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Automotive

Horsepower Calculator

Horsepower from torque and engine speed, plus kilowatts.

Your measurementsEvery assumption editable01
Power outputCalculated live02

Result

342.7hp

342.7 hp at 4500 rpm

Torque
400 lb-ft
Engine speed
4500 rpm
Horsepowertorque × rpm ÷ 5252
342.73 hp
In kilowatts
255.57 kW

Horsepower is not a separate measurement from torque — it is torque multiplied by engine speed. A dyno measures torque and calculates power from it, never the other way round.

The 5252 constant is where the two curves cross on every dyno graph ever printed. At 5,252 rpm the numbers are equal by definition, which is a property of the units rather than of the engine.

This is power at one point. A full curve matters far more than a peak figure, because an engine that makes its power over a wide band is quicker in practice than one with a taller, narrower peak.

Horsepower is not measured separately from torque — it is torque multiplied by engine speed and divided by 5252. A dyno measures torque and calculates power from it, never the other way round.

Why use this tool?What it does differently03

Why use this tool?

The relationship explained

Power is derived from torque, not measured alongside it. Understanding that clears up most dyno confusion.

The 5252 crossover

Why every dyno graph ever printed has its two curves intersect at exactly that rpm.

Kilowatts too

For comparing against metric figures without a second conversion.

Peak versus curve

Flags that a wide band beats a tall peak in practice.

How this worksThe method04

How this horsepower calculator works

Horsepower equals torque in pound-feet times engine speed in rpm, divided by 5252. That constant comes from the definition of horsepower as 33,000 foot-pounds per minute, divided by 2π radians per revolution.

Because of that constant, the torque and horsepower curves on any dyno plot cross at exactly 5,252 rpm. At that speed the two numbers are equal by definition — a property of the units, not of any engine.

This is power at a single point. A full curve matters far more than a peak figure, because an engine making its power across a wide band is quicker in practice than one with a taller but narrower peak.

How to use itStep by step05

How to use it

  1. Step 1: Enter the torque

    In pound-feet, at the engine speed you are interested in.

  2. Step 2: Enter the engine speed

    The rpm at which that torque figure was measured.

  3. Step 3: Read power in both units

    Horsepower and kilowatts.

  4. Step 4: Repeat across the range

    A curve tells you far more than a single peak.

Example usageWorked figures06

Example usage

A torquey V8
400 lb-ft at 4,500 rpm gives 342.7 hp, or 255.6 kW.
A high-revving four
180 lb-ft at 8,000 rpm gives 274.2 hp from far less torque — engine speed doing the work.
At the crossover
300 lb-ft at exactly 5,252 rpm gives exactly 300 hp. That equality is where the two dyno curves always meet.
Frequently asked questionsCommon questions07

Frequently asked questions

How do I convert torque to horsepower?

Multiply torque in pound-feet by rpm, then divide by 5252. The constant comes from the definition of horsepower and the conversion between revolutions and radians.

Why do torque and horsepower cross at 5252 rpm?

Because dividing by 5252 makes the two numerically equal at that speed. It is a property of the units rather than anything about the engine.

Is torque or horsepower more important?

They are the same measurement expressed differently. What matters is the shape of the curve — where the power arrives and how wide the usable band is.

Does a dyno measure horsepower directly?

No. A dyno measures torque against engine speed and calculates power. Any horsepower figure is derived.

How do I convert horsepower to kilowatts?

Multiply by 0.7457. One mechanical horsepower is about 746 watts.

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