This construction calculator adds, subtracts, multiplies and divides feet, inches and fractions: type 5' 7-3/8" + 3' 9-3/4" and it returns 9' 5-1/8", plus decimal feet, inches and millimeters. It reads tape-measure notation, decimal feet and metric in the same box, rounds to the nearest 1/16" (or 1/8, 1/32, 1/64), and has area, volume and rafter modes for quick takeoffs.
It is built for carpenters, framers, trim and cabinet makers, remodelers, architects checking drawings and DIYers who don't own a dedicated feet-inch calculator. It works as a free carpenter calculator, builder's calculator or architectural calculator online: nothing is installed, and it runs on a phone at the job site.
How to use the feet-inch-fraction calculator
- Pick a mode: math for + − × ÷, area for square feet, volume for cubic yards, or rafter for a sloped length from run and pitch.
- Type each measurement the way you'd write it:
5' 7-3/8",12 ft 6 in,3/4",2.5 ftor1.2 m. Put larger units first. - For × or ÷, type a plain number (no unit) to scale a length, or a second length to get an area (×) or a count (÷).
- Choose the fraction precision. 1/16" is the default; the "rounding" line shows how far the rounded fraction is from the exact answer in millimeters.
- Copy the answer or use "Copy link" to share the exact calculation; the inputs are saved in the URL.
What you can type
The parser accepts feet as ', ft, foot or feet; inches as ", '', in, inch or inches; and yd, m, cm and mm. Fractions can be joined with a dash or a space (7-3/8 or 7 3/8). A number after feet with no unit counts as inches, matching how people write 5'7. A minus sign in front makes a negative length.
| You type | Read as (inches) | Feet-inches at 1/16" | Decimal feet | mm |
|---|---|---|---|---|
5' 7-3/8" | 67.375 | 5' 7-3/8" | 5.6146 | 1,711.3 |
12 ft 6 in | 150 | 12' 6" | 12.5 | 3,810 |
3/4" | 0.75 | 3/4" | 0.0625 | 19 |
2.5 ft | 30 | 2' 6" | 2.5 | 762 |
1.2 m | 47.2441 | 3' 11-1/4" | 3.937 | 1,200 |
5'7 | 67 | 5' 7" | 5.5833 | 1,701.8 |
1-1/2" | 1.5 | 1-1/2" | 0.125 | 38.1 |
30 cm | 11.811 | 11-13/16" | 0.9843 | 300 |
150 mm | 5.9055 | 5-7/8" | 0.4921 | 150 |
1 yd 2 ft | 60 | 5' 0" | 5 | 1,524 |
Entries the calculator rejects on purpose: units in the wrong order (3 in 5 ft), a number after inches with no unit (5 in 3), a zero denominator (3/0) and units it doesn't know (5 kg). It tells you which box it couldn't read instead of guessing.
How the math works
Every entry is converted to inches first: feet × 12, yards × 36, and metric through the exact definition 1 inch = 25.4 mm, which NIST's Office of Weights and Measures states has applied since the yard was redefined on July 1, 1959. Addition and subtraction work on inches. Multiplying two lengths gives square inches, divided by 144 for square feet and 1,296 for square yards. Volume divides cubic inches by 1,728 for cubic feet and 46,656 for cubic yards (27 cubic feet per cubic yard, NIST Handbook 44 Appendix C).
The answer is rounded only at the end: inches × 16 (or your chosen denominator) is rounded to the nearest whole number, split into feet, whole inches and a fraction, and the fraction is reduced, so 8/16 shows as 1/2. Because rounding happens once, chaining results through the share link doesn't pile up rounding error.
Worked examples
- Add: 5' 7-3/8" + 3' 9-3/4" = 67.375 + 45.75 = 113.125 in = 9' 5-1/8" (9.4271 ft).
- Subtract: a 12' 6" wall minus a 5' 7-3/8" opening leaves 6' 10-5/8".
- Divide by a number: 8' ÷ 3 = 32 in, which is 2' 8" at 1/16" and 2' 8" at 1/32".
- Length ÷ length: 12' ÷ 16" = 9, so nine 16-inch spaces fill 12 feet exactly.
- Area: 12' 6" × 10' 3" = 128.125 sq ft = 14.24 sq yd.
- Volume: a 20 × 12 ft pad 4" deep = 80 cu ft = 2.96 cu yd.
- Rafter: 12' of run at 6/12 rises 6' 0"; the line length is 13' 5".
Sixteenths of an inch in decimals and millimeters
Tape measures are marked in sixteenths. This table shows the exact decimal and metric value of each one, useful when a drawing gives decimals or millimeters and you need the tape mark.
| Fraction | Decimal inch | mm |
|---|---|---|
| 1/16" | 0.0625 | 1.588 |
| 1/8" | 0.125 | 3.175 |
| 3/16" | 0.1875 | 4.762 |
| 1/4" | 0.25 | 6.35 |
| 5/16" | 0.3125 | 7.938 |
| 3/8" | 0.375 | 9.525 |
| 7/16" | 0.4375 | 11.112 |
| 1/2" | 0.5 | 12.7 |
| 9/16" | 0.5625 | 14.288 |
| 5/8" | 0.625 | 15.875 |
| 11/16" | 0.6875 | 17.463 |
| 3/4" | 0.75 | 19.05 |
| 13/16" | 0.8125 | 20.638 |
| 7/8" | 0.875 | 22.225 |
| 15/16" | 0.9375 | 23.813 |
Decimal feet to feet and inches
Surveys, site plans and many estimating programs use decimal feet. Multiply the decimal part by 12 to get inches. The decimal feet to feet and inches converter does just this conversion; here are common values:
| Decimal feet | Feet-inches (1/16") | Inches |
|---|---|---|
| 0.1 ft | 1-3/16" | 1.2 |
| 0.25 ft | 3" | 3 |
| 0.33 ft | 3-15/16" | 3.96 |
| 0.5 ft | 6" | 6 |
| 0.75 ft | 9" | 9 |
| 1.5 ft | 1' 6" | 18 |
| 2.25 ft | 2' 3" | 27 |
| 3.6 ft | 3' 7-3/16" | 43.2 |
| 7.83 ft | 7' 9-15/16" | 93.96 |
| 10.4 ft | 10' 4-13/16" | 124.8 |
Rafter and diagonal helper
Pitch is inches of rise per 12 inches of run. The helper finds the rise (run × pitch ÷ 12) and the sloped line length with the Pythagorean theorem. It doesn't include overhang, the ridge-board deduction or the birdsmouth seat cut; add those from your plan. For roof area and slope factors, see the roof pitch calculator.
| Pitch | Angle | Rise over 12' run | Rafter line length for 12' run | Length per foot of run |
|---|---|---|---|---|
| 3/12 | 14.04° | 3' 0" | 12' 4-7/16" | 12.369 in |
| 4/12 | 18.43° | 4' 0" | 12' 7-13/16" | 12.649 in |
| 5/12 | 22.62° | 5' 0" | 13' 0" | 13 in |
| 6/12 | 26.57° | 6' 0" | 13' 5" | 13.416 in |
| 7/12 | 30.26° | 7' 0" | 13' 10-11/16" | 13.892 in |
| 8/12 | 33.69° | 8' 0" | 14' 5-1/16" | 14.422 in |
| 9/12 | 36.87° | 9' 0" | 15' 0" | 15 in |
| 10/12 | 39.81° | 10' 0" | 15' 7-7/16" | 15.62 in |
| 12/12 | 45° | 12' 0" | 16' 11-5/8" | 16.971 in |
Layout math on the job
Most job-site arithmetic is one of four moves, and each maps to a mode here. Cut lists are subtraction: an opening minus two jack studs, or a wall length minus the corner blocking. Equal spacing is division by a number: the gap between balusters is the clear span minus the total width of the balusters, divided by the number of gaps (one more than the number of balusters). Layout counts are length ÷ length: a run divided by the on-center spacing tells you how many spaces fit, and the calculator shows the leftover so you can see whether the last bay is short. Takeoffs are multiplication: two lengths give square feet for drywall, sheathing or flooring, and three give cubic yards for concrete or gravel.
Because each result shows feet-inches, decimal feet and millimeters together, it also works as a quick bridge between a metric product data sheet and an imperial plan. Type 600 mm and read 1' 11-5/8"; type 2400 mm and read 7' 10-1/2". For a single conversion with a ruler view, the mm to inches converter goes to 1/64".
When you copy a result into another calculation, copy the decimal-inch value rather than the rounded fraction if the next step multiplies it: a 1/32" rounding error multiplied by 20 pieces becomes 5/8".
Common mistakes
- Treating 5.6 ft as 5' 6". 5.6 ft is 5' 7-3/16". Decimal feet are tenths of a foot, not inches.
- Rounding at every step. Rounding each piece to 1/16 before adding lets the small errors pile up over a long run. Add exact values, then round once.
- Forgetting the kerf. Dividing a board into equal parts ignores the kerf (the width of wood the blade removes) on every cut; subtract the kerfs before dividing.
- Multiplying two lengths and reading feet. Length × length is an area. The calculator labels it in square feet so it isn't mistaken for a length.
- Using 25 mm per inch. The exact factor is 25.4. Over 10 ft the shortcut is off by 48 mm.
Related calculators
Convert a whole lumber list to board feet, total trim with the linear feet calculator, order fill or concrete with the cubic yards calculator, or convert metric hardware sizes with mm to inches.
Frequently asked questions
How do I add feet and inches with fractions?
Convert both to inches, add, then turn the total back into feet and inches. 5' 7-3/8" is 67.375 in and 3' 9-3/4" is 45.75 in, so the sum is 113.125 in, which is 9' 5-1/8". The calculator does this for any mix of feet, inches, fractions and metric.
How do I type 5 feet 7 and 3/8 inches?
Any of 5' 7-3/8", 5'7 3/8, 5 ft 7 3/8 in or 5 feet 7-3/8 inches works. A number typed after feet with no unit is read as inches, so 5'7 means 5' 7".
What does a number with no unit mean?
By default a bare number is inches, so 7 means 7". Switch "Number with no unit means" to feet if you work in decimal feet. In × and ÷ a bare number is a multiplier: 8' ÷ 3 = 2' 8".
How do I divide a board into equal parts?
Divide the length by the number of pieces. An 8 ft board split into 3 equal parts is 2' 8" each to the nearest 1/16 (exactly 32 in), before you allow for saw kerf.
How many 16-inch spaces fit in 12 feet?
Divide one length by the other: 12' ÷ 16" = 9. Length ÷ length gives a count, and the calculator also shows whole pieces and what's left over.
How do I convert decimal feet to feet and inches?
Multiply the decimal part by 12 for inches. 7.83 ft is 7 ft plus 0.83 × 12 = 9.96 in, which rounds to 7' 9-15/16" at 1/16".
Is the metric conversion exact?
Yes. NIST defines the inch as exactly 25.4 mm, so 1 ft is exactly 304.8 mm. 1.2 m is 47.2441 in, or 3' 11-1/4" to the nearest 1/16.
How do I find the square footage of a room in feet and inches?
Multiply length by width. A 12' 6" × 10' 3" room is 128.125 sq ft, or 14.24 sq yd. Choose the area mode, or multiply two lengths in math mode.
How many cubic yards is a 20 × 12 ft slab 4 inches thick?
240 sq ft × 4/12 ft = 80 cu ft, divided by 27 = 2.96 cu yd before waste.
How long is a rafter on a 6/12 roof?
For 12 ft of run, a 6/12 pitch rises 6' 0", so the line length is √(144² + 72²) inches = 13' 5" (26.57°). Add overhang and subtract half the ridge board yourself.
Which fraction should I round to?
1/16" is standard for framing and trim on a tape measure. Use 1/32" or 1/64" for cabinet and machine work, and 1/8" for rough work. Rounding 1/3" to 1/16 gives 5/16", off by -0.53 mm.
Sources & method
- NIST Office of Weights and Measures — SI Units: Length (the inch is exactly 25.4 mm)
- NIST Handbook 44 (2026), Appendix C — General tables of units of measurement (foot, yard, square and cubic yard)
Results are estimates for general information. Found an error? It helps everyone — see our methodology.