How do you calculate engine torque if given horsepower and RPM?

• A. Torque = (Horsepower × 5252) / RPM.
• B. Torque = (Horsepower × RPM) / 5252.
• C. Torque = HP / (RPM × 5252).
• D. Torque = 5252 × (Horsepower / RPM).

Answer: (A)

Power is the result of how much torque the engine produces and how fast that torque is applied. In these units, horsepower equals torque (in pound-feet) times engine speed (RPM) divided by 5252. So the torque can be found by rearranging to torque = horsepower × 5252 ÷ RPM.

The constant 5252 comes from converting the two parts of power: converting RPM to revolutions per second and converting feet-pounds per second to horsepower. Specifically, 5252 = 550 (ft-lb per second per horsepower) × 60 (to convert minutes to seconds) ÷ 2π (to convert from rotations to radians).

For a quick check, if you have 320 HP at 5200 RPM, the torque is 320 × 5252 ÷ 5200 ≈ 323 lb-ft. If RPM increases holding HP fixed, torque must drop proportionally, and if HP increases at fixed RPM, torque rises proportionally.