Suppose a wall run is 5 ft 3 1/8 in and an opening is 2 ft 7 3/4 in. You want the length left after subtracting the opening. The inches in the first measurement are smaller, so borrow one foot.
- Rewrite the wall run as 4 ft 15 1/8 in. The total length is the same.
- Write the fractions with the same denominator: 15 1/8 in − 7 6/8 in = 7 3/8 in.
- Subtract the feet: 4 − 2 = 2. The remaining length is 2 ft 7 3/8 in.
To check the result another way, turn both measurements into inches before subtracting. The starting values are 63 1/8 in and 31 3/4 in. Their difference, 31 3/8 in, is the same 2 ft 7 3/8 in. Keeping the fractions through the subtraction avoids errors from treating the inches as decimal feet.
That is a measured length, not an order quantity. If you are buying trim, decide separately how to account for cut ends, usable offcuts, and the stock lengths you can buy.
I work on the feet and inches calculator. Enter the two original measurements and choose Subtract. The app preserves eighth-inch and quarter-inch fractions exactly, so this example can be checked without a rounding approximation.
Top comments (0)