Horsepower Torque Calculator — HP, Torque, RPM

Real-world use cases

• Read a dyno sheet and fill in the missing number

  A dyno run prints peak torque of 400 lb·ft at 4000 RPM but the horsepower line is smudged. Solve for HP, type 400 and 4000, and you get about 305 HP without redoing the pull. Flip it around if the sheet shows peak HP and you want the torque at that point. The tool covers all three directions so you never have to rearrange the formula by hand at the track.

• Compare an imperial and a metric engine spec

  One brochure lists 300 HP and 400 lb·ft, another lists 220 kW and 540 N·m. Put both on the same footing: 220 kW is about 295 HP and 540 N·m is about 398 lb·ft, so the two engines are within a whisker of each other. The unit toggle and the side-by-side equivalent make the comparison a few taps instead of a spreadsheet.

• Size an electric motor against a gas engine

  An EV motor is rated 150 kW and 310 N·m, and you want to know how it stacks up against the 200 HP gas car it replaces. Convert 150 kW to about 201 HP and 310 N·m to about 229 lb·ft, then see how the torque delivered low in the rev range explains why the electric feels quicker off the line even at similar peak horsepower.

• Teach or learn the 5252 crossover

  Showing a student why every dyno graph crosses at 5252 RPM is easier with live numbers. Set torque to 250 lb·ft and RPM to 5252 and watch the horsepower come out at exactly 250. Nudge the RPM up and down and the two values split apart in real time, which makes the otherwise abstract constant click in a way a static diagram does not.

• Plan a gearing or tuning change

  You are deciding whether to chase peak power higher in the rev range or fatten the midrange torque. Plug in candidate torque and RPM figures and read the horsepower each produces, then pair it with a gear-ratio calculation to see how the change lands at the wheels before you spend money on parts or a tune.

Common pitfalls

• Using 5252 with metric numbers. The 5252 constant only works when torque is in lb·ft. If your torque is in N·m, use 9549 to get kilowatts, or convert the N·m to lb·ft first. Mixing a metric torque with the 5252 formula gives a meaningless answer.

• Confusing peak torque RPM with peak power RPM. They almost never sit at the same engine speed. Plugging peak torque together with the peak-power RPM into the formula computes a horsepower the engine never actually makes at that point. Use the torque and RPM that occur together.

• Treating the 5252 crossover as a real engine event. Nothing physical happens at 5252 RPM. It is only where the imperial HP and lb·ft numbers happen to be equal because of the constant. An engine redlining below 5252 never shows the crossover at all, and metric curves do not cross there.

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