mm to feet and inches converter: both ways, plus feet-and-inch sums
Published · Updated · By Sai Nath
Turn both lengths into inches (feet × 12 + inches), add or subtract, then split the answer back into feet and inches: 5' 7 3/4" + 2' 9 1/2" is 101 1/4", which is 8' 5 1/4". To convert, an inch is exactly 25.4 mm: 3000 mm is 9' 10 1/8", and 5' 8" is 172.72 cm.
Feet and inches calculator
Add or subtract feet, inches and fractions, or convert between feet and inches and millimetres, centimetres or metres. Conversions round the way the MeisterCalc app does.
This calculator needs JavaScript, which is turned off or didn't load. You can still do it by hand: inches = feet × 12 + inches, and an inch is 25.4 mm. The method and worked examples below give the same answers.
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How does the feet and inches calculator work?
It does feet-and-inch sums the way you would on paper, but without rounding until the end:
- To inches: inches = feet × 12 + inches + the fraction. 5' 7 3/4" is 60 + 7.75 = 67.75 in.
- Add or subtract the two totals in inches.
- Back to feet: divide by 12 and keep the whole number for the feet; what's left, under 12, is the inches.
- Round the fraction to the nearest sixteenth (or eighth, or thirty-second): multiply the decimal part of the inches by 16 and round. 16/16 carries into the inch, and 12 inches carries into the foot.
- Reduce: 2/16 reads 1/8, 8/16 reads 1/2. Under an inch, only the fraction shows: 3/16".
For metric, an inch is exactly 25.4 mm (NIST SP 811, Appendix B), so a foot is 304.8 mm: mm = inches × 25.4, and inches = mm ÷ 25.4. Steps 3 to 5 are how MeisterCalc's unit converter shows a millimetre length in feet and inches, at its default precision of 1/16". Going from feet and inches to millimetres adds no rounding of its own; the result is shown to one decimal place, as the app shows millimetres.
Worked example: adding feet and inches
- 5' 7 3/4" = 5 × 12 + 7.75 = 67.75 in.
- 2' 9 1/2" = 2 × 12 + 9.5 = 33.5 in.
- 67.75 + 33.5 = 101.25 in.
- 101.25 ÷ 12 = 8 whole feet (96 in), leaving 5.25 in, so the answer is 8' 5 1/4".
By hand you can add the parts instead: 5' + 2' = 7', and 7 3/4" + 9 1/2" = 17 1/4". 17 1/4" is more than 12", so carry a foot: 8' 5 1/4". That's 8.4375 ft, or 2571.75 mm.
Worked example: subtracting feet and inches
- 10' 0" = 120 in. 3' 4 5/8" = 40.625 in.
- 120 − 40.625 = 79.375 in.
- 79.375 ÷ 12 = 6 whole feet (72 in), leaving 7.375 in: 6' 7 3/8".
By hand, borrow a foot: 10' 0" is 9' 12", and 9' 12" − 3' 4 5/8" = 6' 7 3/8". If the second length is the longer one, the calculator shows the answer with a minus sign: 2' − 3' = -1' 0".
Worked example: 3000 mm to feet and inches
- Inches: 3000 ÷ 25.4 = 118.1102 in.
- Feet: 118.1102 ÷ 12 is 9 whole feet (108 in), leaving 10.1102 in.
- Sixteenths: 0.1102 × 16 = 1.76, which rounds to 2.
- 2/16 reduces to 1/8, so 3000 mm is 9' 10 1/8".
That is 9' 10 1/8" = 118.125 in = 3000.375 mm, so the rounded size is 0.38 mm longer than the 3000 mm you started with. The calculator shows this under "Rounding".
Worked example: 4800 mm, with a carry
- Inches: 4800 ÷ 25.4 = 188.9764 in.
- Feet: 15 whole feet (180 in), leaving 8.9764 in.
- Sixteenths: 0.9764 × 16 = 15.62, which rounds to 16.
- 16 sixteenths is a full inch: 8 in becomes 9 in and the fraction drops off. The answer is 15' 9".
The same carry moves into the feet. 304.7 mm is 11.9961 in, which rounds up to 12 in, so it shows 1' 0" and not 0' 12".
Worked example: feet and inches to mm and cm
- 7' 3 5/16": 7 × 12 + 3 = 87 in, plus 5/16 = 0.3125 in, so 87.3125 in.
- Millimetres: 87.3125 × 25.4 = 2217.7375, shown as 2217.7 mm.
Heights work the same way: 5' 8" is 68 in, and 68 × 2.54 = 172.72 cm. Going from imperial to millimetres is exact to the figure you typed. A sixteenth of an inch is 25.4 ÷ 16 = 1.5875 mm.
