Torque is a way of measuring rotational, twisting or turning force; if you've ever heard people talking about how tight to make the lug nuts on their car tires, that's an example of torque. When measured in English or U.S. standard units, torque is usually expressed in foot-pounds or inch-pounds of force. If you know the length of the lever being used to create the torque and the amount of force being exerted on it, you can calculate torque yourself – and if you understand these concepts, you can also easily convert between units like inch-pounds and foot-pounds.
TL;DR (Too Long; Didn't Read)
Twelve inch-pounds of torque are equal to 1 foot-pound of torque. So to convert from inch-pounds to foot-pounds, divide by 12.
How to Calculate Torque
The basic formula for torque is
where L is the length of the lever arm, F is the force being applied to it at right angles to the object being twisted, and T is the resulting torque.
It's easy to see where the concept of foot-pounds or inch-pounds comes from; if the force is being measured in pounds and the length is being measured in feet, your torque will be expressed in foot-pounds. If the force is being measured in pounds and the length is being measured in inches, your torque will be expressed in inch-pounds.
Converting From Inch-Pounds to Foot-Pounds
So, how do you convert from inch-pounds to foot-pounds, or back the other way? Unlike calculating area, which converts one-dimensional measurements into a two-dimensional measurement, torque remains in just one dimension. So just as 12 inches equal 1 foot, 12 inch-pounds of torque equal 1 foot-pound.
That means that to convert from in lbs to ft lbs, all you have to do is divide by 12. For example, if you've been told that you're exerting 24 inch-pounds of torque, you'll divide by 12 to find the equivalent in foot-pounds:
Note that you didn't need the other information about length and force that would have gone into calculating torque in the first place; all you needed was the appropriate conversion ratio.
Converting From Foot-Pounds to Inch-Pounds
If you want to convert from foot-pounds to inch-pounds, just multiply by 12. For example, if you're exerting 200 foot-pounds of torque, you'd calculate as follows to convert from ft lbs to in lbs:
When to Use Inch-Pounds or Foot-Pounds
Perhaps you've already noticed that some measurements make more sense in inch-pounds, while others make more sense in foot-pounds. Just like measuring the length of something, it's natural to use inch-pounds when you measure smaller amounts of force, and to use foot-pounds when you measure larger amounts of force.
However, there is an exception to this. If you're comparing one thing to another, ideally they should be in the same unit of measure so you can compare apples to apples, as it were. Also, if you're performing calculations that involve multiple measurements of torque, all the measurements must be expressed in the same unit, or your calculations won't come out right.
About the Author
Lisa studied mathematics at the University of Alaska, Anchorage, and spent several years tutoring high school and university students through scary -- but fun! -- math subjects like algebra and calculus.