Converting inches to kilometers.
First load the ‘ezunits’ package.
load (ezunits) $
foo : 1 ` inch;
Units are converted by the double backtick operator; conversions are not carried out automatically.
bar : foo `` km;

$$\frac{127}{5000000}\mathrm{km}$$ 

As usual, Maxima prefers exact numbers to inexact.
We can get a float approximation if we wish.
float (bar);

$$2.54\times {10}^{5}\mathrm{km}$$ 

A volume conversion.
Here ‘baz’ is the volume of one inch of water spread over one acre, while ‘quux’ is the same volume expressed in SI units.
baz : 1 ` (inch * acre);

$$1\mathrm{inch}\mathrm{acre}$$ 

quux : baz `` m^3;

$$\frac{20076201783}{195312500}{\mathrm{m}}^{3}$$ 

float (%);

$$102.79015312896{\mathrm{m}}^{3}$$ 
