When I tried that, it gave me the value of 1 atm in torr, but it still left off the [[torr]] unit; however, I believe I have found the correct way to use units in Maple: Use whichever units you like when assigning values to variables. Maple can do inter-unit calculations just fine, but may display undesired units. Right-click on the value Maple outputs, and convert the units to whatever system you like. This is assuming you are running with(Units).