Why is 5252 used in the torque-to-HP formula?

At 5252 RPM, one lb-ft of torque produces exactly one horsepower. The constant comes from the definition of horsepower (33,000 ft-lb per minute) divided by 2π (to convert revolutions to radians). Horsepower and torque curves always cross at 5252 RPM.

How accurate is Hale's ET formula?

Hale's formula (HP = Weight / (ET / 5.825)^3) gives a reasonable estimate for street-prepped and bracket race cars running on a properly prepared track. It can be off by 5–15% for cars with extreme aerodynamics, traction problems, or unconventional powertrains.

What is trap speed and how do I measure it?

Trap speed is the vehicle's speed at the finish line of a quarter-mile run, measured by timing lights. It is printed on every official drag timing slip. The formula HP = Weight x (Trap Speed / 234)^3 uses the kinetic energy at the finish line to estimate power.

Should I use my car's curb weight or race weight?

Use the actual weight of the vehicle during the run — driver included. Race weight is curb weight plus driver (approximately 180 lb) minus any removed ballast. More accurate weight input yields a more accurate HP estimate.

How do I convert horsepower to kilowatts?

1 mechanical horsepower equals 0.7457 kilowatts. This tool shows the kW equivalent automatically next to every HP result.

Horsepower Calculator

Calculate horsepower from torque and RPM, quarter-mile ET and weight, or trap speed and weight — with instant kW conversion.

Classic dyno method. Enter peak torque and the RPM at which it occurs. Formula: HP = Torque × RPM / 5252

Torque (lb-ft)

RPM

Formula Reference

Method 1 — Torque / RPM

HP = Torque × RPM / 5252

Method 2 — Hale's ET

HP = W / (ET / 5.825)³

Method 3 — Trap Speed

HP = W × (MPH / 234)³

1 HP = 0.7457 kW

Enter values on the left to see the result

Summary

This tool calculates engine or vehicle horsepower using three industry-standard methods. Method 1 uses the mechanical relationship between torque and RPM (HP = Torque x RPM / 5252), ideal for engine dyno data. Method 2 uses Hale's quarter-mile elapsed time formula, a widely used drag racing estimate. Method 3 derives power from the terminal velocity (trap speed) at the end of a quarter mile. All results include an instant conversion to kilowatts.

How it works

Select a calculation method using the tabs at the top.
Enter the required values for that method.
Horsepower and kilowatts are calculated instantly as you type.
Switch between methods any time — each tab remembers your inputs.
Use the Reset button to clear the current method's fields.

Use cases

Convert dyno torque and RPM readings to peak horsepower.
Estimate a drag car's power from its quarter-mile elapsed time.
Back-calculate engine output from a measured trap speed.
Compare horsepower figures in HP and kW for international specs.
Quickly sanity-check published power claims against drag strip data.

Frequently Asked Questions

Last updated: 2026-05-29 · Reviewed by Nham Vu