What Is Foot-pound (ft·lbf)?
A foot-pound is a unit for measuring work or energy. It tells you how much effort is used when a force moves an object through a distance of one foot.
For example, if you lift a small weight using a steady push through one foot, the work you do can be measured in foot-pounds.
Definition
One foot-pound of work happens when a force of one pound-force moves an object by one foot in the same direction as the force.
In simple words, it joins two ideas:
- Force measured in pound-force lb_f
- Distance measured in feet ft
So:
1 foot-pound ft·lbf = 1 pound-force × 1 foot of movement
In the metric system, one foot-pound is about:
1 ft·lbf ≈ 1.3558 joules J
History / Origin
The foot-pound comes from older English and later British Imperial systems of measurement. These systems used feet for length and pounds for weight and force in daily life, building, trade, and engineering.
When scientists and engineers needed a way to measure work and energy before the metric system became common, they combined pound-force with feet. That combination became the foot-pound, a natural choice in English speaking countries where these units were already in use.
Later, the metric system and the joule became the main scientific unit for energy around the world. Even so, the foot-pound stayed popular in the United States and in some industries that still use Imperial or US customary units.
Symbol & Abbreviation
The most precise symbol for the unit of work or energy is:
- ft·lbf read as foot pound force
Other common written forms include:
- ft-lb
- ft lb_f
- foot-pound or foot pound
Important note: The same written forms, like ft-lb, are also used for torque, which is a turning force. In casual use, people may say foot-pounds for both energy and torque. In more careful writing, ft·lbf is used for energy and work, and a different symbol such as lbf·ft can be used for torque. The size of the unit is the same, but the physical meaning is different.
Current Use Around the World
Today, the official SI unit for energy is the joule J, but the foot-pound is still used in several places and fields.
United States
- Used in some physics and engineering problems that follow US customary units
- Appears in ballistics, such as bullet energy for firearms and air rifles
- Used in some mechanical and automotive work, often for torque settings, like tightening bolts
United Kingdom and other countries
- Main science and engineering work uses joules and newton-metres
- Foot-pounds may still appear in older books, manuals, or certain industries
Science and education
- Students may learn foot-pounds when studying older systems of units
- Conversions between foot-pounds and joules help connect Imperial units to SI units
Example Conversions
Use this key fact:
1 ft·lbf ≈ 1.3558 J
and the reverse:
1 J ≈ 0.7376 ft·lbf
Foot-pounds to joules
- 1 ft·lbf ≈ 1.36 J
- 5 ft·lbf ≈ 5 × 1.3558 ≈ 6.78 J
- 10 ft·lbf ≈ 13.56 J
- 50 ft·lbf ≈ 67.79 J
- 100 ft·lbf ≈ 135.58 J
Joules to foot-pounds
- 1 J ≈ 0.74 ft·lbf
- 5 J ≈ 3.69 ft·lbf
- 10 J ≈ 7.38 ft·lbf
- 50 J ≈ 36.88 ft·lbf
- 100 J ≈ 73.76 ft·lbf
Simple real life style example
Imagine you push a 1 pound-force weight straight up by 2 feet. The work you do is:
Work = force × distance = 1 lb_f × 2 ft = 2 ft·lbf
Related Units
Units related to the foot-pound include:
- Joule J main SI unit of energy
- Calorie cal energy unit often used for food and heat 1 small calorie is about 4.184 J
- British thermal unit Btu larger heat energy unit used in heating and cooling 1 Btu ≈ 778 ft·lbf
- Watt W unit of power, which is energy per unit time 1 W = 1 J per second not a direct energy unit but closely linked
- Newton-metre N·m SI unit that can measure work and torque 1 N·m = 1 J exactly
- Inch-pound in·lbf a smaller unit of work or torque where 1 ft·lbf = 12 in·lbf
FAQs
What does a foot-pound measure?
A foot-pound measures work or energy. It tells you how much effort a force uses while moving something through a distance measured in feet.
Is a foot-pound the same as torque?
The size of the unit is the same, but the meaning is different. A foot-pound used for work or energy describes moving something along a path. A foot-pound used for torque describes how strongly something is being twisted around a point, like a bolt being turned. Many people use the same name and symbol in both cases, so the context must make it clear.
How do I convert foot-pounds to joules?
Multiply the number of foot-pounds by 1.3558. For example, 20 ft·lbf ≈ 20 × 1.3558 ≈ 27.12 J.
How do I convert joules to foot-pounds?
Multiply the number of joules by 0.7376. For example, 50 J ≈ 50 × 0.7376 ≈ 36.88 ft·lbf.
Where are foot-pounds still used today?
Foot-pounds are still common in the United States, especially in mechanics, ballistics, some engineering fields, and older technical manuals. Most modern science, however, uses joules.
Why do scientists prefer joules instead of foot-pounds?
Joules are part of the SI system, which is used worldwide. This makes sharing data, building models, and comparing results easier and more consistent across countries and fields.
Is foot-pound a large or small unit of energy?
A foot-pound is fairly small. For example, the energy in a normal chocolate bar is thousands of joules, which is equal to many hundreds of foot-pounds.