How it works

A useful distinction

Equal power can hide different torque

At 100 lb-ft and 3,000 RPM, calculated power equals that at 200 lb-ft and 1,500 RPM. The matching horsepower does not imply matching torque. Compare both operating values when interpreting two machines with similar power labels.

Choosing the power relationship you know

Power can be calculated from torque and rotational speed or from work completed over time under the supported modes. Choose the relationship that matches the measurements available. Check units before comparing horsepower with watts or kilowatts. The calculation converts or combines entered quantities; it does not independently measure an engine's output or account for unentered drivetrain losses.

Torque & RPM: formula and assumptions

Mechanical horsepower = torque(lb-ft)×RPM/5252.113122.

Engine, wheel, and brake horsepower measurements are not interchangeable. This mode requires torque measured at the same RPM.

Force, distance & time: formula and assumptions

Power = force×distance/time; mechanical hp = watts/745.699871582.

Assumes constant force parallel to the displacement.

Power unit converter: formula and assumptions

Converted power = value × input watts factor / output watts factor.

Mechanical, metric, electric and boiler horsepower are distinct definitions. Boiler horsepower uses the rounded NIST value 9,809.50 W. BTU is the International Table definition.

Work with selected units: formula and assumptions

Power = parallel force × distance / time.

Normalize force to newtons, distance to meters and time to seconds. The force must act parallel to displacement.

Worked example

Using the Torque & RPM example:

Inputs

Torque (lb-ft): 300
Rotational speed (RPM): 5000

Method

Mechanical horsepower = torque(lb-ft)×RPM/5252.113122.

Result

Example result

Mechanical horsepower: 285.6 hp

Power: 212.97 kW

These are illustrative inputs. Change the values to explore a different scenario.

What to keep in mind

Quarter-mile performance modes use historical empirical coefficients and do not measure actual crank or wheel power.

Formula references

References explain the underlying methods; they do not imply endorsement or individual professional review of Finflo.

Common questions

Must torque and RPM come from the same operating point?

Yes. Combining peak torque with an unrelated RPM does not describe the power at either actual operating point.

Are all units called horsepower identical?

No. Mechanical, metric, electric and boiler horsepower have different watt definitions. Select the intended definition before converting.

What does the work-based power calculation assume?

It assumes a constant force parallel to the displacement and divides that work by elapsed time after converting the selected units.

Prepared by Finflo. See our methodology and editorial policy.

Horsepower Calculator

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