Thank you very much! This really helps me.

Thanks both for your answers, i found that by using this two commands i get the results that i wanted:

Units[UseSystem](FPS);
 
with(Units[Natural])

Now all the units in my calculations are done correctly.

However a new issue has appeared, i am calculating the moment of inertia of a cross section, where the equation is:

b := 1 ft

d := 1 ft

I= (b*d^3)/12

Result: I= 1 ft^2*s^2

Both  b, and d are variables in feets (ft), so my result should be ft^4, but instead i am getting ft^2*s^2. I have no idea what this s means.