What Is Pound-force inch (lbf·in)?
A pound force inch is a unit used to measure torque, which is a twisting force. It tells you how hard something turns when a force in pounds pushes at the end of a lever that is one inch long.
Definition
Torque is the turning effect of a force. The pound force inch combines two things:
- Pound force (lbf) is a unit of force based on the weight caused by gravity on one pound of mass.
- Inch (in) is a unit of length in the imperial and US customary systems.
One pound force inch is the torque made when a force of one pound force acts at a right angle on the end of a lever that is one inch long from the turning point.
In science and engineering it is a non SI unit of torque. The main SI unit for torque is the newton meter (N·m).
History / Origin
The pound force inch comes from older British and US systems of measurement, where engineers already used pounds and inches for many things.
When people started to measure torque, they combined their familiar units:
- Pounds were used for weight and force.
- Inches were used for length and distance.
By putting these together, they got pound force inch as a simple way to describe how tight a screw or bolt is, or how strong a small rotating part is. It became common in mechanical, automotive, and aerospace work in countries that use imperial or US customary units, especially the United States.
Symbol & Abbreviation
The most common symbol for pound force inch is:
- lbf·in
You may also see these forms in manuals or on tools:
- lbf in
- lbf-in
- lb f in or similar spacing styles
Be careful not to mix this up with:
- lb·in which can mean pound inch using mass instead of force.
- lb in written without the f where people sometimes forget to show that it is force.
In engineering, the small f matters, because torque depends on force, not just mass.
Current Use Around the World
Pound force inch is still widely used in:
- United States for tools like torque screwdrivers, small torque wrenches, and some machine parts.
- Canada and the United Kingdom in some industries that still follow imperial style engineering data.
- Global manufacturing for products made for US markets, such as appliances, electronics, and vehicles.
Typical uses include:
- Setting tightening torque for small screws and bolts.
- Specifying torque on delicate parts in electronics and medical devices.
- Measuring torque on small motors, gears, and shafts.
In science and international engineering, the newton meter is preferred, but pound force inch remains common in day to day technical work in many companies.
Example Conversions
The exact relationship between pound force inch and newton meter is:
- 1 lbf·in ≈ 0.112985 N·m
- 1 N·m ≈ 8.85075 lbf·in
Relationship with pound force foot:
- 1 lbf·ft = 12 lbf·in
- 1 lbf·in = 1/12 lbf·ft ≈ 0.08333 lbf·ft
Here are some simple example conversions:
- 10 lbf·in
- To newton meter: 10 × 0.112985 ≈ 1.13 N·m
- To pound force foot: 10 ÷ 12 ≈ 0.83 lbf·ft
- 50 lbf·in
- To newton meter: 50 × 0.112985 ≈ 5.65 N·m
- To pound force foot: 50 ÷ 12 ≈ 4.17 lbf·ft
- 100 lbf·in
- To newton meter: 100 × 0.112985 ≈ 11.30 N·m
- To pound force foot: 100 ÷ 12 ≈ 8.33 lbf·ft
- 2 N·m
- To pound force inch: 2 × 8.85075 ≈ 17.70 lbf·in
To convert from lbf·in to N·m, multiply by about 0.113. To convert from N·m to lbf·in, multiply by about 8.85.
Related Units
Units related to pound force inch include:
- Newton meter (N·m)
- SI unit of torque.
- Most common in science and international engineering.
- Pound force foot (lbf·ft)
- Another imperial torque unit.
- Used for larger torques, such as car wheel nuts.
- 1 lbf·ft = 12 lbf·in.
- Ounce force inch (ozf·in)
- Smaller torque unit for very light torques.
- 16 ozf·in = 1 lbf·in.
- Dyne centimeter (dyn·cm)
- Torque unit in the older cgs system.
- Much smaller than lbf·in.
FAQs
What does pound force inch actually measure?
It measures how strong a twisting action is. Imagine using a small wrench to turn a bolt. The harder you push and the longer the wrench, the more torque you make. Pound force inch tells you the size of that twist using pounds of force and inches of length.
How is pound force inch different from pound inch?
Pound force inch (lbf·in) uses force caused by gravity and is correct for torque. Pound inch (lb·in) without the f uses mass and is not precise for torque. In practice, many people write lb·in when they really mean lbf·in, but in careful engineering, the f should be there.
Is pound force inch an SI unit?
No, pound force inch is not an SI unit. The SI unit for torque is the newton meter (N·m). However, lbf·in is still widely used in industry, especially in the United States.
Where is pound force inch commonly used?
It is common in mechanical, automotive, and electronics work where small torques matter. Examples include tightening screws in laptops or phones, setting torque on small machine parts, and adjusting delicate instruments.
How do I convert pound force inch to newton meter?
Multiply the value in lbf·in by 0.112985 to get N·m. For quick mental math, you can use 0.113. For example, 20 lbf·in × 0.113 ≈ 2.26 N·m.
How do I convert pound force inch to pound force foot?
Divide by 12, because there are 12 inches in a foot. For example, 60 lbf·in ÷ 12 = 5 lbf·ft.
Why use pound force inch instead of pound force foot?
Pound force inch is better for small torques where pound force foot would give awkward decimal numbers. It keeps the numbers easy to read for light tightening tasks and small parts.
Can torque tools show both lbf·in and N·m?
Yes, many torque screwdrivers and torque wrenches have dual scales. You can read or set the torque in either lbf·in or N·m, depending on the values given in your manual or drawing.