Engine Horsepower Calculator
Adjust the inputs below. Results update as you type.
How it works
HP = Torque (lb·ft) × RPM / 5,252. This constant comes from the definition: 1 HP = 33,000 ft·lb/min = 550 ft·lb/sec. Metric horsepower (PS) = 0.9863 HP. Kilowatts = HP × 0.7457. Peak HP occurs at higher RPM; peak torque at lower RPM—the crossover is always at 5,252 RPM. Dyno-measured wheel horsepower is 10–20% less than engine (flywheel) HP due to drivetrain losses.
Input guidance
- Confirm date/time/unit settings before comparing outputs.
- If a task has optional fields, run both with and without them to understand impact.
- Save scenario variants when making practical decisions.
The formula
Estimated horsepower from a quarter-mile time (Fox's equation): HP ≈ weight ÷ (ET ÷ 5.825)³, where weight is in pounds and ET in seconds. Trap-speed methods also exist.
Worked example
A 3,200 lb car running a 13-second quarter mile estimates roughly 280 horsepower by the elapsed-time method.
More examples to test
- Quick-pass example: run default assumptions for a first estimate.
- Refined example: update one assumption at a time to isolate impact.
How to interpret results
Use outputs as operational estimates and pair them with local constraints, business rules, or provider requirements.
When this can be inaccurate
Utilities can miss local policy details, special-case rules, and environment-specific constraints.
Change history
- July 2026: Quality-reviewed for publication with formula checks and explanatory copy updates.
Site-wide corrections also appear on the corrections log.
Find productivity tools for everyday workflows
General recommendation, not professional advice.
Frequently asked questions
How can a quarter-mile time estimate horsepower?+
Acceleration over a fixed distance reflects power-to-weight, so empirical formulas relate elapsed time and vehicle weight to engine output.
Is this as accurate as a dyno?+
No. A dynamometer measures power directly. Track-based estimates are approximations affected by traction, conditions, and driver skill.
What is the difference between crank and wheel horsepower?+
Crank (engine) horsepower is measured at the engine; wheel horsepower is lower after drivetrain losses, typically 10–20%.
Why are outputs slightly different from another tool?+
Different tools can use different rounding rules, default assumptions, and treatment of edge cases.
How can I improve estimate reliability?+
Use verified inputs from real records and re-run calculations after major assumption changes.