# Torque wrenches use Newtons (correct) and Foot Pounds (Wrong)
**URL:** https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631
**Category:** Factual Questions
**Created:** [October 26, 2003, 7:29am UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631 "2003-10-26T07:29:58Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@wolf\_meister](https://boards.straightdope.com/u/wolf_meister)
#### Post date: [October 26, 2003, 7:29am UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/1 "2003-10-26T07:29:58Z")
Okay, I’ve noticed that on every torque wrench I’ve ever seen the readout is in Newtons (which is a unit of force and torque is a force). However, they also read out in foot pounds which is a unit of ENERGY and that is wrong. POUNDS are a unit of force so why do they say foot-pounds? To me this makes as much sense as having a measuring cup that measures cubic centimeters and square inches.
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### Author:  [@sailor](https://boards.straightdope.com/u/sailor)
#### Post date: [October 26, 2003, 7:45am UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/2 "2003-10-26T07:45:06Z")
You got it all wrong. Torque is not a force but force multipled by the length of the arm. Foot\*pounds is correct. Newtons is not.
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### Author:  [@commasense](https://boards.straightdope.com/u/commasense)
#### Post date: [October 26, 2003, 7:57am UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/3 "2003-10-26T07:57:46Z")
There seems to be widespread confusion between the foot-pound (the unit of work required to raise a mass of one pound a distance of one foot) and the proper SAE unit of torque, which is the pound-foot. A pound-foot is the torque exerted by a pound of mass at a distance of a foot from the axis of torsion.
This confusion apparently extends to many Web-based conversion tables, and the makers of torque wrenches. I have two, from different manufacturers, both of which incorrectly read “foot-pounds.” (Until just a few days ago, when I read an item about the pound-foot in the current issue of _Road & Track_, the confusion also extended to me.)
[Here’s](http://science.howstuffworks.com/fpte3.htm) a site that gets it right.
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### Author:  [@scr4](https://boards.straightdope.com/u/scr4)
#### Post date: [October 26, 2003, 8:02am UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/4 "2003-10-26T08:02:20Z")
> [@](#):
>
> \*Originally posted by commasense \*
> **There seems to be widespread confusion between the foot-pound (the unit of work required to raise a mass of one pound a distance of one foot) and the proper SAE unit of torque, which is the pound-foot.**
Why is this distinction important? It’s the same unit; same exact dimensions and value.
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### Author:  [@Earthling](https://boards.straightdope.com/u/Earthling)
#### Post date: [October 26, 2003, 8:03am UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/5 "2003-10-26T08:03:45Z")
You’d better check those torque wrenches again. The metric measurement should say N-m, for Newton-meters. That’s what it says on my wrench.
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### Author:  [@sailor](https://boards.straightdope.com/u/sailor)
#### Post date: [October 26, 2003, 8:42am UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/6 "2003-10-26T08:42:56Z")
> [@](#):
>
> \*Originally posted by scr4 \*
> Why is this distinction important? It’s the same unit; same exact dimensions and value.
No, not really. When you exert a force of 1 lb for a distance of 1 ft you have done 1 lb\*ft of work. It is a unit of work or energy.
OTOH, torque is not work or energy. It is the force multiplied by the length of the arm. To obtain the work done you cannot multiply the force by the length of the arm but you have to multiply the force by the distance traveled which is the arc of circumference traveled.
If I am turning a wheel with a torque of 1 ft_lb, for every complete turn I have done 2_pi() lb\*ft of work.
Different concepts even if the dimensions are the same.
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### Author:  [@racer72](https://boards.straightdope.com/u/racer72)
#### Post date: [October 26, 2003, 12:09pm UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/7 "2003-10-26T12:09:28Z")
At my job everything is measured in inch/pounds. In fact last week I got me a brand new 15 in/lb cordless screwgun in my tool kit. No more dragging air hoses around the airplane!!!
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### Author:  [@scr4](https://boards.straightdope.com/u/scr4)
#### Post date: [October 26, 2003, 12:39pm UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/8 "2003-10-26T12:39:58Z")
> [@](#):
>
> \*Originally posted by sailor \*
> \*\*Different concepts even if the dimensions are the same. \*\*
OK, I agree they are different concepts, and I realize that SI uses different units for the two (Joule vs. Newton-meter). But I’m still not sure if it’s important to distinguish between “pound-foot” and “foot-pound” - I can’t think of any other compound units where the order matters.
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### Author:  [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)
#### Post date: [October 26, 2003, 12:43pm UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/9 "2003-10-26T12:43:47Z")
It should be N-m, not just N.
The difference between ft-lbf versus lbf-ft seems IMO to be one of parlance. Equation-wise, they have the same dimensions. And in some professions, like mine, parlance wars are the equivalent of “Enterprise versus Star Destroyer”.
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### Author:  [@sailor](https://boards.straightdope.com/u/sailor)
#### Post date: [October 26, 2003, 1:21pm UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/10 "2003-10-26T13:21:19Z")
> [@](#):
>
> \*Originally posted by scr4 \*
> OK, I agree they are different concepts, and I realize that SI uses different units for the two (Joule vs. Newton-meter). But I’m still not sure if it’s important to distinguish between “pound-foot” and “foot-pound” - I can’t think of any other compound units where the order matters.
Conventions exist for a reason. It is only a matter of being clear about whether you are talking about torque or work. As long as it is clear to everybody you can say it whichever way you want but following conventions makes it easier for everybody to understand and avoid mistakes.
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### Author:  [@aerodave](https://boards.straightdope.com/u/aerodave)
#### Post date: [October 26, 2003, 1:36pm UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/11 "2003-10-26T13:36:42Z")
> [@](#):
>
> \*Originally posted by sailor \*
> \*\*Different concepts even if the dimensions are the same. \*\*
But the N-m is the proper metric unit of both work and torque. Different concept, same dimensions…but same unit as well.
