The MKS system of units has the meter, kilogram, and second as its base units.  Although the MKS system does not include units of electricity or magnetism, the SI system, which is also based on the meter, kilogram, and second, does.

When used in conjunction with the UseSystem command, the MKS system sets the following units for the given dimensions.

To enter a unit in 2-D Math input, select the unit from the appropriate Units palette. If the unit you want is not there, select $\mathrm{unit}$ and then enter the unit.

When you edit a unit, double brackets appear around it.

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

$\mathrm{Units}\left[\mathrm{UseSystem}\right]\left('\mathrm{MKS}'\right)$

$5\mathrm{Unit}\left(\frac{'\mathrm{kg}''m'}{'s'}\right)$

$5⟦\frac{\mathrm{kg}m}{s}⟧$

$\frac{\mathrm{Unit}\left('s'\right)}{}$

$5⟦N⟧$

$\frac{{\mathrm{Unit}\left('\mathrm{cm}'\right)}^2}{}$

$50000⟦\mathrm{Pa}⟧$