This HP to torque calculator converts horsepower at a given RPM into torque, in both pound-feet (lb-ft) and Newton-meters (Nm). It's the reverse of the torque-to-HP formula and is useful for working out the twisting force behind a power figure, comparing engines, or filling in a missing value from a spec sheet.
HP to Torque Formula
The constant 5252 comes from converting the definition of horsepower into rotational terms. James Watt defined 1 HP as 33,000 ft·lb of work per minute. Since one revolution covers 2π radians, dividing 33,000 by 2π gives ≈ 5252.1, the exact crossover constant.
To also get torque in Newton-meters, multiply the lb-ft result by 1.3558:
Why RPM Is Essential
Horsepower by itself can't tell you torque — you also need the engine speed where that power is made. Because torque = HP × 5252 ÷ RPM, the same horsepower figure yields high torque at low RPM and low torque at high RPM. This is exactly why high-revving engines can make big horsepower from modest torque, while low-revving diesels make enormous torque for their horsepower. At 5,252 RPM the torque and horsepower numbers are always identical.
Torque for 350 HP at Different RPM
| RPM | Torque (lb-ft) | Torque (Nm) |
|---|---|---|
| 2,000 | 919 | 1,246 |
| 4,000 | 460 | 623 |
| 5,252 | 350 | 475 |
| 6,000 | 306 | 415 |
| 8,000 | 230 | 312 |
Worked Example
Given: A 350 HP engine at 6,000 RPM
Step 1: Apply the formula: Torque = 350 × 5252 / 6000
Step 2: Numerator = 350 × 5252 = 1,838,200
Step 3: Divide: 1,838,200 / 6000 = 306.4 lb-ft
Step 4: Convert to Nm: 306.4 × 1.3558 = 415.4 Nm
This formula gives torque at a single operating point. Real engines produce varying torque across the RPM range. Enter peak HP at its peak RPM for the peak torque figure, or use any operating point for that specific value.
HP to Inch-Pounds at 1,750 RPM
| Horsepower | Torque (lb-ft) | Torque (in-lb) |
|---|---|---|
| 0.5 | 1.5 | 18 |
| 1 | 3.0 | 36 |
| 5 | 15.0 | 180 |
| 10 | 30.0 | 360 |
Torque depends on RPM, so the same horsepower yields more torque at lower speeds — a key reason gear reduction is used to boost torque in compact drives.
Inch-Pounds, Foot-Pounds, Inch-Ounces and Newton-Meters
Small-equipment specifications rotate between four torque units, and mixing them up is easy because the numbers are all in similar ranges. These are the conversions worth memorising:
| From | To | Multiply by | Where each is used |
|---|---|---|---|
| Foot-pounds | Inch-pounds | 12 | Automotive to small-motor specifications |
| Inch-pounds | Inch-ounces | 16 | Stepper motors and instrument drives |
| Inch-pounds | Newton-meters | 0.11298 | Imperial to metric datasheets |
| Newton-meters | Inch-pounds | 8.8507 | Metric to imperial datasheets |
| Foot-pounds | Newton-meters | 1.35582 | Engine and fastener specifications |
Inch-ounces catch people out most often. A stepper motor rated at 200 oz-in is only 12.5 in-lb, so treating the two as equivalent overstates the motor by a factor of sixteen. Where a figure looks implausibly large for the physical size of the device, an ounce-inch rating read as inch-pounds is usually the explanation. The Nm to lb-ft converter handles the metric side directly.
Why Fastener Specifications Use Inch-Pounds
Torque wrench specifications for small fasteners are almost always given in inch-pounds rather than foot-pounds, and the reason is resolution rather than tradition. A bolt specified at 7.5 lb-ft becomes 90 in-lb, and a whole number is far easier to set accurately on a click-type wrench than a decimal.
This is also why inch-pound torque wrenches exist as a separate tool class. A wrench with a range of 40 to 200 in-lb covers small fasteners with far better accuracy than a foot-pound wrench operating at the very bottom of its scale, where click-type mechanisms are least reliable.
Frequently Asked Questions
The 5252 constant derives from converting James Watt's definition of horsepower (33,000 ft·lb/min) into rotational terms. One revolution equals 2π radians, so 33,000 ÷ (2π) ≈ 5,252.1. This gives us: Torque (lb-ft) = HP × 5252 / RPM. The crossover point where HP equals torque numerically is exactly 5,252 RPM.
Torque is the rotational force an engine produces; horsepower is how fast it can do work. They're related through RPM: HP = Torque × RPM / 5252. An engine can produce high torque at low RPM (like a diesel) or high HP at high RPM with less torque (like a sports car engine). Neither is "better" — they serve different purposes.
Horsepower and torque (in lb-ft) are always numerically equal at exactly 5,252 RPM. This is because when RPM = 5252, the formula becomes HP = Torque × 5252 / 5252 = Torque. On a dyno chart, the HP and torque curves always cross at this RPM. Above 5,252 RPM, HP exceeds torque; below it, torque exceeds HP.
A foot-pound (lb-ft) is a unit of torque equal to the force of 1 pound applied at a 1-foot radius. For example, if you use a 1-foot wrench and apply 100 pounds of force, you're creating 100 lb-ft of torque. The metric equivalent is the Newton-meter (Nm): 1 lb-ft = 1.3558 Nm, or 1 Nm = 0.7376 lb-ft.
Because torque = (HP × 5252) ÷ RPM. At higher RPM the same horsepower corresponds to less torque, which is why high-revving engines make big power from modest torque.