Horsepower Calculator
Calculate horsepower from torque and RPM, or convert between HP, kW, watts, and BTU/hr. Includes engine power estimation and common machine power reference.
N·m
RPM
Enter your values above to see the results.
Tips & Notes
- ✓Mechanical horsepower (hp) = 745.7 W. Metric horsepower (PS or CV) = 735.5 W — about 1.4% less. Electric horsepower = 746 W exactly. Always clarify which standard is used when comparing international specifications.
- ✓Torque-to-power relationship: HP = Torque (ft·lb) × RPM / 5,252. At 5,252 RPM, torque in ft·lb and power in hp are numerically equal — this is why torque and power curves cross at 5,252 RPM on engine dyno charts.
- ✓Peak power occurs at higher RPM than peak torque for most combustion engines. An engine making peak torque at 3,500 RPM and peak power at 6,500 RPM is still making the same power at 3,500 RPM as physics allows — it just makes more at 6,500 RPM.
- ✓Electric motors produce maximum torque at zero RPM, unlike combustion engines. A 200 kW electric motor produces more towing force from a standstill than a 200 kW combustion engine which needs RPM to develop torque.
- ✓Brake horsepower (BHP) is measured at the engine output shaft using a dynamometer. Wheel horsepower (WHP) is measured at the driven wheels and is 10-20% lower due to drivetrain losses (transmission, driveshaft, differential friction).
Common Mistakes
- ✗Confusing mechanical and metric horsepower — US vehicles use mechanical hp (745.7 W); European spec sheets often use PS (735.5 W). A 200 PS European car has 197 US hp. The difference (1.4%) is small but significant when comparing specifications.
- ✗Calculating HP from torque using the wrong constant — HP = Torque (ft·lb) × RPM / 5,252 for US units. Using SI units: HP = Torque (N·m) × RPM / 9,549 (since 745.7 W = N·m × rad/s = N·m × RPM × 2π/60, and 745.7 × 60/(2π) = 7,121, divided by 9.8... this simplifies to Torque(N·m) × RPM / 9,549).
- ✗Assuming rated horsepower equals available horsepower at all RPM — a 400 hp engine produces 400 hp only at its peak power RPM (e.g., 6,500 RPM). At 2,000 RPM idle, it may produce only 80-100 hp.
- ✗Ignoring efficiency when sizing drive motors — a pump requiring 50 hp output must be driven by a motor providing 50 hp / motor efficiency. At 90% motor efficiency: motor input = 50 / 0.90 = 55.6 hp required.
- ✗Treating horsepower and torque as equivalent — they are related but measure different things. High torque at low RPM makes vehicles good at towing; high power at high RPM makes them fast at top speed. Both matter, in different contexts.
Horsepower Calculator Overview
Horsepower is the most widely recognized power unit in transportation and mechanical engineering — every vehicle, motor, and engine is rated by it, and understanding the relationship between horsepower, torque, and RPM is fundamental to mechanical engineering.
Horsepower from torque and RPM:
HP = T (ft·lb) × RPM / 5,252 | HP = T (N·m) × RPM / 9,549 | Power (W) = T (N·m) × 2π × RPM / 60
EX: Engine makes 350 ft·lb torque at 4,500 RPM → HP = 350 × 4,500 / 5,252 = 299.9 hp ≈ 300 hp. In SI: 350 ft·lb = 474.6 N·m → HP = 474.6 × 4,500 / 9,549 = 223.7 kW = 299.9 hp (consistent)Power unit conversions:
1 hp = 745.7 W = 0.7457 kW = 550 ft·lb/s = 2,545 BTU/hr | 1 kW = 1.341 hp = 3,412 BTU/hr
EX: 200 kW electric motor → 200 × 1.341 = 268.2 hp. 150 hp gasoline engine → 150 × 0.7457 = 111.9 kW. A 1 hp motor running 1 hour consumes 0.7457 kWh of energy.HP and torque at different RPM (same engine):
| RPM | Torque (ft·lb) | Horsepower | Notes |
|---|---|---|---|
| 1,000 | 280 | 53 hp | Low-load idle zone |
| 2,500 | 350 | 167 hp | Typical cruising RPM |
| 3,500 | 400 | 267 hp | Near peak torque |
