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physical-quantitiesmath

This is a library for handling physical quantities. It supports physical units, values and uncertainties.

It has good documentation. Here is an example from the README:

POFTHEDAY> (let (m c m)
             ;; Define the mass
             (setf m #q(1.00 +/- 0.01 kg))
             ;; The speed of light
             (setf c #q(299792458 m / s))
             ;; e = m * c^2
             (setf e (q* m (qpow c 2)))
             ;; Print e, converted to petajoule
             (print #q(e -> PJ)))

#<QUANTITY 89.87552 +/- 1.0 % petajoule {...}>

As you can see, physical-quantities not only able to tag values with units but also is able to convert from one unit to another:

POFTHEDAY> (q* #q(2 +/- 0.1 m)
               #q(3 +/- 0.1 m))
#<QUANTITY 6 +/- 0.36055514 metre ^ 2 {10021734A3}>

POFTHEDAY> #q(* -> mm ^ 2)
#<QUANTITY 6000000 +/- 360555.16 millimetre ^ 2 {10023A6AF3}>

You can find more examples in its documentation:

http://quickdocs.org/physical-quantities/