Horsepower Calculator

Multiply torque (in lb-ft) by RPM, then divide by 5252: HP = torque x RPM / 5252. For example, an engine making 300 lb-ft of torque at 5000 RPM produces 300 x 5000 / 5252 = 285.6 HP. The number 5252 is the RPM where the torque and horsepower curves cross — it comes from the exact definition of mechanical horsepower (33,000 ft-lb per minute). If your torque is in Newton-meters, divide by 1.356 first to convert to lb-ft; the calculator above handles both units.

It is not magic — it is arithmetic. One mechanical horsepower is defined as 33,000 foot-pounds of work per minute. Engine power equals torque times angular velocity, and converting revolutions-per-minute to radians-per-minute introduces a factor of 2*pi, giving 33,000 / (2*pi) = 5252.11 (rounded to 5252). That is why a torque curve and a horsepower curve always cross at 5252 RPM. The formula works in reverse too: torque (lb-ft) = HP x 5252 / RPM, handy for checking what torque a target power needs.

Use Hale's formula: HP = vehicle weight (lb) / (ET / 5.825)^3. A 3500-lb car running a 13.5-second quarter mile makes about 3500 / (13.5 / 5.825)^3 = 281 crank HP. For 1/8-mile runs use the constant 3.745 instead of 5.825. The formula assumes optimal traction and a well-geared car, so it estimates crank horsepower under ideal conditions — a bad launch or spinning tires will make the ET slower than the formula predicts. The calculator above also works in reverse, predicting ET from weight and power.

HP = weight (lb) x (trap speed in MPH / 234)^3. A 3500-lb car trapping 100 MPH in the quarter mile makes about 3500 x (100/234)^3 = 273 HP. Trap speed is often considered a slightly better power indicator than ET because it is less affected by launch technique — ET rewards traction and reaction, while trap speed mostly reflects how hard the engine pulls. The calculator above accepts MPH or km/h and works in reverse too: enter weight and power to predict your trap speed.

A calculator gives an estimate, typically within 5-10% of a chassis dyno for a well-sorted car — a dyno measures actual power under load, which is the real number. Calculators based on ET or trap speed assume ideal traction and gearing; torque-x-RPM math is exact but only as good as your torque reading. Treat every number on this page as a planning estimate. If you are buying parts, tuning, or making claims, get a dyno pull for the definitive figure.

Drivetrain loss is the power eaten by the transmission, differential, and axles before it reaches the wheels — the commonly quoted rules of thumb are 10% for front-wheel drive, 15% for rear-wheel drive, and 20% for all-wheel drive. Wheel HP = crank HP x (1 - loss): a 400-crank-HP rear-drive car puts down about 400 x 0.85 = 340 wheel HP. To go the other way, crank HP = wheel HP / (1 - loss), so 340 wheel HP on an AWD car implies about 425 crank HP. The toolbox above converts both directions for all three drivetrains.

Injector flow (lb/hr per injector) = HP x BSFC / (number of injectors x duty cycle). BSFC (brake specific fuel consumption) is roughly 0.45 for naturally aspirated gasoline, 0.55 for forced induction, and 0.65 for E85; keep duty cycle at 0.80 or below for injector longevity. Example: a 400-HP V8 with 8 injectors needs 400 x 0.45 / (8 x 0.80) = 28.1 lb/hr per injector, about 295 cc/min. The injector calculator above covers NA, boosted, and E85 builds and reports both lb/hr and cc/min.

A rough rule of thumb: boosted HP = NA HP x (14.7 + boost psi) / 14.7 x efficiency. Atmospheric pressure is 14.7 psi, so 10 psi of boost raises the intake pressure ratio to 24.7/14.7 = 1.68x; multiply by a real-world efficiency of about 0.92 for turbos or 0.88 for superchargers (which eat crank power to spin). Example: a 332-HP small block with 10 psi from a turbo makes about 332 x 1.68 x 0.92 = 514 HP. The boost tab above runs this with your own NA baseline, boost pressure, and induction type.