| 5,252 | 320 | 320 hp | Torque = HP numerically at 5,252 RPM |
| 6,500 | 260 | 322 hp | Near peak power |
| 7,000 | 235 | 314 hp | Past peak power, falling |
| Application | Typical Power | Horsepower |
|---|---|---|
| Electric drill (portable) | 300–700 W | 0.4–0.9 hp |
| Residential HVAC compressor | 1.5–7.5 kW | 2–10 hp |
| Economy passenger car | 75–110 kW | 100–148 hp |
| Performance sports car | 300–600 kW | 400–800 hp |
| Diesel locomotive | 2,200–4,500 kW | 3,000–6,000 hp |
| Large commercial jet (per engine) | ~75,000 kW | ~100,000 hp equiv. |
Frequently Asked Questions
HP = Torque (ft·lb) × RPM / 5,252 (US units). In SI: HP = Torque (N·m) × RPM / 9,549. Example: engine producing 300 ft·lb at 4,000 RPM → HP = 300 × 4,000 / 5,252 = 228 hp. Or in metric: 407 N·m at 4,000 RPM → HP = 407 × 4,000 / 9,549 = 170 hp (metric) = 228 US hp. The constant 5,252 comes from: 1 hp = 550 ft·lb/s = 550 × 60 ft·lb/min = 33,000 ft·lb/min; HP = T × RPM × 2π / 33,000 → HP = T × RPM / (33,000/(2π)) = T × RPM / 5,252.
1 mechanical hp = 0.7457 kW = 745.7 W. 1 kW = 1.341 hp. Examples: 250 hp = 250 × 0.7457 = 186.4 kW. 150 kW = 150 × 1.341 = 201.1 hp. Quick mental estimate: multiply hp by 0.75 to get approximate kW (off by 0.7%). For electric motors: 1 electric hp = 746 W exactly (IEEE standard). European car specs use PS (metric hp): 1 PS = 735.5 W = 0.9863 US hp; so 200 PS ≈ 197 US hp ≈ 147 kW.
Crank HP (CHp): theoretical power calculated from engine specs without accounting for friction losses. BHP (Brake Horsepower): measured at the engine output shaft (crankshaft) using a brake dynamometer. This is the standard for engine ratings. WHP (Wheel Horsepower): measured at the driven wheels using a chassis dynamometer. WHP is lower than BHP due to drivetrain losses: manual transmission 10-12%, automatic 15-20%, AWD systems 20-25%. Example: a 400 BHP engine in a car with a 15% drivetrain loss: WHP = 400 × 0.85 = 340 WHP.
Common power requirements: electric drill (1/4 to 1/2 hp); table saw (1 to 3 hp); air compressor (1 to 5 hp for portable; 10-25 hp for shop); water pump (1/3 to 2 hp for residential); HVAC compressor (1 to 5 hp); electric car motor (100 to 600 hp peak); passenger car engine (100 to 500+ hp); commercial truck diesel (300 to 600 hp); large ship diesel (10,000 to 100,000 hp); jet aircraft engine (equivalent to 10,000-50,000 hp); large power plant generator (100,000+ hp equivalent). A standard US household uses about 2-5 kW average power = 2.7 to 6.7 hp continuously.
Power determines top speed and acceleration at high speeds; torque determines acceleration at low speeds and towing capacity. For a vehicle: F_drive = HP × 5,252 / (Tire radius × RPM) (simplified). Quarter-mile time correlates with power-to-weight ratio: approximately ET (seconds) ≈ 5.825 × (Vehicle weight / HP)^0.333. A 3,000 lb car with 300 hp: (3000/300)^0.333 = 10^0.333 = 2.154 → ET ≈ 5.825 × 2.154 = 12.5 seconds. This formula (Hale equation) gives a rough estimate — real times vary with traction, gearing, and launch technique.
James Watt coined "horsepower" in the 1780s to compare his steam engines to horses, allowing customers to quantify the value of replacing animal power with mechanical power. He defined 1 hp as 33,000 ft·lb/min based on his measurements of draft horse work capacity. The unit persisted in the US and UK through automotive and industrial standardization. Most of the world uses kilowatts for motor and generator ratings. The US automotive market and some industrial sectors continue using hp because it is familiar to buyers and embedded in decades of product marketing and consumer expectation. Both units are valid; the conversion is exact: 1 hp = 745.69987 W.