CAD tip # 5980:

Instead of entering a specific number (length, angle) you can use an arithmetic or conversion (e.g. inches-mm) expression in the dimension text (the blue text). Examples:

  
=1360/3  
=810-45mm
=120*30%  
=2'-3"  
=sqrt(16)  
=88^2  
=atan(1.0)  

You can also combine units or use metric units in an imperial project or vice versa - like =5000-7'5'':

Imperial units (feet/inches) can be entered with single or double quotes as 1'-2'', or 1'2", or 1'2, or just 1 2 in imperial projects.