What Is Pound-force foot (lbf·ft)?
A pound-force foot is a unit used to measure torque, which is the turning force that causes something to rotate. Torque tells us how hard something is being twisted around a point. When you use a wrench to tighten a bolt, the amount of twist you apply is measured in units like pound-force feet.
In simple words, pound-force foot shows how much force in pounds is applied at the end of a one foot long lever or wrench.
Definition
One pound-force foot is the torque created when a force of one pound-force is applied at a right angle to the end of a lever that is exactly one foot long from the turning point.
So, if you push with a force of 1 pound-force at the end of a 1 foot long wrench, you are creating a torque of 1 pound-force foot.
In the metric system, the matching unit for torque is the newton meter, written as N m.
The exact conversion is:
- 1 pound-force foot ≈ 1.35582 newton meters (N m)
- 1 newton meter ≈ 0.73756 pound-force feet (lbf ft)
History / Origin
The pound-force foot comes from older British and American systems of measurement. These systems used pounds for force and feet for length.
Engineers needed a way to measure twisting force for things like engines, wheels and machines. They already used pounds to measure force and feet to measure distance. Putting them together gave the unit pound-force foot.
Over time, scientists created the SI system, which uses the newton meter for torque. However, many industries in the United States and some other countries kept using pound-force foot because it was already common and familiar in tools, manuals and workplaces.
Symbol & Abbreviation
The most common ways to write pound-force foot are:
- lbf·ft or lbf ft this clearly means pound-force times foot
- Sometimes people write lb ft in short, but this can be confusing
Important note about symbols:
- lbf·ft or lbf ft is a unit of torque
- ft·lbf or ft lbf is often used for energy and is called a foot pound
The order of the words and symbols matters. Pound-force foot is for turning force. Foot pound is usually for work or energy. Many people mix these up, so it is good to be careful.
Current Use Around the World
Today, pound-force foot is still widely used in:
- The United States for cars, trucks, bikes and tools
- Some industries in Canada and the United Kingdom, especially when using older tools or manuals
- Engineering fields that follow the Imperial or US customary system
You will often see torque values in pound-force feet when:
- Looking at car or truck engine torque ratings
- Reading a repair manual that tells you how tight to make a bolt
- Using a torque wrench that shows lbf ft on its scale
In science and most of the world outside these areas, the standard unit for torque is the newton meter. Many modern tools and manuals show both units so people can choose the one they know best.
Example Conversions
Here are some common example conversions between pound-force foot and other torque units.
Between pound-force foot and newton meter
- 1 lbf ft ≈ 1.35582 N m
- 5 lbf ft ≈ 6.7791 N m
- 10 lbf ft ≈ 13.5582 N m
- 50 lbf ft ≈ 67.791 N m
- 100 lbf ft ≈ 135.582 N m
Between newton meter and pound-force foot
- 1 N m ≈ 0.73756 lbf ft
- 5 N m ≈ 3.6878 lbf ft
- 10 N m ≈ 7.3756 lbf ft
- 50 N m ≈ 36.878 lbf ft
Between pound-force foot and pound-force inch
There are 12 inches in 1 foot, so for torque:
- 1 lbf ft = 12 lbf in
- 5 lbf ft = 60 lbf in
- 10 lbf ft = 120 lbf in
Simple example you can imagine
If you push down with a force of 20 pounds on the end of a 1 foot long wrench, the torque is:
- 20 lbf ft
- ≈ 27.116 N m
Related Units
Units closely related to the pound-force foot include:
- Newton meter (N m) the main SI unit for torque used in science and most countries
- Pound-force inch (lbf in) a smaller torque unit, used for small screws and delicate parts
- Foot pound or foot pound-force (ft lbf) commonly used as a unit of energy, such as in ballistics and physics
- Joule (J) the SI unit for energy, often used instead of foot pound in modern science
Remember, torque units are about turning force, while energy units are about work done. They can look similar but are used in different ways.
FAQs
Q1. What does pound-force mean in pound-force foot?
A. Pound-force is a unit of force. It is the force caused by the weight of one pound of mass near the surface of the Earth. When this force is applied at the end of a lever that is one foot long, it makes one pound-force foot of torque.
Q2. Is pound-force foot the same as foot pound?
A. No, they are usually used for different things. Pound-force foot, written as lbf ft, is mostly used for torque. Foot pound, written as ft lbf, is often used for energy. They have the same basic units inside, but the meaning and common use are different. People sometimes mix them up, so it is important to read carefully.
Q3. Why do car manuals use pound-force feet instead of newton meters?
A. Many car makers and repair shops in the United States and some other places have used pound-force feet for a long time. Mechanics and tools there are used to this unit, so manuals keep using it. However, many modern manuals now show both lbf ft and N m.
Q4. How do I quickly change lbf ft to N m in my head?
A. A simple trick is to multiply by about 1.36. For example, 20 lbf ft is about 20 × 1.36 = 27.2 N m, which is close to the more exact 27.116 N m.
Q5. Where do I see pound-force foot in real life?
A. You will most often see pound-force feet on:
- Torque wrenches
- Car and truck engine specs
- Bike and motorcycle repair guides
- Mechanical and engineering handbooks that use Imperial or US units
Q6. Can I use pound-force foot in school science projects?
A. You can, but most school science classes prefer SI units like the newton meter. If you start with pound-force feet, it is a good idea to also show the value in newton meters so everyone can understand and compare.
Q7. Why does the order of the letters lbf and ft matter?
A. The order reminds people what the unit is used for. Lbf ft is widely taken as torque. Ft lbf is often used for energy. Some books and tools are not strict about this, but using the correct order helps avoid confusion and keeps your work clear.