This calculator converts electrical current (A) and voltage (V) into delivered mechanical power expressed in horsepower (HP), plus watts and kilowatts. It supports single-phase and three-phase systems and allows you to include motor efficiency and power factor for realistic estimates. Use it to estimate motor output, size wiring, or quickly compare equipment ratings.

How to use this calculator

1. Choose the electrical Phase: Single-Phase or Three-Phase.
2. Enter the motor Current (Amps).
3. Enter the Voltage (Volts) the motor runs on (e.g., 120, 230, 480).
4. (Optional) Enter Efficiency (%): if you leave blank, the tool assumes 100%.
5. (Optional) Enter Power Factor (PF) for AC motors (value between 0 and 1); default is 1.
6. Click Calculate to show: Horsepower (HP), Kilowatts (kW), Watts (W) and expanded electrical metrics (input kW, apparent kVA, reactive kVAr, losses).

What the calculator does

• It first computes electrical input power (watts) from Amps × Volts, adjusted for power factor and the √3 factor for three-phase systems. Then it converts watts into mechanical horsepower while accounting for motor efficiency. This is the standard engineering approach used across reference tools.
• Defaults: If you omit efficiency, the calculator assumes 100% (no losses). If you omit power factor, it uses PF = 1 (purely resistive load). These defaults give the theoretical maximum output; realistic motors will be less (efficiency < 100%, PF < 1).

Formulas

Single-phase (AC or DC with PF = 1 for DC)

• Apparent power (VA): S = I × V
• Input electrical power (W): P_in = I × V × PF
• Output mechanical power (W) accounting for efficiency η (fraction):
  P_out = P_in × η
• Horsepower (HP): HP = P_out / 746 (1 HP ≈ 746 W; many references use 746 W for electrical HP conversions).

Three-phase (balanced, line-to-line voltage)

• Apparent power (VA): S = √3 × I × V
• Input electrical power (W): P_in = √3 × I × V × PF
• Output mechanical power (W): P_out = P_in × η
• Horsepower: HP = P_out / 746 (same watt→HP conversion).

DC (direct current) quick equivalent

• A commonly used direct conversion formula for DC circuits is:
  HP ≈ 0.00134 × V × I
  That expression is algebraically equivalent to HP = (V × I) / 746 with slight rounding differences used in practical references. Use the efficiency factor if you want mechanical output.

Example

Example: single-phase: 10 A at 230 V, PF = 1, efficiency 90% (0.90)

• P_in = 10 × 230 × 1 = 2300 W
• P_out = 2300 × 0.90 = 2070 W
• HP = 2070 / 746 = 2.776 HP → ≈ 2.776 HP

Example: three-phase: 10 A at 415 V, PF = 0.9, efficiency 92%

• P_in = √3 × 10 × 415 × 0.9 ≈ 6,500 W
• P_out ≈ 6,500 × 0.92 ≈ 5,980 W
• HP = 5,980 / 746 ≈ 8.02 HP.

Extra outputs our calculator provides (and why they matter)

• kW (output) – common engineering unit for power.
• Input kW – shows how much electrical power is drawn before losses. Useful for estimating running costs and breaker sizing.
• Apparent power (kVA) – required transformer/breaker sizing considers kVA, not just kW.
• Reactive power (kVAr) – indicates reactive load; important for power factor correction and utility charges.
• Losses (W) – the difference between input and output power; helps estimate heat and cooling needs.

Practical tips & engineering notes

• Always enter efficiency for realistic motor output nameplate efficiencies vary with size and loading. If unknown, use manufacturer datasheets or typical ranges (small motors ≈ 70–85%, larger motors ≈ 85–95%).
• When sizing cables and breakers, use input current at PF = 1 and then apply correction factors; consult NEC/local regulations for ampacity and safety margins.
• For inductive loads (motors), include power factor (PF < 1); neglecting PF overestimates real output.
• Remember rounding: 1 HP is commonly treated as 746 W in electrical conversions; mechanical definitions sometimes use 745.7 W the difference is negligible for most practical sizing but keep consistency.

FAQ

Q1. Can I convert amps to HP without efficiency or PF?

A: You can get a theoretical maximum by assuming efficiency = 100% and PF = 1, but real motors are less efficient and often have PF < 1. Use the defaults for quick estimates only.

Q2. Why is √3 used for three-phase calculations?

A: For balanced three-phase systems, line voltages and phase relationships introduce a √3 factor when converting between phase and line quantities; it’s standard in power calculations.

Q3. Which conversion factor do you use for HP → W?

A: This tool uses 1 HP = 746 W, the standard electrical conversion used by most engineering calculators and references.

Q4. Is the calculator suitable for DC motors?

A: Yes — for DC, set PF = 1 (no reactive component) and apply the single-phase formula or the direct DC formula HP ≈ 0.00134 × V × I. Include efficiency to get mechanical output.

Q5. When should I use apparent power (kVA) vs. real power (kW)?

A: Use kVA for equipment and transformer sizing (it includes reactive demand). Use kW for energy consumption and cost calculations (real power delivered).

Citations / references

Most of the standard formulas, the 746 W conversion, and typical calculator behavior follow established references and engineering utilities, including InchCalculator, OmniCalculator, Calculator.Academy, EngineeringToolBox and Electrical4U.