Engineering 2026-05-03 3 min read By Konvertibly Team

Horsepower and Torque Calculator: RPM, HP, and kW Relationships

Calculate engine horsepower from torque and RPM, or find torque from known HP. Understand the relationship between peak torque and peak power in internal combustion and electric motors.

The HP–Torque–RPM Triangle

Horsepower is derived from torque and speed. Peak torque and peak power always occur at different RPM — knowing both is essential for drivetrain design and engine selection.

Formulas

HP (imperial) = Torque (ft·lb) × RPM / 5252
kW (metric)   = Torque (N·m)  × RPM / 9549

Rearranged:
Torque (ft·lb) = HP × 5252 / RPM
Torque (N·m)   = kW × 9549 / RPM

Worked Examples

Engine: 300 ft·lb at 4000 RPM
HP = 300 × 4000 / 5252 = 228 hp

Electric motor: 200 kW at 3000 RPM
Torque = 200 × 9549 / 3000 = 636.6 N·m

Why HP and Torque Cross at 5252 RPM

At exactly 5252 RPM, HP = Torque (in ft·lb)
At any RPM:
  If peak torque is below peak-power RPM: engine is torquey/lug-friendly
  If spread is narrow: high-revving engine

Real-World Specs

Diesel truck: 2000 N·m at 1000 RPM, 450 kW at 1800 RPM
Tesla Model 3 LR: 493 N·m, 358 kW (flat torque from 0 RPM)
Honda Civic Si: 220 N·m at 6000 RPM, 134 kW at 7000 RPM

Calculate HP from torque: Free HP-Torque Calculator

Horsepower and Torque Relationship

Power (HP) = Torque (lb·ft) × RPM / 5252. The constant 5252 = 33,000 / (2π) and comes from the original definition of horsepower (James Watt: 1 HP = 33,000 ft·lbf/min). In SI: Power (kW) = Torque (N·m) × RPM / 9549. Consequence: at exactly 5,252 RPM, horsepower and torque are numerically equal in imperial units. Below 5,252 RPM, torque exceeds HP number; above it, HP exceeds torque number. This is why power curves and torque curves always cross at 5,252 RPM on any dynamometer graph.

Conversion Reference

1 horsepower (hp): = 745.7 W = 0.7457 kW
1 PS (Pferdestärke, metric HP): = 735.5 W (slightly less than imperial HP)
1 lb·ft: = 1.3558 N·m
100 hp at 3,000 RPM: = 175 lb·ft = 237 N·m
100 kW at 4,000 RPM: = 239 N·m

Frequently Asked Questions

Why do diesels have more torque than petrol engines of equal power?

Diesels produce peak torque at lower RPM (typically 1,500–2,500 RPM) due to higher compression ratios and longer power strokes. Petrol engines produce peak torque at higher RPM (3,000–5,000 RPM). Since power = torque × RPM, a diesel achieving the same peak power as a petrol engine must produce more torque at its lower RPM. High low-RPM torque makes diesels feel strong for towing and heavy loads where high RPM is impractical.

What is mechanical horsepower vs electrical horsepower?

Mechanical (imperial) horsepower: exactly 550 ft·lbf/s = 745.69987 W. Electrical horsepower (used in motor ratings in the US): exactly 746 W (essentially the same). Metric horsepower (PS, CV, pk): 75 kgf·m/s = 735.499 W. Boiler horsepower (steam): 9.810 kW (used only for steam boiler capacity). For practical purposes, 1 HP ≈ 0.746 kW with less than 0.15% error between definitions.

How do I size a motor for a given mechanical load?

Calculate required shaft power: P = T × ω. Apply service factor (1.15–1.5 for variable loads, shock loads, or uncertain duty). Account for drivetrain efficiency (belt drive 95%, chain drive 97%, gearbox 95–99% per stage). Choose the next standard motor size above the calculated requirement. Verify starting torque meets load breakaway requirement — some loads (compressors, heavy conveyors) need 150–200% of running torque at startup.