# Boost Users :

Subject: Re: [Boost-users] boost::units - converting from one derived_dimension to another across systems (imperial to metric)
From: Leon Mlakar (leon_at_[hidden])
Date: 2018-11-16 09:57:11

On 15.11.2018 17:27, Michael Powell via Boost-users wrote:
> On Thu, Nov 15, 2018 at 11:11 AM Roger Leigh via Boost-users
> <boost-users_at_[hidden]> wrote:
>> On 15/11/2018 15:18, Steven Watanabe via Boost-users wrote:
>>> AMDG
>>>
>>> On 11/14/2018 05:04 AM, Matt Vinson via Boost-users wrote:
>>>> I am having trouble converting from one derived_dimension to another across systems (imperial to metric). I'd like to convert, say, lb/in to, say, kg/m. I thought I would do the following but get a compiler error #:
>>> It's not going to work. lb is a unit of
>>> force, but kg is a unit of mass. They are not
>>> compatible and there is no conversion between
>>> them.
>> lbÂ·f (foot-pounds) is force; lb is mass.
> No, lb-f is a unit of Torque. lb is a unit of Force.
>
Hmm, that pound is primarily unit of force is a news to me. The
following is from the UK's Weights and Measures Act 1963:

"Theyard <https://en.wikipedia.org/wiki/Yard>or themetre
<https://en.wikipedia.org/wiki/Metre>shall be the unit of measurement
oflength <https://en.wikipedia.org/wiki/Length>and the pound or
thekilogram <https://en.wikipedia.org/wiki/Kilogram>shall be the unit of
measurement of mass by reference to which any measurement involving a
measurement of length ormass <https://en.wikipedia.org/wiki/Mass>shall
be made in the United Kingdom; and- (a) the yard shall be 0.9144 metre
exactly; (b) the pound shall be0.45359237kilogram exactly."

To my knowledge, the US still adhere to 1894's update to Mendenhall
Order which definedÂ  that there shall be 2.20462234pounds to a kilogram.

On the other hand, lbf is often used to denote "pound force" unit, with
1 lbf being approximately 4.448N or 0.454 kp.

So Boost.Units should then not be used for cooking recipes as the
conversions from pound based to gram based quantities would only work on
Earth?

Cheers,

Leon