Amps to Horsepower Calculator
Reverse calculate your motor's output power in Horsepower based on the current it draws from the line. Perfect for verifying equipment specifications in the field.
Motor Power Audit
Amperage → Output HP
Shaft horsepower based on electrical input.
The Conversion Formula
Motor output horsepower is calculated from electrical input — efficiency and power factor determine how much energy becomes mechanical work.
DC Motors
HP = (I × V × Eff) ÷ 746 Electrical input power (I × V) multiplied by efficiency gives output watts, which divides by 746 to yield horsepower.
1-Phase AC Motors
HP = (I × V × Eff × PF) ÷ 746 AC motors require power factor — only the real power portion (V × I × PF) converts to mechanical output.
3-Phase AC Motors
HP = (√3 × I × V × Eff × PF) ÷ 746 The √3 factor accounts for power distribution across three phases. Three-phase motors deliver more power per amp than single-phase.
How to Convert Amps to Horsepower
Converting amps to horsepower lets you determine a motor's mechanical output from its electrical input measurements. This is useful when the motor nameplate is damaged or missing, when verifying motor performance, or when comparing measured current draw to the rated HP. The key insight: not all electrical input becomes mechanical output — efficiency and power factor dictate how much is lost as heat.
Understanding Motor Efficiency
Motor efficiency is the ratio of mechanical output power to electrical input power. A 5 HP motor drawing 15A at 240V and 0.85 PF consumes about 3,060W of real input power. At 90% efficiency, it delivers 2,754W (≈ 3.69 HP) of mechanical output. The rest becomes heat in the windings. Premium-efficiency motors minimize this loss.
When to Use This Calculation
- Verifying Motor Output: Measure FLA with a clamp meter and calculate HP to confirm the motor is delivering its rated output.
- Troubleshooting Overloads: If the calculated HP is significantly below rated, the motor may be worn, undersized, or operating with a degraded power factor.
- Pump and Fan Selection: When replacing driven equipment, knowing actual HP output (not just nameplate) prevents under-sizing the new load.
- Energy Audits: Calculate actual HP output versus nameplate rating to identify inefficient motors worth replacing with premium-efficiency units.
Step-by-Step Conversion
- Measure or Find the Current: Use a clamp meter for measured FLA, or use the nameplate value.
- Note the Supply Voltage: Use line-to-line voltage for three-phase; line-to-neutral for single-phase.
- Multiply by Efficiency: Motor efficiency in decimal form (85% = 0.85).
- Multiply by Power Factor (AC): Typical induction motors run 0.80–0.92 PF at full load.
- Divide by 746: Convert watts to horsepower using the 746 W/HP constant.