Convert Feet and Fractional Inches to Millimeters
An inch is 25.4 mm and a foot is 304.8 mm, both exact. So millimetres = (feet × 304.8) + (inches × 25.4), and a fraction is just inches with a decimal point: 3/8" is 0.375 × 25.4 = 9.525 mm.
Open the feet & inches to millimeters calculator →Worked example
5' 7 3/8" = (5 × 304.8) + (7.375 × 25.4) = 1524 + 187.325 = 1711.325 mm, which on a drawing you would write as 1711 mm. Type 5 and 7 3/8 in the ft and in fields of the calculator.Conversion reference
| Fraction | Millimetres |
|---|---|
| 1/16" | 1.5875 mm |
| 1/8" | 3.175 mm |
| 3/16" | 4.7625 mm |
| 1/4" | 6.35 mm |
| 5/16" | 7.9375 mm |
| 3/8" | 9.525 mm |
| 7/16" | 11.1125 mm |
| 1/2" | 12.7 mm |
| 9/16" | 14.2875 mm |
| 5/8" | 15.875 mm |
| 11/16" | 17.4625 mm |
| 3/4" | 19.05 mm |
| 13/16" | 20.6375 mm |
| 7/8" | 22.225 mm |
| 15/16" | 23.8125 mm |
| 1" | 25.4 mm |
Feet
| Feet | Millimetres |
|---|---|
| 1' | 304.8 mm |
| 2' | 609.6 mm |
| 3' | 914.4 mm |
| 4' | 1219.2 mm |
| 5' | 1524 mm |
| 6' | 1828.8 mm |
| 8' | 2438.4 mm |
| 10' | 3048 mm |
| 20' | 6096 mm |
| 100' | 30480 mm |
Where imperial habits meet a metric supply chain
| Imperial dimension | What the metric market sells |
|---|---|
| 1/2" gypsum board (12.7 mm) | 12.5 mm · 0.2 mm gap |
| 5/8" gypsum board (15.875 mm) | 16 mm · 0.125 mm gap |
| 2x4 stud, actual 1 1/2" x 3 1/2" (38.1 x 88.9 mm) | 38 x 89 mm · 0.1 mm gap |
| 16" stud spacing (406.4 mm) | 400 mm · 6.4 mm gap |
| 24" stud spacing (609.6 mm) | 600 mm · 9.6 mm gap |
| 3'-0" door opening (914.4 mm) | 900 mm · 14.4 mm gap |
| 6'-8" door height (2032 mm) | 2040 mm leaf · 8 mm gap |
| 4' x 8' sheet (1219.2 x 2438.4 mm) | 1200 x 2400 mm · 19.2 / 38.4 mm gap |
| 8'-0" ceiling (2438.4 mm) | 2400 mm · 38.4 mm gap |
Read that table top to bottom and the pattern is hard to miss. Board thickness matches to a tenth of a millimetre and a stud is basically identical. By the time you are at sheet sizes and ceiling heights the gap is 20 to 40 mm, which is why dropping metric products into an imperial layout feels fine at the detail and falls apart at the grid.
Where to round
The arithmetic is exact, so anything wrong in a converted set came from a rounding call somebody made. Round building dimensions to the nearest millimetre: half a millimetre of error is inside any construction tolerance, and 1711.325 mm on a plan is not precision, it is clutter. Keep a decimal place for shop and setting-out work, where a fabricator is holding to it.
Rounding to the nearest 5 or 10 is a different thing entirely. That changes the building, and it needs to be a decision made across the whole set rather than one dimension at a time.
And do not round twice. Convert to metric, round, hand it on, someone converts back for a US supplier and rounds again, and a 3'-0" opening quietly becomes 2'-11 3/4". Always go back to the original imperial number.
Reading millimetres back as fractions
Inches = mm ÷ 25.4, and sixteenths = mm ÷ 1.5875. A millimetre is 0.03937", slightly under a sixteenth, so metric dimensions almost never land on a clean fraction. 2400 mm is 94.488", which you can call 7'-10 1/2" if you are willing to lose about a third of a millimetre. Sometimes you are. Say so rather than letting the number drift.
Most converters make you translate fractional inches to decimal first. Arch Convert takes 3 1/2 or 3/8 directly, rounds to 64ths, and carries inches to feet automatically. Working at a drawing scale rather than a dimension? See architectural scale factors. These tables are generated from conversion-tables, our open dataset.
Frequently asked
- How many millimetres are in 5 feet 3 1/2 inches?
- 5 feet 3 1/2 inches equals 1612.9 mm. The math: 5 ft x 304.8 mm/ft = 1524 mm, plus 3.5 in x 25.4 mm/in = 88.9 mm, total 1612.9 mm. In Arch Convert you can type 5 in the ft field and 3 1/2 in the in field of the Feet and Fractional Inches row, and the millimetres row updates instantly.
- How do I enter feet and fractional inches like 5' 3 1/2"?
- Use the Feet and Fractional Inches row. Type whole feet in the ft field (5) and a fractional or mixed inch value in the in field (3 1/2). Values round to 64ths and carry over automatically when inches reach 12, so 11 15/16" plus 1/16" becomes 1' 0". You can also use just a fraction (3/8) or a decimal (3.5).
- What is the conversion factor from feet to millimetres?
- 1 foot equals exactly 304.8 mm, derived from 1 inch = 25.4 mm times 12 inches per foot. For architectural drawings, hold the exact value through every step and round only on the final dimension shown on the sheet so tolerances do not stack.
- What is 1/16 of an inch in mm?
- 1.5875 mm, or 1.6 mm for practical purposes. A thirty-second is 0.79375 mm and a sixty-fourth is 0.396875 mm.
- Is 8 feet the same as 2400 mm?
- No. 8'-0" is 2438.4 mm. 2400 mm is a bit under 7'-10 1/2". That 38.4 mm difference is the usual reason a metric ceiling system does not fit an imperial shell, and it is why substituting metric products into an imperial layout works at the detail and fails at the grid.
- How many decimal places should a converted drawing carry?
- Whole millimetres for building work, since half a millimetre of error sits inside any construction tolerance and 1711.325 mm on a plan is clutter rather than precision. Keep one decimal for shop and setting-out dimensions, where a fabricator is actually holding to it.