Foot Pound to Joule Converter
Convert foot pound joule between units instantly. Includes conversion tables, common equivalents, and calculation formulas.
Foot Pound to Joule Converter
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Formula: J = ft-lbf x 1.35582 | 1 BTU = 1055.06 J | 1 cal = 4.184 J
Worked example — 350 ft-lbf = 474.54 J = 474.54 Nm
Formula
J = ft-lbf x 1.35582 | 1 BTU = 1055.06 J | 1 cal = 4.184 J
One foot-pound equals 1.35582 joules. The joule is the SI unit of energy defined as the work done by a one-newton force through one meter. Key relationships: 1 BTU = 1,055.06 J, 1 calorie = 4.184 J, 1 kWh = 3,600,000 J. The foot-pound measures work in Imperial units (one pound of force through one foot).
Worked Examples
Example 1: Engine Torque Conversion
Problem:An engine produces 350 ft-lbf of torque. Convert to joules and newton-meters.
Solution:Joules: 350 x 1.35582 = 474.537 J Newton-meters: 474.537 Nm (same as joules for energy) Kilojoules: 474.537 / 1000 = 0.4745 kJ BTU: 474.537 / 1055.06 = 0.4498 BTU
Result:350 ft-lbf = 474.54 J = 474.54 Nm
Example 2: Energy Comparison
Problem:Convert 5,000 joules to foot-pounds, calories, and BTU.
Solution:Foot-pounds: 5,000 / 1.35582 = 3,688.10 ft-lbf Calories: 5,000 / 4.184 = 1,195.22 cal Kilocalories: 5,000 / 4,184 = 1.195 kcal BTU: 5,000 / 1,055.06 = 4.739 BTU
Result:5,000 J = 3,688.1 ft-lbf = 1,195.2 cal = 4.74 BTU
Frequently Asked Questions
How do I convert foot-pounds to joules?
To convert foot-pounds to joules, multiply by 1.35582. One foot-pound (ft-lbf) is the energy needed to exert a force of one pound through a distance of one foot. For example, 100 foot-pounds equals 135.582 joules. The foot-pound is commonly used in US engineering for torque specifications on engines and fasteners, while the joule is the SI standard energy unit used internationally.
What is the difference between a foot-pound of energy and a foot-pound of torque?
A foot-pound of energy (ft-lbf) and a foot-pound of torque (lb-ft) have the same dimensions but different physical meanings. Energy foot-pounds measure work done when a force moves through a distance. Torque foot-pounds (often written lb-ft in automotive contexts) measure rotational force around an axis. Numerically they are equivalent, but conventionally energy uses ft-lbf while torque uses lb-ft to distinguish the concepts.
How many joules are in a BTU?
One British thermal unit (BTU) equals approximately 1,055.06 joules or about 778.17 foot-pounds. The BTU is defined as the amount of heat needed to raise the temperature of one pound of water by one degree Fahrenheit. BTUs are widely used in the US for rating heating and cooling equipment, natural gas pricing, and energy efficiency of appliances. In metric countries, the equivalent calculations use kilojoules or kilocalories instead.
What is an erg and how does it relate to a joule?
An erg is the CGS (centimeter-gram-second) unit of energy equal to exactly 0.0000001 joules (10^-7 J). One joule equals 10 million ergs. The erg is defined as the work done by a force of one dyne acting through one centimeter. While largely replaced by the joule in modern science, ergs still appear in astrophysics literature where extremely large energies are expressed in ergs. A supernova releases about 10^44 joules or 10^51 ergs of energy.
What is the difference between the calorie used here and a food Calorie?
This converter uses the thermochemical calorie of exactly 4.184 joules. The food Calorie printed on nutrition labels is written with a capital C and is really a kilocalorie, equal to 1,000 thermochemical calories or 4,184 joules. So a 250 Calorie snack carries about 1,046,000 joules, which is roughly 771,000 foot-pounds. Select the kilocalorie option rather than the calorie option whenever your figure came from a nutrition label.
How large is an electronvolt compared with a joule?
One electronvolt is about 1.602 x 10^-19 joules, the energy an electron gains crossing a potential difference of one volt. It is the natural unit for atomic, molecular, and particle physics, where joules would produce unreadable exponents. Chemical bond energies sit in the range of a few electronvolts per bond, while nuclear processes run into millions of electronvolts. Converting eV to foot-pounds is rarely useful physically, but the tool supports it for completeness.
How many foot-pounds are in one kilowatt-hour?
One kilowatt-hour is 3,600,000 joules, since a kilowatt sustained for 3,600 seconds delivers that much energy. Dividing by 1.35582 gives about 2.655 million foot-pounds. That enormous number is why domestic energy is billed in kilowatt-hours rather than in mechanical work units. It also shows the scale gap between everyday electricity use and the hand-tool torque figures that foot-pounds normally describe.
How do you calculate a projectile's kinetic energy in foot-pounds?
Use KE = one half x m x v squared with consistent Imperial units. Mass must be in slugs, which you get by dividing the weight in pounds by the standard gravity of 32.174 ft/s squared, and velocity in feet per second. The result then comes out directly in foot-pounds of force. Ballistics tables often list projectile mass in grains, so divide grains by 7,000 first to reach pounds before converting to slugs.
What is a foot-poundal and why is it not the same as a foot-pound?
The foot-poundal belongs to the absolute foot-pound-second system, where force is defined by the poundal, the force that accelerates one pound of mass at one foot per second squared. One poundal is about 0.138255 newtons, so one foot-poundal is about 0.04214 joules. The familiar foot-pound force is larger by the factor of standard gravity, about 32.174. Mixing the two silently introduces a factor-of-32 error in older engineering texts.
How do BTU and calories relate to each other directly?
Both were originally defined by heating water, which is why they are close cousins. One BTU raises one pound of water by one degree Fahrenheit; one calorie raises one gram of water by one degree Celsius. Working through this tool's factors, 1,055.06 joules divided by 4.184 joules gives about 252 calories per BTU. That ratio comes from combining the pound-to-gram mass ratio with the Fahrenheit-to-Celsius degree size ratio.
References
Background & Theory
1 foot-pound = 1.35581794833 joules exactly, so 1 joule = 0.737562149 foot-pounds. The exact value is a foot of exactly 0.3048 m multiplied by a pound-force of exactly 4.4482216152605 N.
Conversion reference table
| Conversion | Equivalent | Exact or rounded? |
|---|---|---|
| 1 foot-pound (energy) | 1.35581794833 joules | Exact by definition |
| 1 joule | 0.737562149 foot-pounds | Derived, rounded |
| 1 inch-pound | 0.112984829028 joules | Exact |
| 1 BTU (IT) | 1,055.05585262 joules = 778.169 ft-lb | Exact / derived |
| 1 calorie (thermochemical) | 4.184 joules = 3.08596 ft-lb | Exact / derived |
| 1 kilowatt-hour | 3,600,000 joules = 2,655,224 ft-lb | Exact / derived |
The foot-pound doubles as both an energy unit and a torque unit and the two are not interchangeable in use even though they share the same numerical factor: as energy it is a force acting through a distance along the line of action, and as torque it is a force acting at a perpendicular distance from an axis. Careful writing distinguishes them as ft-lb for energy and lbf-ft for torque, and firearms ballistics uses the energy sense - a 500 ft-lb muzzle energy is 678 joules.
Reviewed for accuracy by Manoj Kumar, Mathematics Educator · Editorial policy
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