It's a limitation of the way explicit was implemented. Mathcad has two calculation engines, symbolic and numeric, and they handles units differently. The symbolic processor is not units aware, and "handles" them by simply treating them the same way it treats any undefined variable. When you use the explicit keyword you are using the symbolic processor. A numeric assignment results in a value that is stored internally in SI units, regardless of the unit system chosen. So when you calculate tau the value is stored as 4.597*18^8, with units of Pa. On the other hand, sigma is simply an assignment, not a calculated value. The number is stored internally in SI units, but when you do a symbolic evaluation it is not used. The symbolic processor uses the expression 12*ksi. Richard
