Horsepower Calculator - HP, Torque, RPM & Power Converter
Free online horsepower calculator. Compute horsepower from engine torque (lb-ft or N·m) and RPM, calculate physical power output from applied force, displacement, and elapsed time, or convert power between Mechanical HP, Metric HP (PS/CV), Kilowatts (kW), Watts, and BTU/hour.
Horsepower Calculator (HP & Power Converter)
Calculate mechanical horsepower from torque & RPM, work & time physical definition, or convert between power units.
Torque & Engine Speed Parameters
Horsepower & Mechanical Power Guide: Torque, RPM, Work Definition & Energy Standards
Horsepower is the standard unit of measurement for mechanical power output across internal combustion engines, electric motors, turbines, and steam engines. It defines the rate at which physical work is performed over time.
The Mathematical Relationship Between Torque and Horsepower
Horsepower is mathematically derived from rotational torque and angular engine speed (RPM):
HP = (Torque in lb-ft × RPM) / 5,252
In SI Metric units: HP = (Torque in N·m × RPM) / 7,127
- At 5,252 RPM, a dyno torque curve and horsepower curve always cross each other on a graph.
- Torque measures the instantaneous rotational twist, while horsepower measures how rapidly that torque is applied.
Step-by-Step Instructions
Select Calculation Mode
Choose to calculate Horsepower from Torque & RPM (`HP = (Torque * RPM) / 5252`), Vehicle Weight & Quarter-Mile Trap Speed, or ET (Elapsed Time).
Input Mechanical or Drag Racing Metrics
Enter engine torque (lb-ft or N-m) and engine speed (RPM), or vehicle curb weight (lbs) and 1/4 mile trap speed (mph).
Calculate True Horsepower
The calculator evaluates Mechanical Horsepower (HP), Metric Horsepower (PS / CV), and Electrical Kilowatts (kW).
Review Drivetrain Parasitic Loss
Estimate Wheel Horsepower (WHP) vs Crankshaft Brake Horsepower (BHP) factoring in FWD (10%), RWD (15%), or AWD (20%) drivetrain losses.
Key Features & Capabilities
- Torque to Horsepower engine: `HP = (Torque_lbft * RPM) / 5252` and Metric `kW = (Torque_Nm * RPM) / 9549`
- Quarter-mile drag strip formulas: Hale Trap Speed Formula (`HP = Weight * (Speed / 234)^3`) and Fox ET Formula (`HP = Weight / (ET / 5.825)^3`)
- Power unit converter: Mechanical HP (imperial hp), Metric Horsepower (`PS` / `CV` / `pk`), Kilowatts (`kW`), and Foot-Pounds/sec
- Drivetrain parasitic loss estimator: computes Crank BHP from Wheel Dyno WHP (FWD 10-12%, RWD 15%, AWD 20-25%)
- Power-to-weight ratio calculator outputting Horsepower per Ton and Pounds per Horsepower (`lbs/HP`)
- Real-time calculations with step-by-step mechanical engineering explanations
Frequently Asked Questions
Why do Torque and Horsepower curves always cross at exactly 5,252 RPM?
Horsepower is defined as `(Torque in lb-ft * RPM) / 5,252`. When engine speed is exactly 5,252 RPM, the formula simplifies to `HP = (Torque * 5252) / 5252 = Torque`. Therefore, on any standard dyno chart plotted on identical scales in lb-ft and HP, torque and horsepower curves MUST intersect at 5,252 RPM.
What is the difference between Wheel Horsepower (WHP) and Brake Horsepower (BHP)?
Brake Horsepower (BHP) is the power measured directly at the engine crankshaft without transmission or drivetrain components. Wheel Horsepower (WHP) is the power delivered to the tires measured on a chassis dynamometer, which is 10%–25% lower due to frictional drag in the transmission, driveshaft, differential, and wheel bearings.
How do you calculate horsepower from quarter-mile trap speed?
Using Peter Hale's drag racing formula: `Horsepower = Vehicle Weight (lbs) * (Trap Speed in mph / 234)^3`. For example, a 3,500 lb car crossing the quarter mile at 115 mph produces `3500 * (115 / 234)^3 ≈ 3500 * 0.1186 = 415 HP` at the wheels.
What is the conversion factor between Horsepower (HP) and Kilowatts (kW)?
One mechanical horsepower (HP) equals approximately `0.7457 Kilowatts (kW)` (or `1 kW = 1.341 HP`). One metric horsepower (PS / Pferdestärke) equals `0.7355 kW` (or `1 HP = 1.0139 PS`).
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