How do you add feet and inches?
Convert both measurements to total inches, do the arithmetic, then convert back. 8 ft 4-1/2 in is (8 × 12) + 4.5 = 100.5 inches. 2 ft 7-3/8 in is 31.375 inches. Added, that is 131.875 inches, which is 10 ft 11-7/8 in.
The step people get wrong is the fraction. Sixteenths do not add cleanly in your head once you mix eighths and quarters — 7/16 plus 5/8 is 17/16, which is 1-1/16, and the carry into the inches column is what gets dropped on site.
Decimal feet is a separate answer, not the same one. 10 ft 11-7/8 in is 10.9896 decimal feet, not 10.11. Estimating software and takeoff sheets want decimal feet; the tape wants sixteenths. Mixing the two is the single most common measurement error in a takeoff.
How the calculation works
Everything is converted to sixteenths of an inch, added or subtracted as whole numbers, then converted back. Working in integer sixteenths avoids the floating-point drift you get from adding 0.4375 and 0.625 repeatedly — over a 40-piece cut list that drift is visible on the last board.
The result is rounded to the nearest 1/16 in because that is the finest graduation on a standard tape. Fractions are reduced, so 8/16 prints as 1/2 and 12/16 as 3/4.
Decimal feet = total inches ÷ 12. Metric = total inches × 25.4 mm.
Kerf is applied between pieces, not after the last one: n pieces need n − 1 cuts. A 1/8 in blade across 20 pieces removes 2-3/8 in of stock, which is why a cut list that fits exactly on paper comes up short in the shop.
What contractors normally pay for this arithmetic
| Material | Unit | Typical range |
|---|---|---|
| Construction Master Pro (handheld)The industry-standard feet-inch calculator | each | $40.00 – $75.00 |
| Construction Master Pro (phone app) | one-off | $20.00 – $30.00 |
| Feet-inch tape measure | each | $12.00 – $40.00 |
| Laser distance measureMost read decimal feet — convert before you cut | each | $30.00 – $200 |
| This calculatorFree, and it shows both answers at once | each | $0.00 – $0.00 |
Calculated Industries has sold the Construction Master line since 1985 largely on this one feature. The arithmetic is not hard — it is just tedious and easy to fumble with cold hands on a roof.
What goes wrong on this job
- Write the fraction, not the decimal, on the board. A pencil mark reading 4.6875 means nothing to the person making the cut; 4-11/16 means exactly one thing.
- Set the kerf before you break down a sheet or a cut list. Twenty cuts at 1/8 in is nearly two and a half inches of material that exists on your list and not in the pile.
- Decimal feet and feet-inches look dangerously similar in a takeoff. 10.5 ft is 10 ft 6 in, not 10 ft 5 in. If your estimate and your tape disagree by about half an inch per item, this is why.
- When subtracting for a fit, decide first whether you want a tight fit or a reveal. Cabinets and trim usually want 1/16 to 1/8 of clearance; decking wants a deliberate gap. Subtract it here rather than "taking a bit off" at the saw.
- A laser measure that reads decimal feet is not wrong, it is just speaking a different language. Convert once, at the start, rather than eyeballing it at every cut.
Frequently asked questions
How do I add feet and inches together?
Convert both to total inches, add them, then divide by 12 for feet with the remainder as inches. 8 ft 4-1/2 in plus 2 ft 7-3/8 in is 100.5 + 31.375 = 131.875 inches, which is 10 ft 11-7/8 in.
What is 10 feet 11 inches in decimal feet?
10.9167 decimal feet. Divide the inches by 12 and add them to the feet — never write 10.11, which is a different length entirely.
How do I subtract inches from feet?
Borrow 12 inches from the feet column, exactly like borrowing in normal subtraction. 6 ft 2 in minus 9 in becomes 5 ft 14 in minus 9 in = 5 ft 5 in.
What is a saw kerf and does it matter?
The kerf is the width of material the blade removes, typically 1/8 in on a circular saw and 3/32 in on a fine crosscut blade. On one cut it is irrelevant; on a 20-piece cut list it costs you 2-3/8 in, which is usually the last piece.
How many sixteenths are in an inch?
Sixteen. That is the finest graduation on a standard tape measure, which is why results here are rounded to the nearest 1/16 rather than to a decimal you cannot find on the tape.