Torque / Power Converter

Understanding Torque and Power

Torque and power are the two fundamental metrics that define engine and motor performance. Torque measures rotational force — how hard the engine can twist the crankshaft — while power measures the rate of doing work, combining torque with rotational speed.

The relationship P = τ × ω (power = torque × angular velocity) means that the same power can come from high torque at low RPM (diesel engines, electric motors) or lower torque at high RPM (naturally aspirated gasoline engines). This is why electric vehicles feel so responsive — they deliver peak torque from 0 RPM.

Different regions use different units: N·m and kW (SI/metric), ft·lbf and HP (US/UK), kgf·m and PS (Japan/Germany legacy). Converting correctly between these systems is essential for comparing specifications across markets.

When comparing engines, always note the RPM at which peak torque and peak power are measured. A diesel engine producing 500 N·m at 1800 RPM delivers very different driveability than a gasoline turbo producing 500 N·m at 4000 RPM, even though the peak torque number is identical.

Formula: Power (kW) = Torque (N·m) × RPM / 9549 1 HP = 0.7457 kW = 1.0139 PS 1 N·m = 0.10197 kgf·m = 0.7376 ft·lbf

Example Calculation

An engine produces 350 N·m at 5500 RPM. Power = 350 × 5500 / 9549 = 201.6 kW = 270.3 HP = 274.1 PS. In ft·lbf, the torque is 350 × 0.7376 = 258.2 ft·lbf.

When to Use This Calculator

• Comparing engine specifications from different markets — e.g., a Japanese-spec PS rating vs. a US-spec HP rating for the same engine
• Converting dyno results from one unit system to another for tuning documentation or customer reports
• Translating torque specs when sourcing fasteners or tooling — e.g., converting a bolt torque spec from ft·lbf to N·m for a metric torque wrench
• Verifying powertrain calculations during drivetrain component selection, matching motor output to gearbox input requirements

Common Mistakes to Avoid

• Confusing HP (mechanical/imperial horsepower, 745.7 W) with PS (metric horsepower, 735.5 W) — the 1.4% difference matters for regulatory compliance and marketing accuracy
• Mixing up kgf·m (kilogram-force meter) with N·m — 1 kgf·m = 9.807 N·m, not 1:1; this is a common error that can cause 10× bolt torque mistakes
• Forgetting that power depends on RPM — two engines with the same peak torque produce different peak power if they reach peak torque at different RPM points
• Using conversion factors for the wrong HP variant — electrical horsepower (746 W), boiler horsepower (9810 W), and mechanical HP (745.7 W) are all different quantities

How to Interpret Results

• All three output values represent the same physical torque — the result is an exact unit conversion, not an approximation
• Use N·m for SI engineering calculations, kgf·m for legacy Japanese and European workshop documentation, and ft·lbf for US fastener and engine specifications
• When setting torque wrenches, always verify the scale printed on the wrench handle — mixing up kgf·m and N·m produces a near-10× error (1 kgf·m = 9.807 N·m)
• For power calculations, pair torque in N·m with speed in rad/s, or use P(kW) = Torque(N·m) × RPM / 9549 as a practical shortcut

Related Standards & References

• SAE J670 — Vehicle Dynamics Terminology; defines torque and force conventions used in chassis and powertrain engineering
• ISO 80000-4 — Quantities and units for mechanics; defines torque (moment of force) in SI with the base unit N·m
• SAE J1349 — Engine Power Test Code for spark-ignition and compression-ignition engines; specifies how peak torque and power are measured and reported
• ECE R85 — UN regulation on measurement of net power and maximum 30-minute power for internal combustion engines installed in motor vehicles

Frequently Asked Questions

What is the difference between HP and PS?

HP (mechanical horsepower) is defined as 550 ft·lbf/s = 745.7 W. PS (Pferdestärke, metric horsepower) is defined as 75 kgf·m/s = 735.5 W. PS is about 1.4% less than HP. Japanese and German specifications traditionally use PS, while US/UK use HP.

Why do electric motors have higher torque than combustion engines?

Electric motors produce maximum torque at 0 RPM because the electromagnetic force is strongest when the rotor is stationary. Combustion engines need to reach a specific RPM range for optimal air-fuel mixing and combustion timing to produce peak torque.

What is brake horsepower (BHP) vs. wheel horsepower (WHP)?

BHP is measured at the engine crankshaft (flywheel) before drivetrain losses. WHP is measured at the wheels on a dynamometer and accounts for losses in the transmission, differential, and axles — typically 10-20% lower than BHP. Manufacturer specs are always BHP/PS; aftermarket dyno results show WHP.