# Feet down below his knee

May 27, 2009 at 9:44 AM by Dr. Drang

Following up this post from yesterday, I downloaded and compiled GNU `units`

to give it a test run. Although it’s feature-packed, it won’t let me use units like $\sqrt{\text{in}}$. Also, the inclusion of addition/subtraction and the change to equal precedence for multiplication and division make it, I think, a little *less* usable than the traditional `units`

. For example, in traditional `units`

, `btu/hr ft degF`

is interpreted as

because multiplication has higher precedence than division. In GNU `units`

, you’d have to do that as `btu/hr/ft/degF`

. Since you’d say the unit as “BTU per hour foot degree Fahrenheit,” I think the traditional method is better.

I made a couple of additions to my local `units.lib`

file to handle, in a limited way, the $\sqrt{\text{in}}$ and $\sqrt{\text{m}}$ units that come up in fracture toughness values.

```
rootm !k!
rootin 0.1593737745 rootm
```

This defines `rootm`

as a fundamental unit and `rootin`

as a derived unit. The proportionality constant is the square root of 0.0254. Now I can do things like

```
You have: 25 MPa rootm
You want: ksi rootin
* 22.751193
/ 0.04395374
```

which is pretty much all I need the root units for.