How to type it
- Feet and inches: 5' 7 3/8" or 5 ft 7 3/8 in. A number right after feet counts as inches, so 5' 7 works too.
- Inches only: 7 3/8" or 0.375". A fraction is one number, a slash and another number, with no spaces.
- Metric: 300 mm, 30 cm, 2.5 m. Yards work too, as 1 yd.
- Signs: + and − as usual, x or × to multiply, ÷ to divide. A slash with a space on each side, like 96" / 16", also divides.
- Plain numbers have no unit, like the 4 in 10' 6" ÷ 4. Add one to a length and it's read as inches.
Worked examples
- Two boards end to end:
5' 7 3/8" + 2' 4 1/2"= 7′ 11 7/8″. - Cutting a bit off:
16' − 3/4"= 15′ 11 1/4″. - Splitting a wall into four equal bays:
10' 6" ÷ 4= 2′ 7 1/2″. - Four pieces of the same length:
4 x 2' 6"= 10′ 0″. - How many 16″ spaces fit in 16 feet:
16' ÷ 16"= 12. - A floor area:
12' x 14'= 168 ft². - A slab, 4 inches thick:
10' x 10' x 4"= ≈ 33.33 ft³. The calculator also shows it in cubic yards. - Mixing metric and imperial:
300 mm + 2' 3"= ≈ 3′ 2 13/16″. The ≈ means it was rounded to the nearest 1/16″.
Multiplying two lengths gives an area, and three give a volume. Dividing a length by a length gives a plain count. Adding a length to an area doesn't mean anything, so the calculator tells you instead of guessing.
Adding fractions of an inch by hand
Put both fractions over the same bottom number first. For 7 3/8″ + 4 1/2″, sixteenths work: 3/8 is 6/16 and 1/2 is 8/16. Add the whole inches and the sixteenths separately to get 11 14/16″, then reduce 14/16 to 7/8. That's 11 7/8″.
With feet, carry over every 12 inches. In the first example, 7 3/8″ and 4 1/2″ make 11 7/8″, which is under a foot, so the feet just add up: 5′ and 2′ make 7′, and the answer is 7′ 11 7/8″. If the inches had come to 13 1/4″, you'd keep 1 1/4″ and carry 1 foot over to the feet.
How the rounding works
Every step is done with exact fractions, so a long chain of sums doesn't drift. Only the answer is rounded, to the nearest mark you pick in Round to, and a mark exactly halfway rounds away from zero. When the answer had to be rounded, you'll see ≈ in front of it and the exact decimal under it. For a table of every 64th with its decimal and millimeters, see the inch fraction chart.
Questions
How do I convert decimal feet to feet and inches?
Type it with a foot mark, like 5.75', and the answer shows 5.75' = 5′ 9″. By hand, multiply the decimal part by 12: 0.75 × 12 is 9 inches.
How do I convert millimeters to feet and inches?
Type the millimeters with mm after them, like 2400 mm, and you'll get 2400 mm = ≈ 7′ 10 1/2″. The exact decimal shows under it.
Why does my answer have a ≈ sign?
The exact answer falls between two marks at the precision you picked, so it was rounded to the nearer one. Pick a finer mark in Round to, like 1/32″, to get closer.
Can I use it for square feet?
Yes. Multiply two lengths, like 12' x 14', and the answer is in square feet, with square inches and square meters under it.
Is there an app for this?
Yes. My Construction Calculator for Android uses the same rules on a keypad made for feet, inches and fractions, and keeps a tape of your past answers.
Construction Calculator
Feet, inches and fractions with exact math, rounded to the mark you choose.