Just need the answer? The free feet and inches calculator takes measurements the way you read them off the tape, like 12' 4 3/8", and answers to the nearest sixteenth.
The one rule that trips people up
Everything else in tape math is ordinary arithmetic. The catch is that inches carry at 12, not 10. Seventeen inches is 1 foot 5 inches. And fractions of an inch carry at their own bottom number: 9/8 of an inch is 1 1/8 inches.
So the method is always the same. Add (or subtract) each part on its own, feet with feet, inches with inches, fractions with fractions. Then carry from the smallest part up.
Adding: a worked example
Two pieces of trim, 5' 7 3/8" and 3' 9 3/4". How long together?
- Fractions first. 3/8 and 3/4 need the same bottom number. 3/4 is 6/8, so 3/8 + 6/8 = 9/8, which is 1 1/8.
- Inches. 7 + 9 = 16, plus the 1 carried from the fractions makes 17 1/8.
- Feet. 5 + 3 = 8.
- Carry the inches. 17 1/8 inches is 1 foot and 5 1/8 inches, so the total is 9' 5 1/8".
If finding a common bottom number is the slow part, count in sixteenths. Every mark on a tape is some number of sixteenths: 3/8 is 6/16 and 3/4 is 12/16. The fraction to decimal chart lists them all, with millimeters.
Subtracting, and borrowing a foot
Cutting 4' 7 5/8" off a 10' 2 1/4" board. What's left?
The inches on top (2 1/4) are smaller than the inches you're taking away (7 5/8), so borrow a foot: 10' 2 1/4" is the same as 9' 14 1/4". Now subtract. 14 1/4 minus 7 5/8 is 14 2/8 minus 7 5/8, which is 6 5/8. And 9 minus 4 is 5. What's left is 5' 6 5/8", before the saw takes its share.
That's the piece people forget. A saw blade turns a strip of wood into dust, often around 1/8" wide. Cut a 12' board into three pieces and the two cuts take about 1/4" between them. Three pieces can't each be 4' long.
Dividing into equal parts
For spacing balusters, shelves or fence boards, turn everything into inches first. Division is much easier on one unit.
Four equal spaces across 10' 3": that's 123 inches, and 123 / 4 is 30.75, or 30 3/4" each (2' 6 3/4").
It doesn't always come out even. Five equal parts of 15' 7" is 187 / 5 = 37.4 inches. A tape doesn't have tenths, so round to the nearest sixteenth: 0.4 of an inch is 6.4 sixteenths, call it 6/16, which is 3/8. Each part is about 3' 1 3/8". Mark them from the same end, adding as you go, so the rounding doesn't stack up.
Decimal feet, and back again
Plans, lumber yards and some calculators work in decimal feet. To get there, divide the inches by 12. 6" is 0.5', 3" is 0.25', and 9' 5 1/8" is 9 plus 5.125 / 12, about 9.427'.
Going the other way, multiply the decimal part by 12 to get inches, then multiply what's left of that by 16 to get sixteenths. 0.43' is 5.16 inches, and 0.16 × 16 is about 2.6, so call it 3/16: 5 3/16".
For metric, one inch is exactly 25.4 mm.
The mistakes that cost a board
- Reading decimal feet as feet and inches. 5.6' is not 5' 6". It's about 5' 7 3/16". The difference is over an inch.
- Carrying at 10. A calculator that doesn't know about feet will happily tell you 5.7 + 3.9 = 9.6, and that number means nothing on a tape.
- Adding the fraction tops and bottoms. 3/8 + 3/4 is not 6/12. Get a common bottom number first.
- Forgetting the kerf. Every cut eats a blade's width.
- Rounding at every step. Keep the exact fractions until the end and round once.
On a job site
Handyman Calc, our app, has the same feet-and-inches keypad, plus calculators for concrete, drywall, decking, stairs, rafters, paint and tile. It keeps exact fractions the whole way through, so it never rounds until the answer. It is free on the App Store and Google Play.
Frequently asked questions
How do you add feet and inches together?
Add the feet, add the inches, and add the fractions. Then tidy up: a fraction over one becomes an inch, and every 12 inches becomes a foot. 5' 7 3/8" plus 3' 9 3/4" is 8' 17 1/8", which is 9' 5 1/8".
How do you convert inches to decimal feet?
Divide the inches by 12. 6 inches is 0.5 feet, 3 inches is 0.25 feet, and 5 1/8 inches is about 0.427 feet. Turn the fraction into a decimal first: 1/8 is 0.125.
Is 5.6 feet the same as 5 feet 6 inches?
No. 5.6 feet is 5 feet and six tenths of a foot, which is 7.2 inches, so about 5' 7 3/16". 5 feet 6 inches is 5.5 feet.
How do you divide a length into equal parts?
Turn it all into inches, divide, and turn it back. 10' 3" is 123 inches, so four equal spaces are 30 3/4" each. Round to the nearest sixteenth when it doesn't come out even.