Foot Pounds Calculator: Torque & Energy Explained

This foot pounds calculator converts between foot-pounds and other common units of torque or energy. You enter a value and select the unit you have, and the calculator returns the equivalent in foot-pounds, plus newton-meters, inch-pounds, and joules. It works for both torque (a twisting force) and energy (work done), since the foot-pound is used for both, though the conversions differ slightly.

Formula

The foot-pound (symbol: ft-lb or lbf·ft) is a unit of work or torque in the US customary and imperial systems. The conversion formulas are:

• Torque: 1 foot-pound = 12 inch-pounds = 1.35582 newton-meters
• Energy/work: 1 foot-pound = 1.35582 joules

For the calculator above, the base value is foot-pounds. The conversion factor u is the multiplier that converts your input unit to foot-pounds. For example, if you enter 100 in-lb, u = 0.083333, so the result is 100 × 0.083333 = 8.33 ft-lb.

Worked Example

Using the default values: Value = 100, Unit = ft-lb. Then u = 1, so:

1. Foot-pounds = 100 × 1 = 100 ft-lb
2. Newton-meters = 100 × 1 × 1.35582 = 135.58 N·m
3. Inch-pounds = 100 × 1 × 12 = 1200 in-lb
4. Joules = 100 × 1 × 1.35582 = 135.58 J

The calculator shows 100 ft-lb as the headline answer, which matches.

Foot-Pounds vs. Pound-Feet: What’s the Difference?

In everyday use, “foot-pounds� and “pound-feet� are often used interchangeably, but they are technically different. The foot-pound (ft-lb) is a unit of work or energy, while the pound-foot (lbf·ft) is a unit of torque. However, in the US, torque specs on cars and tools are commonly called “foot-pounds� even though they are pound-feet. The numerical value is the same: 1 ft-lb = 1 lbf·ft. The difference is only in the order of the words and the physical concept. For most practical purposes, treat them as equal.

When to Use Foot-Pounds: Torque vs. Energy

Understanding the context is key to using the calculator correctly.

• Torque (twisting force): Used in automotive repair (lug nuts, cylinder head bolts), bicycle maintenance, and any fastener that needs a specific tightness. Torque wrenches are calibrated in ft-lb or N·m.
• Energy/work: Used in physics, engineering, and ballistics. For example, the muzzle energy of a firearm is often quoted in foot-pounds. A .22 LR bullet might have ~100 ft-lb of energy, while a .45 ACP has ~400 ft-lb.
• Conversion caution: When converting energy, use joules (1 ft-lb = 1.35582 J). When converting torque, use newton-meters (1 ft-lb = 1.35582 N·m). The number is the same, but the unit name changes.

Common Mistakes and Misconceptions

One frequent error is mixing up foot-pounds with inch-pounds. A foot-pound is 12 times larger than an inch-pound. For example, 10 ft-lb = 120 in-lb. Another mistake is using the same conversion factor for energy and torque without realizing they are different physical quantities. The calculator above handles both correctly because the joule and newton-meter are numerically equal (1 J = 1 N·m), but the context matters when you apply the result.

Also, beware of “foot-poundal� – an old unit of energy that is not the same as a foot-pound. One foot-poundal is about 0.0421 joules, which is why we did not include it in the calculator. Stick with ft-lb, in-lb, N·m, and J.

How to Use a Torque Wrench with Foot-Pounds

If you’re using a torque wrench that reads in ft-lb, follow these steps:

1. Set the wrench to the specified ft-lb value (e.g., 80 ft-lb for lug nuts).
2. Attach the correct socket and place it on the fastener.
3. Pull smoothly until you hear a click or feel a release. Do not continue pulling after the click.
4. Store the wrench at its lowest setting to maintain accuracy.

Always check the manufacturer’s torque spec for your specific vehicle or equipment. Over-tightening can strip threads or warp parts; under-tightening can cause failures.

Practical Examples of Foot-Pounds in Real Life

• Car lug nuts: Typically 80–100 ft-lb (108–135 N·m).
• Bicycle crank bolts: Often 30–50 ft-lb (41–68 N·m).
• Muzzle energy: A 9mm handgun round might have ~350 ft-lb of energy at the muzzle.
• Household: A 10-lb weight lifted 1 foot does 10 ft-lb of work.

These examples show the range of values you might encounter, from small fasteners to ballistic energy.

Limitations of the Calculator

This calculator only converts units; it does not account for friction, temperature, or the elasticity of materials. Torque specs are affected by lubrication, thread pitch, and fastener material. For critical applications, always refer to the equipment manual. Also, the conversion factor 1.35582 is exact for the international foot (1 ft = 0.3048 m) and the standard gravity (9.80665 m/s²). In practice, rounding to 1.356 is fine.

Frequently Asked Questions

How many inch-pounds are in a foot-pound?

There are 12 inch-pounds in one foot-pound. This is because 1 foot = 12 inches, and torque is force times distance. So 1 ft-lb = 12 in-lb. To convert, multiply the ft-lb value by 12.

Is foot-pound the same as pound-foot?

In numeric value, yes – 1 ft-lb = 1 lbf·ft. The difference is semantic: foot-pound is used for energy, pound-foot for torque. But in the US, “foot-pound� is commonly used for torque as well. For conversions, treat them as identical.

How do I convert foot-pounds to newton-meters?

Multiply the foot-pound value by 1.35582 to get newton-meters. For example, 50 ft-lb × 1.35582 = 67.79 N·m. This works for both torque and energy because 1 N·m = 1 J.

What is the difference between foot-pounds and joules?

They measure the same thing (energy), but use different systems. 1 foot-pound = 1.35582 joules. To convert ft-lb to J, multiply by 1.35582. To convert J to ft-lb, divide by 1.35582. For example, 100 J ÷ 1.35582 = 73.76 ft-lb.

Can I use this calculator for muzzle energy?

Yes, if you know the energy in foot-pounds, you can convert to joules or other units. But to calculate muzzle energy from bullet weight and velocity, you need the formula: Energy (ft-lb) = (bullet weight in grains × velocity² in fps) / 450,240. This calculator does not compute that; it only converts existing values.

Why does my torque wrench show both ft-lb and N·m?

Many torque wrenches have dual scales because cars and tools use different unit systems. The conversion is built into the tool. The calculator above helps you when you only have one scale or need to convert a spec.

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in v | Value | - | 100 | 0
sel u | Unit | ft-lb=1, in-lb=0.083333, N-m=0.737562, J=0.737562, ft-lal=1.35582
out ftlb | Foot-pounds | ft-lb | v * u | 2
out nm | Newton-meters | N·m | v * u * 1.35582 | 2
out inlb | Inch-pounds | in-lb | v * u * 12 | 1
out j | Joules | J | v * u * 1.35582 | 2