Horsepower Calculator
Engine hp and kW from torque and RPM with vehicle context.
What it does
Horsepower links twist to speed. Torque turns the crank, RPM repeats it, and their product rates how fast an engine can work.
The tool above divides that product by a constant. Three hundred pound feet at 4000 RPM gives 228.5 hp, or 170.4 kW, in family SUV range. The metric match is 407 N m for 228.6 hp.
Constants carry the units. Imperial divides by 5252, metric newton meters by 7120.9, and kilowatts follow at 0.7457 each. Same engine, same answer either way.
Bands translate numbers to cars. Under 100 hp suits small cars, under 250 family SUVs, under 400 sport sedans, and above runs to performance metal.
Peak hp never tells the whole drive. Gearing, weight, and the shape of the curve decide passing and towing far more than one point.
How to use
- Pick imperial (lb ft) or metric (N m).
- Enter torque: 300 or 407 suits the default.
- Enter RPM: 4000 suits the default.
- Read hp first.
- Read kW next.
- Read band for context.
Formula
- torque
- Twist at the crank.
- RPM
- Revolutions per minute.
The divisors fold unit conversions into one constant. Metric kW also equals N m x RPM / 9549 directly.
Worked example
Example: 300 lb ft at 4000 RPM
Imperial defaults: 300 lb ft at 4000 RPM. These are also the default values, so the page opens on this answer.
- Torque: 300 lb ft at 4000 RPM.
- Multiply: 300 x 4000 = 1,200,000.
- Divide: 1,200,000 / 5252 = 228.5 hp.
- kW: 228.5 x 0.7457 = 170.4 kW.
- Band: Family SUV range.
- Tool shows 228.5 hp, 170.4 kW, Family SUV range.
Reference table
| Torque | RPM | HP |
|---|---|---|
| 200 lb ft | 3000 | 114.2 hp |
| 300 lb ft | 4000 | 228.5 hp |
| 400 lb ft | 5000 | 380.8 hp |
| 407 N m | 4000 | 228.6 hp |
Limits
- Point math only. Curves matter more.
- Ignores drivetrain loss to wheels.
- Maker dyno sheets rule disputes.
- Bands are context, not ratings.
Common mistakes
- Comparing crank hp to wheel hp directly.
- Reading peak with no RPM context.
- Mixing lb ft and N m in one divisor.
- Sizing mods from one point.
- Ignoring weight and gearing.
Try it yourself
Quick checks. Answers sit tucked under each question.
200 lb ft at 3000 RPM?
200 x 3000 / 5252 = 114.2 hp. kW is 114.2 x 0.7457 = 85.2 kW. Band is Family SUV range.
150 N m at 5000 RPM?
150 x 5000 / 7120.9 = 105.3 hp. kW is 150 x 5000 / 9549 = 78.5 kW.
Questions
How do torque and RPM make hp?
Multiply torque by RPM, then divide by 5252. Here 300 x 4000 / 5252 = 228.5 hp.
hp to kW?
Multiply hp by 0.7457. Here 228.5 x 0.7457 = 170.4 kW.
What do the bands mean?
Under 100 small car, under 250 family SUV, under 400 sport sedan, above performance. Here 228.5 is family SUV.
Metric torque?
Use newton meters with 7120.9 divisor. Here 407 x 4000 / 7120.9 = 228.6 hp.
Does hp set top speed?
No. Gearing, weight, and curves decide speed, not peak hp alone.
Dyno or math?
Yes. Maker dyno sheets beat any single point math.