How to properly use unknown variables in an equation?

The short answer is: you can't, because your statement is not true.

The true answer is (assuming that you wanted L to represent a length of 18 inches):

f = 2.187 * P / m, or f = 0.556 * P / in.

But with P unknown, mathcad does not know how to calculate the numeric value of f

Unless you define f as a function of P, like:

You can have mathcad work with unknown variables such as P, using its symbolics. But the symbolics of mathcad don't know units. So it will treat 'in' as an unknown variable. Then what you get is this:

Hope this helps.

Success!

Luc

Here are some other ways to get something similar to what you demand.

But if you intend to evaluate f later in your sheet, probably with different values of P, I would chose the function f(P) as shown by Luc. You can display f(1) as 1/18 instead of 0,056 if you change the number format for this region to "Fraction" (as in my second example below).