Length Adding
Add feet and inches.
About the Length Adding
Add measurements in feet and inches without converting everything to decimals and back, which is where mistakes creep in.
How to use it
- Enter each length in feet and inches.
- Add as many as you need.
- The total comes back in feet and inches.
The formula
Feet and inches are base-12 arithmetic. Add the inches, carry a foot every time you reach twelve, and the awkwardness disappears.
Total inches = (Feet × 12) + Inches
Result = Total ÷ 12 as feet, remainder as inches
So 3 ft 8 in plus 2 ft 9 in is 44 + 33 = 77 inches, which is 6 feet with 5 left over: 6 ft 5 in. Converting everything to inches first, adding, then converting back is more reliable than carrying as you go, and it is exactly how this calculator works.
Fractional inches follow the same principle with a common denominator. Sixteenths handle almost all construction work: 3/8 becomes 6/16, so 6/16 + 5/16 = 11/16 with no conversion between denominators required.
Worked examples
| First length | Second length | In inches | Total |
|---|---|---|---|
| 3 ft 8 in | 2 ft 9 in | 44 + 33 = 77 | 6 ft 5 in |
| 5 ft 6 in | 5 ft 6 in | 66 + 66 = 132 | 11 ft 0 in |
| 8 ft 11 in | 1 ft 1 in | 107 + 13 = 120 | 10 ft 0 in |
| 2 ft 7½ in | 3 ft 9½ in | 31.5 + 45.5 = 77 | 6 ft 5 in |
Rows three and four are worth noticing for different reasons. The third lands exactly on ten feet, which is the sort of clean result that makes you suspect an error and is simply how the carry worked out. The fourth reaches the same total as the first through halves — a useful check that fractional entry is behaving, since two halves should always resolve into a whole inch.
Common mistakes
- Treating inches as decimals of a foot. 6 inches is 0.5 feet, not 0.6. Adding 3.8 and 2.9 as decimals gives 6.7 feet, which is neither 6 ft 5 in nor anything else useful.
- Forgetting to carry at twelve. 8 in plus 9 in is 17 in, which is 1 ft 5 in. Leaving it as 17 inches inside a feet-and-inches answer is the most common slip.
- Mixing fractional denominators. You cannot add 3/8 to 5/16 directly. Convert to sixteenths first — 6/16 + 5/16 = 11/16.
- Ignoring the width of the cut. A saw blade removes roughly 1/8 inch per cut. Across several pieces from one board the kerf adds up, and a run measured without it comes out short.
Terms explained
- Foot
- Twelve inches. The carry point in all of this arithmetic.
- Sixteenth
- The standard fine division on a construction tape. Finer than most work requires and coarse enough to read quickly.
- Kerf
- The material a saw blade removes, typically about 1/8 inch. Must be accounted for when cutting multiple pieces from one length.
- Running measurement
- Marking cumulatively from one end rather than piece to piece, so small errors do not accumulate.
- On centre
- Spacing measured from the centre of one member to the centre of the next. Standard for studs and joists.
- Linear feet
- Length only, disregarding width or thickness. How trim and lumber are usually sold.
Common questions
- How do I add feet and inches?
- Convert both to inches, add, then divide by twelve. The quotient is feet and the remainder is inches — 44 + 33 = 77 inches, or 6 ft 5 in.
- Why can I not just add them as decimals?
- Because inches are twelfths, not tenths. 3 ft 8 in is 3.667 feet, not 3.8, so decimal addition gives an answer that is simply wrong.
- How do I add fractional inches?
- Put them over a common denominator, usually sixteenths. 3/8 is 6/16, so adding 5/16 gives 11/16. Carry a whole inch whenever the total reaches 16/16.
- How do I convert inches to decimal feet?
- Divide by twelve. 8 inches is 0.667 feet. This is the form you want for area calculations and for most spreadsheets.
- What is kerf and why does it matter?
- The width of material a blade removes, about 1/8 inch. Cutting five pieces from one board loses over half an inch to sawdust, which is enough to leave the last piece short.
- Should I measure each piece separately or cumulatively?
- Cumulatively, from a single reference point, whenever pieces must line up. Measuring piece to piece lets each small error compound into the next.
- How do I convert feet and inches to metric?
- One inch is exactly 25.4 mm. Convert the whole length to inches, then multiply — 77 inches is 1,955.8 mm.
- Why do tapes mark sixteenths rather than tenths?
- Because halving is easy and repeatable: a half, a quarter, an eighth, a sixteenth. Finding a tenth of an inch by eye is far harder than folding a distance in half twice.