The combine/units function combines, if possible, expressions involving units into a unit-free expression multiplied by a Unit object.

The conversion $\mathrm{convert}\left(\mathrm{expr}\,'\mathrm{metric}'\right)$ can be implemented as combine(expr, 'units', 'mode'='natural').

The opts argument can contain one or more of the following equations that set options for how units are interpreted and combined.

'mode'=standard or natural or simple

The mode standard indicates that Maple uses routines and operations from the Units[Standard] environment, whereas the mode natural indicates routines and operations from the Units[Natural] environment and simple indicates routines and operations from the Units[Simple] environment.

The default mode is set by the UseMode command, which is called if you load Units[Standard], Units[Natural], or Units[Simple]. If the command hasn't been called, the default mode is standard.

'system'=symbol

By default, any result is converted to the default system of units, which by default is SI.  By specifying a different system of units, the result is converted, if possible, to that system.

$\mathrm{combine}\left(4'\mathrm{ft}'+3'\mathrm{inches}'\,'\mathrm{units}'\,'\mathrm{mode}'='\mathrm{natural}'\right)$

$\frac{6477}{5000}⟦m⟧$

$\mathrm{combine}\left('\mathrm{yards}'\,'\mathrm{units}'\,'\mathrm{mode}'='\mathrm{natural}'\right)$

$\frac{1143}{1250}⟦m⟧$

$\mathrm{combine}\left(5'\mathrm{gallons}\left[\mathrm{UK}\right]'\,'\mathrm{units}'\,'\mathrm{mode}'='\mathrm{natural}'\right)$

$\frac{454609}{20000000}⟦m^3⟧$

$\mathrm{combine}\left(5'\mathrm{gallons}\left[\mathrm{US\_liquid}\right]'\,'\mathrm{units}'\,'\mathrm{mode}'='\mathrm{natural}'\right)$

$\frac{473176473}{25000000000}⟦m^3⟧$

$\mathrm{with}\left(\mathrm{Units}\left[\mathrm{Natural}\right]\right)\:$

$\mathrm{d1}≔3\mathrm{ft}$

$\mathrm{d1}≔3⟦\mathrm{ft}⟧$

$\mathrm{t1}≔13s$

$\mathrm{t1}≔13⟦s⟧$

$\mathrm{speed}≔\mathrm{subs}\left(\mathrm{dx}=\mathrm{d1}\,\mathrm{dt}=\mathrm{t1}\,\frac{\mathrm{dx}}{\mathrm{dt}}\right)$

$\mathrm{speed}≔\frac{3}{13⟦s⟧}⟦\mathrm{ft}⟧$

$\mathrm{combine}\left(\mathrm{speed}\,'\mathrm{units}'\right)$

$\frac{1143}{16250}⟦\frac{m}{s}⟧$

$\mathrm{combine}\left(\mathrm{speed}\,'\mathrm{units}'\,'\mathrm{system}'='\mathrm{FPS}'\right)$

$\frac{3}{13}⟦\frac{\mathrm{ft}}{s}⟧$