a lightly loaded motor draws fewer amps. Also, your efficiency and PF guesses may differ from the motor's real values — check the nameplate for both.
No — electrical code requires the standardized NEC full-load-current tables, not calculations. This tool is for understanding and load estimation; wiring is a licensed electrician's job.