How to use it
Enter the length and width in feet and inches. You get the floor area in square feet, the perimeter, and the diagonal to check that the corners are square. Add a height, or a slab thickness, to get the volume in cubic feet and cubic yards and the total wall area.
Worked example
A room 12' 6" by 10' 3" is 128.13 sq ft with a perimeter of 45' 6". The corner-to-corner diagonal is about 16' 2"; if both diagonals measure the same, the room is square. With 8' walls the volume is 1,025 cu ft and the four walls total 364 sq ft before deducting doors and windows.
What it is good for
- Flooring, tile and carpet: use the area, then add your waste allowance.
- Concrete: enter the slab thickness as the height, for example
4", and read the cubic yards. - Paint and drywall: use the wall area.
- Baseboard and trim: use the perimeter.
- Layout: use the diagonal to square a foundation or deck frame.
Typing measurements
Every box takes feet, inches and fractions the way you would write them: 9' 1 1/8", 7 1/2", 2.5' or 300mm. Put a space between whole inches and a fraction. A box also takes a whole calculation, so 14' 6" / 2 works anywhere.
Questions
How do I calculate square footage from feet and inches?
Convert each measurement to feet, then multiply. 12' 6" is 12.5 feet and 10' 3" is 10.25 feet, so the area is 128.13 square feet. The calculator does the conversion for you.
How do I work out cubic yards of concrete for a slab?
Multiply length by width by thickness in feet to get cubic feet, then divide by 27. Enter the thickness in the Height box, for example 4", and the cubic yards appear in the results.
How do I check that a room or foundation is square?
Measure both diagonals. If they match each other, the corners are square. The calculator shows what the diagonal should be for your length and width.
How precise is the answer?
The math is done at full precision and only the displayed fraction is rounded, to the nearest 1/16 inch by default. You can change that from 1/2 down to 1/64. When rounding changes the value, the answer is marked with ≈.
More calculators
- Feet & inches calculatorAdd, subtract, multiply and divide feet, inches and fractions. Free, nothing to install.
- Roof pitch & rafter length calculatorEnter any two of pitch, rise, run and diagonal. The other two fill in.
- Stair calculatorEnter the floor-to-floor rise to get risers, treads, total run and stringer length.
- Circle & arc calculatorEnter a diameter for circumference and area. Add an angle for arc length and chord.
- Stud & on-center spacing calculatorEnter a length and a spacing to get the member count, the last bay and the equal spacing.