Common sizes in feet, inches and sixteenths
Each millimetre size is converted and rounded to the nearest 1/16". The last column is the rounded size minus the size in the first column: it is never more than 0.79 mm either way, half of a 1.5875 mm sixteenth. The table says what the millimetres come to, not what a product is called.
| mm | Feet, inches, fraction (nearest 1/16") | Decimal inches | Rounding (mm) |
|---|---|---|---|
| 3 | 1/8" | 0.1181 | +0.17 |
| 6 | 1/4" | 0.2362 | +0.35 |
| 9 | 3/8" | 0.3543 | +0.52 |
| 12 | 1/2" | 0.4724 | +0.70 |
| 16 | 5/8" | 0.6299 | -0.13 |
| 19 | 3/4" | 0.7480 | +0.05 |
| 25 | 1" | 0.9843 | +0.40 |
| 32 | 1 1/4" | 1.2598 | -0.25 |
| 38 | 1 1/2" | 1.4961 | +0.10 |
| 45 | 1 3/4" | 1.7717 | -0.55 |
| 50 | 1 15/16" | 1.9685 | -0.79 |
| 70 | 2 3/4" | 2.7559 | -0.15 |
| 90 | 3 9/16" | 3.5433 | +0.49 |
| 100 | 3 15/16" | 3.9370 | +0.01 |
| 140 | 5 1/2" | 5.5118 | -0.30 |
| 190 | 7 1/2" | 7.4803 | +0.50 |
| 200 | 7 7/8" | 7.8740 | +0.02 |
| 300 | 11 13/16" | 11.8110 | +0.04 |
| 450 | 1' 5 11/16" | 17.7165 | -0.74 |
| 600 | 1' 11 5/8" | 23.6220 | +0.07 |
| 900 | 2' 11 7/16" | 35.4331 | +0.11 |
| 1000 | 3' 3 3/8" | 39.3701 | +0.13 |
| 1200 | 3' 11 1/4" | 47.2441 | +0.15 |
| 1800 | 5' 10 7/8" | 70.8661 | +0.22 |
| 2400 | 7' 10 1/2" | 94.4882 | +0.30 |
| 2440 | 8' 0 1/16" | 96.0630 | -0.01 |
| 2700 | 8' 10 5/16" | 106.2992 | +0.34 |
| 3000 | 9' 10 1/8" | 118.1102 | +0.38 |
| 3600 | 11' 9 3/4" | 141.7323 | +0.45 |
| 4800 | 15' 9" | 188.9764 | +0.60 |
Common imperial sizes in millimetres
Imperial to millimetres is exact, so these are shown in full, up to three decimal places. The converter above shows one, as the app does.
| Feet, inches, fraction | Millimetres |
|---|---|
| 1/8" | 3.175 |
| 1/4" | 6.35 |
| 3/8" | 9.525 |
| 1/2" | 12.7 |
| 5/8" | 15.875 |
| 3/4" | 19.05 |
| 1" | 25.4 |
| 1 1/2" | 38.1 |
| 2" | 50.8 |
| 3 1/2" | 88.9 |
| 5 1/2" | 139.7 |
| 7 1/4" | 184.15 |
| 11 7/8" | 301.625 |
| 1' 0" | 304.8 |
| 2' 0" | 609.6 |
| 4' 0" | 1219.2 |
| 8' 0" | 2438.4 |
Imperial sizes convert exactly: 1 ft is 304.8 mm and 8 ft is 2438.4 mm. A sheet sold as 2400 mm is not 8 feet, it is 7' 10 1/2". 2440 mm is 8' 0 1/16", a hair over.
How accurate is a sixteenth?
Rounding to the nearest sixteenth can be out by up to half a sixteenth: 1/32", or 0.79 mm. For framing and setting out that is finer than most tape reading. For a tight fit between two parts, stay in millimetres. Each conversion carries its own rounding, so convert the total, not the parts: ten lengths of 450 mm are each 17 11/16", which adds to 176 7/8", but the 4500 mm total is 177 3/16", about 8 mm more. The add-and-subtract mode avoids this by adding the exact inches and rounding only the answer.
Questions people ask
How do I add feet and inches?
Add the feet, add the inches, and carry 12 inches into a foot: 5' 7 3/4" + 2' 9 1/2" = 7' 17 1/4" = 8' 5 1/4". Or turn both into inches first, as the calculator does.
How do I convert decimal feet to feet and inches?
Keep the whole feet and multiply the decimal part by 12: 8.4375 ft is 8 ft and 0.4375 × 12 = 5.25 in, so 8' 5 1/4".
How many mm in an inch?
25.4, exactly. A foot is exactly 304.8 mm.
What is 1000 mm in feet and inches?
1000 ÷ 25.4 = 39.3701 in, which is 3' 3 3/8" to the nearest sixteenth.
How many cm is 5 feet 8 inches?
68 in × 2.54 = 172.72 cm.
What this calculator doesn't do
- Two lengths at a time for adding and subtracting. It doesn't multiply or divide lengths, or work out areas.
- Inputs can't be negative; only a subtraction can give a negative answer.
- For a negative millimetre length the app's own display isn't meaningful (it reads -25.4 mm as 11"), so this calculator deliberately differs there: it puts a minus sign in front of the size.
- Lengths over 1000 km are refused.
- It doesn't say what any building rule allows. Sizes and tolerances come from the drawings, the building code and your council.
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Feet and inches in MeisterCalc
MeisterCalc is metric first. Settings ▸ Units ▸ Default units ▸ Imperial (ft/in) turns every calculator over to feet, inches and fractions, with Fraction display and Fraction precision settings. It keeps every value in millimetres inside and converts at display, as above. Since version 1.1 it also has a Feet & Inches calculator that adds and subtracts feet, inches and fractions and converts them to mm, cm or m. It runs on Android 7.0 or later, works offline, and three of its calculators (Concrete, Stairs and Check Square) are free; the other 13 unlock with a weekly subscription that starts with a 3-day free trial, or a one-time lifetime unlock, with Google Play showing the prices in your currency. For the long-hand method, see how to convert millimetres to feet, inches and sixteenths.