# Estimate Engine Output from Torque and RPM

> Calculate engine power from measured torque and speed, with displacement-specific output and an explicit legacy airflow fallback.

Page: https://askcalculators.com/us/engine-horsepower/
Definition: https://askcalculators.com/us/engine-horsepower/definition.json
JSON Schema: https://askcalculators.com/us/engine-horsepower/schema.json
Version: 20.13.0
Maintained by: AskCalculators (software project; not a credentialed reviewer)
Qualified review: pending. Automated checks do not establish professional approval.
Page status: Live
Review status: Pending

## Use
Open the calculator page and enter values. Query-string parameters can restore and bookmark a calculation.
Example: https://askcalculators.com/us/engine-horsepower/?displacement=2&cylinders=4&rpm=4000&efficiency=0.85&torqueNm=300

## Inputs
- displacement: Displacement (L); type=number; required; default=5; min=0.01
- cylinders: Cylinders; type=number; required; default=8; min=1
- rpm: Engine speed (RPM); type=number; required; default=6000; min=0
- efficiency: Legacy airflow efficiency (used only without torque); type=number; required; default=0.85; min=0.01
- torqueNm: Measured torque at entered RPM (N·m, optional); type=number; optional; min=0

## Outputs
- horsepower: Mechanical horsepower; format=number; primary
- kilowatts: Power (kW); format=number
- horsepowerPerLitre: Horsepower per litre; format=number
- litresPerCylinder: Litres per cylinder; format=number

## Formula
Power (W) = torque (N·m) × RPM × 2π / 60; mechanical hp = W / 745.699871582.

Without torque: legacy airflow hp = litres × 61.0237 × RPM × efficiency / 3456 / 1.4.

## Rules
- Empty optional inputs are undefined, not zero.
- Hidden conditional inputs are ignored.
- Do not invent missing required inputs.