Millimeters to Fractional Inches
No whole millimetre equals a clean fraction of an inch, and that is not a rounding problem you can solve. One inch is exactly 25.4 mm, and fractions of an inch are repeated halvings, so they fall on 12.7, 6.35, 3.175 and 1.5875 mm. Whole millimetres never land there. Every answer below is the nearest thirty-second and the error you are accepting by using it.
Open the length calculator →Worked example
5 mm. Divide by 25.4 to get 0.19685 inches. Multiply by 32 to get 6.3, which rounds to 6, so the answer is 6/32 and that reduces to 3/16".
3/16" is 4.7625 mm. You asked for 5 mm, so you are 0.238 mm short. On one joint that is invisible. Across a twenty-course run it is nearly 5 mm, which is a course.
Conversion reference
| Millimetres | Nearest 1/32" (exact mm, error) |
|---|---|
| 1 mm | 1/32" · 0.794 mm (+0.206) |
| 2 mm | 3/32" · 2.381 mm (-0.381) |
| 3 mm | 1/8" · 3.175 mm (-0.175) |
| 4 mm | 5/32" · 3.969 mm (+0.031) |
| 5 mm | 3/16" · 4.762 mm (+0.238) |
| 6 mm | 1/4" · 6.350 mm (-0.350) |
| 7 mm | 9/32" · 7.144 mm (-0.144) |
| 8 mm | 5/16" · 7.938 mm (+0.062) |
| 9 mm | 11/32" · 8.731 mm (+0.269) |
| 10 mm | 13/32" · 10.319 mm (-0.319) |
| 11 mm | 7/16" · 11.112 mm (-0.112) |
| 12 mm | 15/32" · 11.906 mm (+0.094) |
| 13 mm | 1/2" · 12.700 mm (+0.300) |
| 14 mm | 9/16" · 14.287 mm (-0.287) |
| 15 mm | 19/32" · 15.081 mm (-0.081) |
| 16 mm | 5/8" · 15.875 mm (+0.125) |
| 17 mm | 21/32" · 16.669 mm (+0.331) |
| 18 mm | 23/32" · 18.256 mm (-0.256) |
| 19 mm | 3/4" · 19.050 mm (-0.050) |
| 20 mm | 25/32" · 19.844 mm (+0.156) |
| 21 mm | 13/16" · 20.637 mm (+0.363) |
| 22 mm | 7/8" · 22.225 mm (-0.225) |
| 23 mm | 29/32" · 23.019 mm (-0.019) |
| 24 mm | 15/16" · 23.812 mm (+0.188) |
| 25 mm | 31/32" · 24.606 mm (+0.394) |
The error column is the point of the table. It is not noise, it is the size of the decision you are making, and it does not behave the way you would expect. 4 mm converts to within 0.031 mm of 5/32". 5 mm, the very next value, is 0.238 mm out. Nearly eight times the error, one millimetre apart. The worst in the range below 10 mm is 2 mm, at 0.381 mm from 3/32". This is why a blanket "call it a quarter inch" habit fails unpredictably rather than consistently: the error does not grow with the number, it depends on where the value happens to fall between two halvings.
Fractional inches to millimetres, exact
| Fraction | Millimetres |
|---|---|
| 1/32" | 0.7937 mm |
| 1/16" | 1.587 mm |
| 1/8" | 3.175 mm |
| 3/16" | 4.762 mm |
| 1/4" | 6.35 mm |
| 5/16" | 7.938 mm |
| 3/8" | 9.525 mm |
| 1/2" | 12.7 mm |
| 5/8" | 15.88 mm |
| 3/4" | 19.05 mm |
| 7/8" | 22.22 mm |
| 1" | 25.4 mm |
Read that table in the other direction and the values are exact, because going from inches to millimetres is just multiplication by a defined constant. The rounding only appears going the other way, when you have to pick which fraction to land on. So if a drawing set has to end up in fractions, do the arithmetic in millimetres and round once, at the end. Rounding at each step compounds.
Arch Convert takes fractional input directly, so 5' 3 1/2" is a valid entry rather than something you decimalise first. See feet and inches to millimetres for the full-length case, and architectural scale factors for the drawing-scale version of the same imperial-metric gap.
Frequently asked
- What fraction of an inch is 5mm?
- None of them, exactly. 5 mm is 0.19685 inches. The nearest sixteenth is 3/16", which is 4.7625 mm, so it runs 0.238 mm small. The nearest thirty-second is the same 3/16". If the tolerance matters, work in millimetres and convert once at the end.
- What is 6 mm in fractional inches?
- 6 mm is 0.23622 inches. Call it 1/4", which is 6.35 mm and therefore 0.35 mm large. That is one of the worse conversions in the small range, and it is the reason a 6 mm reveal drawn as a quarter inch opens up over a run.
- What is 2mm in fractional inches?
- 2 mm is 0.07874 inches. Nearest thirty-second is 3/32" at 2.381 mm, which is 0.381 mm large. Nearest sixteenth is 1/16" at 1.5875 mm, 0.413 mm small. Neither is close. Below about 3 mm, fractions stop being useful.
- What are fractional inches?
- Inches divided into halves, quarters, eighths, sixteenths and thirty-seconds rather than decimals. Tape measures, drawing sets and lumber dimensions in the US and Canada are written this way, so 5' 3 1/2" means five feet plus three and a half inches.
- Why does no round millimetre equal a round fraction?
- Because one inch is exactly 25.4 mm. Fractions of an inch are halvings, so they land on multiples of 25.4 divided by a power of two: 12.7, 6.35, 3.175, 1.5875. None of those are whole millimetres. The only place the two systems meet cleanly is the inch itself.
- Should I round to 1/16 or 1/32?
- Sixteenths for framing and anything read off a tape. Thirty-seconds for machined or shop-fabricated parts where somebody owns a rule that fine. Rounding to a precision nobody can measure just moves the error somewhere less visible.