Torque to HP Formula
This is the fundamental relationship between rotational force (torque), rotational speed (RPM), and power output (HP). The constant 5,252 converts the imperial units into consistent power output. The same relationship in metric is: kW = Nm × RPM / 9,549.
Once you have HP, convert to kilowatts with:
Worked Example
Given: An engine produces 450 lb-ft of torque at 4,500 RPM
Step 1: HP = 450 × 4500 / 5252
Step 2: Numerator = 450 × 4500 = 2,025,000
Step 3: HP = 2,025,000 / 5252 = 385.6 HP
Step 4: kW = 385.6 × 0.7457 = 287.5 kW
This formula calculates power at a specific operating point. Peak HP is achieved where the product of torque × RPM is maximized, not necessarily at peak torque RPM.
Frequently Asked Questions
Multiply torque (lb-ft) by RPM, then divide by 5,252. For example, 400 lb-ft at 4,500 RPM = 400 × 4500 / 5252 = 342.7 HP. If your torque is in Newton-meters, convert first: lb-ft = Nm × 0.7376.
Yes. First calculate power in kW using kW = Nm × RPM / 9,549, then convert to HP: HP = kW × 1.3410. Alternatively, convert Nm to lb-ft (multiply by 0.7376) and use the standard formula. Our Torque to kW calculator handles Nm directly.
Because HP = Torque × RPM / 5252. Even after peak torque, as long as torque doesn't fall fast enough, rising RPM keeps increasing HP. Peak HP occurs at the RPM where Torque × RPM is maximized. Once the engine can no longer maintain torque faster than RPM rises, HP peaks and starts dropping.
Use HP = (torque in lb-ft × RPM) ÷ 5252. You need both the torque figure and the RPM at which it occurs.
No. Horsepower depends on how fast the torque is delivered, so RPM is required. The same torque at higher RPM produces more horsepower.