Key takeaways
- Horsepower = Torque × RPM ÷ 5252 — torque in lb-ft, speed in RPM.
- The 5252 constant is 33,000 ÷ 2π, baked into the definition of one horsepower.
- HP and torque are numerically equal at 5,252 rpm, so every dyno chart crosses there.
- Multiply horsepower by 0.7457 to get kilowatts.
How to calculate horsepower from torque
Horsepower measures how fast an engine does work, and work is force over distance. Torque is the rotational force, and RPM is how fast that force spins, so combining them gives power. The relationship is one of the most-used equations in the automotive world:
That mysterious 5252 is not arbitrary. One horsepower is defined as 33,000 lb-ft of work per minute. To turn torque (which acts over the circumference of a circle) into linear work, you divide by the 2π radians in one revolution: 33,000 ÷ (2 × π) = 5252. It is a unit-conversion constant, nothing more.
Worked example: 380 lb-ft at 5,500 rpm
Horsepower = 380 × 5500 ÷ 5252 = 397.9 hp. In metric units that's 397.9 × 0.7457 = 296.7 kW. Run the reverse formula to check it: 397.9 × 5252 ÷ 5500 ≈ 380 lb-ft, right back to where we started.
Horsepower the 380 lb-ft engine makes at each RPM
| Engine speed | Torque | Horsepower |
|---|---|---|
| 3,000 rpm | 380 lb-ft | 217.1 hp |
| 4,000 rpm | 380 lb-ft | 289.4 hp |
| 5,252 rpm | 380 lb-ft | 380.0 hp |
| 6,000 rpm | 380 lb-ft | 434.1 hp |
| 7,000 rpm | 380 lb-ft | 506.5 hp |
Notice the row at 5,252 rpm: with 380 lb-ft held constant, horsepower equals 380 exactly. That is the dyno crossover — because HP = torque × RPM ÷ 5252, whenever RPM hits 5,252 the formula collapses to HP = torque, so the two curves on any dyno chart must intersect at that single speed regardless of the engine.
Horsepower vs torque, and gearing
Torque is what you feel off the line; horsepower is what carries you at the top end. Neither is "better" — they're two views of the same curve, and gearing multiplies torque to put the power where you need it. To see how your final drive and transmission turn engine RPM into wheel speed, use the gear ratio calculator.
Frequently asked questions
How do I calculate horsepower from torque?
HP = torque (lb-ft) × RPM ÷ 5252. For example 380 lb-ft at 5,500 rpm ≈ 397.9 hp.
What is the 5252 constant?
It's 33,000 lb-ft/min (one horsepower) divided by 2π radians per revolution: 33,000 ÷ 2π = 5252. A unit conversion factor.
Why do HP and torque cross at 5252 rpm?
Because HP = torque × RPM ÷ 5252, at exactly 5,252 rpm the equation becomes HP = torque, so the curves always meet there.
Horsepower vs torque — which matters?
Torque gives low-end pull; horsepower governs top-end speed. They're the same curve viewed two ways, linked by RPM.
How do I convert hp to kW?
Multiply horsepower by 0.7457. So 397.9 hp ≈ 296.7 kW; divide by 0.7457 to go back.
Is this crank or wheel horsepower?
Whatever your torque figure is. Crank torque gives crank hp; chassis-dyno wheel torque gives wheel hp, usually 10–20% lower.
The HP = torque × RPM ÷ 5252 relationship is standard physics derived from the definition of one horsepower as 33,000 lb-ft per minute — see the horsepower definition. The 0.7457 kW-per-hp conversion is exact for mechanical horsepower.
Last reviewed June 2026