Results are returned in the unit system that you choose for the worksheet. When you add two inches together in the SI unit system, the result is 0.051m. When you change the unit system from SI to USCS, the result becomes 0.167ft. You can manually replace the feet to inches so that the result becomes 2in. In each case, the numerical result is automatically updated to match the new units.

When the new units are incompatible with the expected dimension, PTC Mathcad adds a unit term to the left of the unit placeholder to keep the units balanced. For example, evaluate 1 Newton:

When you replace the Newton by meter, the unit term (kg/s2) is added to balance the units:

You can replace part of the units. For example, you can replace the kilogram by Coulomb in which case the unit term (kg/s·A) is added to balance the units: