How to use the construction calculator
- Type each length exactly as you read it: feet with ', inches with ", and fractions with a slash.
- Choose add or subtract for each line, and how many times it repeats (for example 2 identical rails).
- Divide or multiply the total if you need equal spacing or several runs.
- Pick the precision: 1/16" for most work.
How the calculator works
Every length is converted to inches: feet × 12 + inches + fraction.
Lines are added or subtracted, then multiplied or divided.
The result is turned back into feet (÷ 12), inches and the nearest fraction, reduced to lowest terms.
Worked example
10' 6 1/2" + 2 × 4' 3 3/4" − 1' 7 1/8"
- 10' 6 1/2" = 126.5 in. 4' 3 3/4" = 51.75 in, × 2 = 103.5 in. 1' 7 1/8" = 19.125 in.
- 126.5 + 103.5 − 19.125 = 210.875 in.
- 210.875 ÷ 12 = 17 ft with 6.875 in left = 17' 6 7/8" (17.573 ft, 5.356 m).
- Divided into 4 equal parts: 52.719 in = 4' 4 3/4" to the nearest 1/16".
Fractions of an inch as decimals
| Fraction | Decimal inch | Fraction | Decimal inch |
|---|---|---|---|
| 1/16 | 0.0625 | 9/16 | 0.5625 |
| 1/8 | 0.125 | 5/8 | 0.625 |
| 3/16 | 0.1875 | 11/16 | 0.6875 |
| 1/4 | 0.25 | 3/4 | 0.75 |
| 5/16 | 0.3125 | 13/16 | 0.8125 |
| 3/8 | 0.375 | 7/8 | 0.875 |
| 7/16 | 0.4375 | 15/16 | 0.9375 |
| 1/2 | 0.5 | 1 | 1.0 |
Inches to decimal feet
| Inches | Feet | Inches | Feet |
|---|---|---|---|
| 1 | 0.083 | 7 | 0.583 |
| 2 | 0.167 | 8 | 0.667 |
| 3 | 0.25 | 9 | 0.75 |
| 4 | 0.333 | 10 | 0.833 |
| 5 | 0.417 | 11 | 0.917 |
| 6 | 0.5 | 12 | 1.0 |
Common mistakes to avoid
- Treating 5.6 ft as 5' 6". 5.6 ft is 5' 7 3/16". Decimal feet are tenths, not inches.
- Forgetting the saw kerf. Each cut removes about 1/8"; leave room for it when dividing a board.
- Rounding each step. Round only the final answer to avoid adding up errors.
Frequently asked questions
How do I add feet and inches?
Add the feet and the inches separately, then carry every 12 inches into a foot. 10' 6 1/2" + 4' 3 3/4" = 14 feet and 10 1/4 inches (6 1/2 + 3 3/4 = 10 1/4), so 14' 10 1/4". When the inches pass 12, carry 1 foot.
How do I subtract fractions of an inch?
Convert to the same denominator: 7 1/8" − 2 3/4" = 7 2/16" − 2 12/16" = 4 6/16" = 4 3/8". Type both lengths with a minus on the second line.
How do I divide a length into equal parts?
Add the lengths, choose "Divide into equal parts" and enter how many. 17' 6 7/8" divided by 4 is 4' 4 23/32", or 4' 4 3/4" to the nearest sixteenth.
How do I convert inches to feet and inches?
Divide by 12: the whole number is feet and the remainder is inches. 63.5" = 5 feet with 3.5" left over, so 5' 3 1/2". Type 63.5 here and read the result.
How do I convert decimal feet to inches?
Multiply the decimal part by 12. 5.75 ft = 5 ft + 0.75 × 12 = 5' 9".
What formats can I type?
10' 6 1/2", 10ft 6 1/2in, 126 1/2", 126.5, 7/8, 10.54', 3.2 m, 85 cm and 600 mm. Numbers with no unit are read as inches unless you change it.