Naturally aspirated engines lose roughly 3% of their power for every 1000 feet of elevation, because thinner air means less oxygen per intake stroke: a 300-HP engine makes about 300 x (1 - 0.03 x 5) = 255 HP at 5000 feet. This rule of thumb is related to the SAE J1349 atmospheric correction used on dynos. Turbocharged cars lose much less, since the turbo can spin harder to compensate — one reason boost is popular in the mountains. The altitude corrector above applies the 3%-per-1000-ft rule to any baseline.

First compute displacement: CID = (bore^2 x pi/4) x stroke x cylinders. A classic 350 small-block Chevy (4.00" bore, 3.48" stroke, 8 cylinders) is 349.8 cubic inches, or 5.73 liters. Then multiply by a horsepower-per-cubic-inch factor for the build level: about 0.95 for a mild street build (332 HP), 1.15 for street/strip, 1.35 for a race build on pump gas. Higher compression ratios add a few percent (modeled from ideal Otto-cycle efficiency). The engine-build tab computes displacement in cubic inches and liters, applies your build level and compression ratio, and can stack boost on top.

Multiply kilowatts by 1.34102 to get mechanical horsepower: a 150 kW EV motor makes 201 HP. Metric horsepower (PS, common on European spec sheets) uses 1.35962 instead — 150 kW = 204 PS. The EV & units tab converts kW, HP, and PS in every direction. Note that EV motor ratings are usually peak electrical input power, so real wheel power is a bit lower after inverter and drivetrain losses; the 0-60 estimator has an EV mode with gentler loss assumptions.

This calculator uses a simplified physics model: the kinetic energy needed to reach 60 MPH (1/2 x mass x 26.82 m/s^2) divided by the average usable wheel power (crank HP x 0.7457 x drivetrain efficiency x a utilization factor of 0.55 for combustion engines, 0.80 for EVs), plus a launch and shift overhead (1.0 s for manuals/automatics, 0.3 s for EVs). Example: a 300-HP, 3500-lb rear-drive car estimates at about 6.5 seconds — realistic for that class. It is an estimate, not a substitute for a Dragy run: gearing, tires, and launch technique swing real times by half a second or more.

Invert Hale's formula: HP = weight (lb) / (target ET / 5.825)^3. Want a 12-second pass in a 3500-lb car? You need about 3500 / (12/5.825)^3 = 401 crank HP. For a trap-speed target, HP = weight x (target MPH / 234)^3 — trapping 110 MPH in the same car needs about 358 HP. The "HP needed" toolbox above runs both, so you can size an engine build to your drag-racing goal.

Yes for the physics-based methods: torque x RPM, ET, trap speed, drivetrain loss, and altitude all work for diesels and motorcycles — just use the correct weight (wet weight with rider for bikes) and note that diesels make big torque at low RPM, so the torque formula still applies exactly. For engine-build estimates, diesels and bike engines have very different HP-per-cubic-inch ranges than the gasoline V8 tiers in the calculator, so treat that tab as gasoline-engine specific.

It is the simplest way to compare how hard an engine is working for its size: HP divided by displacement in cubic inches. A mild street small block at 332 HP / 350 CID = 0.95 HP per cubic inch; a serious naturally aspirated race build pushes about 1.35. In metric terms that is roughly 55-80 HP per liter for pump-gas street builds (a 2.0L engine at 140 HP = 70 HP/L). The engine-build tab reports both HP/CI and HP/L alongside the estimate so you can sanity-check how aggressive a combo really is.

A realistic horsepower gain from bolt-on mods is modest: a cold-air intake and cat-back exhaust together typically add 2-5% on a modern engine, a tune on a naturally aspirated car another 3-8%, and a tune on a turbo car 15-30% because boost can be raised. Claims of big gains from bolt-ons alone are usually marketing. Use the boost tab to model forced-induction changes honestly, the drivetrain section to keep crank-versus-wheel numbers straight, and a dyno to verify anything you spend money on.