And yes, the OP should notice that his torque wrench, unless very poorly designed, has N-m on it. Force times Length
The difference between work and torque (even using the same dimenstions) becomes clear when working in vector math. For the unfamiliar, vectors are quantities having both magnitude and direction.
Torque is the **cross product** of force and the length of the moment arm. The result is the torque vector, which is perpendicular to both.
**T** = **F** x **r**
Work is the **dot product** of force and the distance over which it is applied. The result is a **scalar**. That is, it has magnitude, but no direction.
_W_ = **F** • **d**
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### Author:  [@Shade](https://boards.straightdope.com/u/Shade)
#### Post date: [October 26, 2003, 1:39pm UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/12 "2003-10-26T13:39:01Z")
> [@](#):
>
> There seems to be widespread confusion between the foot-pound (the unit of work required to raise a mass of one pound a distance of one foot) and the proper SAE unit of torque, which is the pound-foot. A pound-foot is the torque exerted by a pound of mass at a distance of a foot from the axis of torsion.
Nitpick: don’t you need to specify g? Torque and Energy should be force x distance not mass x distance iirc.
Anyway, after some googling, it appears that:
1. They do in fact have the same units.
2. But the convention is to use J or foot-pounds for energy and Nm or pound-feet for torque, to try and avoid confusion.
3. While it’s normal to gloss over this, torque is defined by physicists as a vector, (while energy is definately a scalar) so they can’t be compared.
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### Author:  [@j666](https://boards.straightdope.com/u/j666)
#### Post date: [October 26, 2003, 2:20pm UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/13 "2003-10-26T14:20:01Z")
I don’t know why I opened this post, but I am really glad I did; I wish my physics instructor had explained this as neatly as Shade.
So this is why my pruning sheers and bolt cutters have such long handles and why I hold my hammer at the far end rather than at the _forte_, right?
If I finally understand torque, it’s why levers work, right?
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### Author:  [@wolf\_meister](https://boards.straightdope.com/u/wolf_meister)
#### Post date: [October 26, 2003, 6:20pm UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/14 "2003-10-26T18:20:13Z")
Thanks to all for the quick replies.
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### Author:  [@Saltire](https://boards.straightdope.com/u/Saltire)
#### Post date: [October 26, 2003, 6:37pm UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/15 "2003-10-26T18:37:55Z")
> [@](#):
>
> \*Originally posted by Shade \*
> \*\*Nitpick: don’t you need to specify g? Torque and Energy should be force x distance not mass x distance iirc. \*\*
Actually, no. Though most people tend to think of the pound as the American system’s unit of mass, it is actually the unit of force, analogous to S.I.'s newton. Mass in the American system is measured in slugs.
Almost no one needs to know this, since almost any application where the distinction is important would use S.I. units.
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### Author:  [@Shade](https://boards.straightdope.com/u/Shade)
#### Post date: [October 26, 2003, 6:44pm UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/16 "2003-10-26T18:44:00Z")
OK, fair enough. But I think “a mass of one pound” is not the clearest way to express “a mass weighing one pound” as opposed to “a pound-mass.”
**j66** : Thanks. Yes, that’s why levers work 🙂
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### Author:  [@Achernar](https://boards.straightdope.com/u/Achernar)
#### Post date: [October 26, 2003, 6:48pm UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/17 "2003-10-26T18:48:44Z")
> [@](#):
>
> \*Originally posted by Shade \*
>
> 1. They do in fact have the same units.
> 2. But the convention is to use J or foot-pounds for energy and Nm or pound-feet for torque, to try and avoid confusion.
> 3. While it’s normal to gloss over this, torque is defined by physicists as a vector, (while energy is definately a scalar) so they can’t be compared.
Torque is actually a pseudovector, but so what? Velocity is a vector and the speed of light is a scalar, but they can still be compared. And they still use the same units.
For a magnetic dipole mu in a magnetic field, the torque is given by:
[ul]tau = mu × B[/ul]and the interaction energy is given by:
[ul]U = -mu · B[/ul]How could they possibly not have the same units?
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### Author:  [@swansont](https://boards.straightdope.com/u/swansont)
#### Post date: [October 26, 2003, 7:23pm UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/18 "2003-10-26T19:23:52Z")
> [@](#):
>
> \*Originally posted by audilover \*
> \*\*Torque is the **cross product** of force and the length of the moment arm. The result is the torque vector, which is perpendicular to both.
> **T** = **F** x **r**
> \*\*
**T** = **r** x **F**
The resultant vector ends up in the opposite direction if you do it backwards.
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### Author:  [@j666](https://boards.straightdope.com/u/j666)
#### Post date: [October 26, 2003, 7:48pm UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/19 "2003-10-26T19:48:02Z")
Psuedo vectors?!!
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### Author:  [@aerodave](https://boards.straightdope.com/u/aerodave)
#### Post date: [October 26, 2003, 7:58pm UTC](https://boards.straightdope.com/t/torque-wrenches-use-newtons-correct-and-foot-pounds-wrong/209631/20 "2003-10-26T19:58:19Z")
> [@](#):
>
> \*Originally posted by swansont \*
> \*\ ***T** = **r** x **F**
>
> The resultant vector ends up in the opposite direction if you do it backwards. \*\*
You are correct. Thanks for the friendly reminder. I was spouting from memory, and wasn’t thinking about the non-commutativity of vector multiplication.
Then again, it would have been more conventional to write **t** (“tau” if it doesn’t format right on everyone’s machines) instead of **T** , as well.
But I think my point was still clear.
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