I've not seen any units convertions that do the above.

The codecogs system has a number of base types for common quantities, i.e.

• distance
• flow
• density
• pressure
• etc.

They then have a recognize unit system, that merely matches to one of these quantities. Lets see if I can embed there 'calculator' here: <iframe src="http://www.codecogs.com/ic-269&u=6" frameborder="0" scrolling="no" width="235" height="193"></iframe>

I've got the source code, which seems good; but doesn't go as far as we need.

--- Not sure how best to proceed. But from what I've read in the past (can't find the references now), we need to express every base unit (perhaps all those that CodeCogs has listed as a start) in its raw universal type of (length, mass, time), which in SI is (m,kg,s). You also work out in advance the conversion factor that takes it into SI (again CodeCogs I think have done this).

Then given a new type, you run through each unit and covert it into one of the base units, keeping a tab of the overall conversion factor. Do this twice for both the initial and target units and you should have the complete conversion.

However, I would also like to be able match these powers of (m,kg,s), back to the more common quantities, so if the system converts say psi into N/m^2=kg/(m s^2), then it can list out Pascals as being of a same type. In addition it would be nice to do error checking, making sure that two types are equivalent.

For this reason we'd need to be able to simplify the base types, however I don't think that should be too complicated.