Actually I guess you'd need 2 IF
formulas.
If column A was Feet and B inches
(formatted as a fraction) (the two together being the first length)
column C was Feet and D inches (fomatted as a fraction)(those two together being
the second length) then you could have:
A(Feet) B(Inches) C(Feet) D(Inches) E(Total Feet)
F(Total Inches)
Then column E could add the feet (+1 if the
inches totaled more than 12):
=IF(B1+D1<12,A1+C1,A1+C1+1)
and F would add the inches and keep them under 12
(the feet would already have been incremented if the inches were >
12):
=IF(B1+D1<12,B1+D1,B1+D1-12)