Construction Calculator: Feet, Inches & Fractions
A free construction calculator that works in feet, inches and fractions the way a tape measure does. Add and subtract lengths, multiply two lengths for square feet, add a thickness for cubic yards of concrete. No app, no subscription, no sign-up.
Try it: the screen already holds a room, 12' 6" × 14' 3". Change a number and the square feet update as you type.
How to use the construction calculator
Type a length the way you would say it: 5' 6, 4' 3-7/8", 7 1/2 in, 1.5 m. A bare number after feet is read as inches, and a fraction can sit next to the whole number with a space or a dash. Then put +, -, x or / between lengths. The answer updates as you type. Press = (or Enter) to keep the line on the tape below, where you can tap it to use it again.
On a phone the keypad saves you from hunting for the ' and " keys: tap Feet after the feet, Inch after the inches, and space between a whole number and its fraction.
Examples (tap one to try it)
- 8' − 2 3/8" = 7' 9-5/8": a stud cut from an 8-footer.
- 16' 4 5/8" ÷ 3 = 5' 5-9/16": three equal spaces on a wall.
- 12' 6" × 14' 3" = 178.13 sq ft: flooring for a room.
- 10' × 12' × 4" = 1.48 cu yd (40 cu ft, about 67 bags of 80 lb concrete): a shed slab.
Feet and inches calculator with area and volume
The calculator keeps track of what kind of number you have. Length plus length is a length, shown in feet, inches and the nearest fraction, along with decimal inches, decimal feet and metric. Length times length becomes an area in square feet, square yards, square meters and roofing squares. Multiply by a third length and you get a volume in cubic yards and cubic feet, plus board feet and an estimate of 80 lb concrete bags. Divide a length by a length and you get a plain number, handy for "how many 16-inch spaces fit in this wall".
Fractions round to the nearest 1/16" by default. Pick 1/8" for rough framing or 1/32" for finish work in the "Round to" menu. The decimal rows are not rounded.
Why not just use the app?
The handheld construction calculators are great, but their phone apps now cost around $40 a year, and the reviews are full of people who only needed one calculation. This page does the everyday part: feet-inch-fraction math, area, volume and unit conversion. It runs in your browser, so once it has loaded it keeps working with no signal. Nothing you type leaves your device.
More construction calculators
- Rafter calculator: rafter length, ridge deduction, overhang, plumb cut angle and birdsmouth from span and pitch.
- Stair calculator: number of steps, riser height, total run and stringer length from the floor-to-floor height.
FAQ
How do I add feet and inches?
Add the inches first. If they come to 12 or more, carry 12 inches as one foot. 5' 8" + 3' 7" is 8' 15", which is 9' 3". The calculator does the carrying for you: type 5' 8" + 3' 7".
How do I convert decimal feet to feet and inches?
Multiply the decimal part by 12. 7.35 ft is 7 ft plus 0.35 × 12 = 4.2 inches, about 7' 4-3/16". Type 7.35' to see it.
How many cubic yards of concrete do I need?
Multiply length × width × thickness and divide cubic feet by 27. A 10' × 12' slab, 4" thick, is 40 cu ft, or 1.48 cu yd. Order about 10% extra for spillage and an uneven base.
Does it work in metric?
Yes. Type mm, cm or m after a number, and you can mix them with feet and inches in one calculation. Every answer also shows millimeters, centimeters and meters.