Horsepower Calculator
Compute engine horsepower from torque and RPM, or estimate quarter-mile trap speed.
Results update as you type.
About this calculator
Horsepower measures the rate of doing work, and this horsepower calculator finds it two ways. In the first mode, engine power follows directly from torque and rotational speed: HP = torque (lb-ft) × RPM ÷ 5252. The magic number 5,252 comes from the definition of horsepower (33,000 ft-lb per minute) and the 2π radians in a revolution, and it is also the RPM at which the horsepower and torque curves always intersect on a dyno chart. The tool reports the metric equivalent too, using 1 hp = 0.7457 kW.
As a worked example, an engine making 300 lb-ft of torque at 5,000 RPM produces 300 × 5000 ÷ 5252 ≈ 286 hp, which is about 213 kW. Because power is torque times speed, the same 300 lb-ft makes far more horsepower at high RPM than at low — which is why high-revving engines chase big peak-power figures even with modest torque.
Switch to the quarter-mile mode to estimate drag-strip performance from power and vehicle weight using Roger Huntington’s classic empirical formulas: trap speed ≈ 234 × (hp / weight)^⅓ mph and elapsed time ≈ 5.825 × (weight / hp)^⅓ seconds. A 300 hp car weighing 3,500 lb comes out around 103 mph and a 13.2-second quarter mile. These are rough guides that ignore traction, aerodynamics and driver skill, but they are a quick sanity check for a build or a way to back out power from a known trap speed.
Frequently asked questions
Why is 5252 in the horsepower formula?
Horsepower is defined as 33,000 ft-lb per minute, and one revolution sweeps 2π radians. Combining the constants gives HP = torque × RPM ÷ 5252, which is also why the horsepower and torque curves always intersect at 5252 RPM on a dyno chart.
How do you estimate horsepower from the quarter mile?
Roger Huntington’s formula links trap speed and weight to power: trap speed ≈ 234 × (hp/weight)^⅓ mph. The calculator inverts it to estimate trap speed and elapsed time from your power and weight, a rough guide that ignores traction and aerodynamics.
How many watts is one horsepower?
One mechanical horsepower is about 745.7 watts, so 300 hp is roughly 224 kW. The calculator shows the kilowatt equivalent alongside horsepower.
Why does the same torque give different horsepower at different RPM?
Because power is torque times rotational speed. HP = torque × RPM ÷ 5252, so 300 lb-ft makes about 286 hp at 5,000 RPM but only 143 hp at 2,500 RPM — the reason engines are revved to reach peak power.
What is a good power-to-weight ratio for a fast quarter mile?
Trap speed scales with the cube root of horsepower per pound, so weight matters as much as power. Around 0.1 hp per pound (a 350 hp, 3,500 lb car) puts you near 110 mph traps; shedding weight helps just as much as adding power.
Can I estimate horsepower from my trap speed?
The quarter-mile mode runs Huntington’s formula from power and weight to a trap speed and ET. To go the other way, adjust the horsepower input until the predicted trap speed matches your measured one — that horsepower is the rough estimate.
Are the quarter-mile estimates accurate?
They are ballpark figures. Huntington’s formulas ignore traction, aerodynamics, gearing, launch technique and driveline losses, so a real car can be several tenths of a second off. Treat them as a guide, not a substitute for a timed run.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/horsepower/curl
curl "https://calculator.free/api/v1/horsepower/?torque=300&rpm=5000&hp=300&weight=3500&_mode=power"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/horsepower/?" + new URLSearchParams({
"torque": "300",
"rpm": "5000",
"hp": "300",
"weight": "3500",
"_mode": "power"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.