Type 12' 6 1/2" + 3' 4 3/4" and the construction calculator returns 15' 11 1/4": each length is turned into inches (150.5 in and 40.75 in), added to 191.25 in and written back as feet, inches and a fraction rounded to 1/16 in.
How the construction calculator works
Result in inches = sum of (feet × 12 + inches + fraction), then ÷ 12 for feet and rounded to the nearest 1/16 in
You can type on a keyboard or tap the keypad. The apostrophe key means feet and the quote key means inches, so 12' 6 1/2" is 12 feet, 6 and a half inches. A fraction bar written without spaces (3/8) is a fraction; a slash with spaces around it, or the divide key, divides. A number that follows feet with no unit is read as inches, so 12' 6 is 12 ft 6 in. Words work too: 12 ft 6 in, 2.5 m, 30 cm, 800 mm, 1 yd 2 ft.
Multiplying builds a shape. One length times one length is an area, three lengths make a volume, and a length divided by a length is a plain number. Adding a length to a plain number is refused with a message, because 5 could mean 5 feet or 5 inches.
Feet-inch-fraction examples
- Add: 12' 6 1/2" + 3' 4 3/4" = 15' 11 1/4" (191.25 in).
- Subtract: 8' - 3' 7 5/8" = 4' 4 3/8", the board left after a cut.
- Divide: 10' 3" ÷ 4 = 2' 6 3/4" for four equal spaces.
- Multiply: 12' × 10' = 120 sq ft, and × 4" more gives 40 cu ft, which is 1.48 cubic yards. Check the yards with the cubic yard calculator.
Decimal feet and metric
Every answer also shows decimal feet, decimal inches, meters, centimeters, millimeters and yards. 15' 11 1/4" is 15.9375 ft or 4.8578 m. Switch the header toggle to metric and the big number is in meters, square meters or cubic meters; you can still type in feet and inches, and 2.5 m + 30 cm gives 2.8 m. For a plain conversion with several values, the feet and inches calculator has its own add, subtract and convert modes.
One-tap construction functions
Pitch
A 6/12 roof rises 6 in for every 12 in of run: 26.57 degrees, a 50% slope, and each foot of run takes 13.42 in of rafter (the multiplier is 1.118). The roof pitch calculator goes further with roof area.
Rise, run and diagonal
Give any two, get the third. A rise of 3' and a run of 4' give a diagonal of 5', the 3-4-5 check for a square corner.
Rafter
A 12' run at 6/12 needs a common rafter of 13' 5" to the ridge centre, or 14' 6 7/16" with a 1' level overhang. A hip or valley rafter for a square corner on that run is 18'. Work out the count and the board size with the rafter calculator.
Stairs
A total rise of 8' 9" (105 in) with a 10 in tread gives 14 risers of 7.5 in, 13 treads, a run of 10' 10" and a stringer 13' 11 1/8" long along the slope. The IRC caps the riser at 7 3/4 in and asks for a 10 in tread (R311.7.5). Lay out the cuts with the stair stringer calculator.
Circle
A 24" diameter has a circumference of 6' 3 3/8" and an area of 3.14 sq ft. Give the radius or the circumference instead if that is what you measured.
Area and volume
Length, width and an optional depth return the area, the perimeter and the volume in cubic feet and cubic yards, for example the 12' × 10' × 4" slab above.
Why 1/16 inch
Tape measures mark down to 1/16 in, and framing lumber and finish carpentry are cut to that. The calculator rounds only the displayed answer; it carries the exact value through the next step, so adding ten boards does not stack ten rounding errors. Pick 1/8 or 1/32 for a coarser or finer reading.
| Fraction | Decimal inches | Millimeters |
|---|---|---|
| 1/16" | 0.0625 in | 1.59 mm |
| 1/8" | 0.125 in | 3.18 mm |
| 1/4" | 0.25 in | 6.35 mm |
| 3/8" | 0.375 in | 9.53 mm |
| 1/2" | 0.5 in | 12.7 mm |
An inch is exactly 25.4 mm (NIST Handbook 44, Appendix C), so these are exact before rounding.
Construction calculator on your phone
The page works as a construction calculator app in the browser, with nothing to install: the keypad keeps the phone keyboard closed, the result stays under the display, and your last results are kept on the page while it is open. Save the page to your home screen if you use it every day.
What to work out next
Areas from the result go to the square footage calculator, long runs of board to the linear feet calculator, and the rest of the tools are listed under measuring and planning calculators.