Machinist Conversion Chart
Quick shop conversion
Enter a decimal to get the nearest 1/64" fraction and exact millimetre value.
Nearest 1/64"
awaiting inputAwaiting input.
Enter a decimal to convert.
- Millimetres
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- Rounding error
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This page combines the three references a machinist reaches for most: decimal-fraction equivalents, standard surface finish values, and everyday tolerance context. 5/16" = 0.3125" = 7.9375 mm, and a ground surface typically runs 8–32 μin Ra.
Everything the Shop Floor Needs at Once
A single job often touches all three of these reference types — a fractional print dimension, a metric equivalent for an imported part, and a surface finish callout — and switching between three separate charts mid-job is exactly what this page is built to avoid.
For the complete, exhaustive versions of each dataset, see the full decimal equivalent chart, drill size chart, and the tap drill and thread size charts in this cluster's "more in this category" section below.
Decimal, Fraction and Metric
The same universal conversion underlies every value on this page.
i is the nearest 64th's numerator for a decimal value V. MM is that same decimal converted to millimetres.
- VThe input decimal inch value.
- iThe nearest sixty-fourth's numerator: round(V × 64).
- MMThe exact millimetre equivalent: V × 25.4.
Worked calculation — 0.3125"
- 0.3125 × 64 = 20Already an exact 64th
- 20/64 = 5/16Simplify the fraction
- 0.3125 × 25.4 = 7.9375Convert to millimetres
0.3125" = 5/16" = 7.9375 mm
Sixteenths, Decimal & MM
| Fraction | Decimal | MM |
|---|---|---|
| 1/16" | 0.0625" | 1.587 mm |
| 1/8" | 0.125" | 3.175 mm |
| 3/16" | 0.1875" | 4.762 mm |
| 1/4" | 0.25" | 6.35 mm |
| 5/16" | 0.3125" | 7.938 mm |
| 3/8" | 0.375" | 9.525 mm |
| 7/16" | 0.4375" | 11.112 mm |
| 1/2" | 0.5" | 12.7 mm |
| 9/16" | 0.5625" | 14.287 mm |
| 5/8" | 0.625" | 15.875 mm |
| 11/16" | 0.6875" | 17.462 mm |
| 3/4" | 0.75" | 19.05 mm |
| 13/16" | 0.8125" | 20.637 mm |
| 7/8" | 0.875" | 22.225 mm |
| 15/16" | 0.9375" | 23.813 mm |
| 1" | 1" | 25.4 mm |
Typical Ra by Process
Achievable surface roughness (Ra) ranges by common manufacturing process.
| Process | Typical Ra |
|---|---|
| Sand casting | 500–1000 μin |
| Rough turning / milling | 125–500 μin |
| Finish turning / milling | 32–125 μin |
| Grinding | 8–32 μin |
| Honing | 4–16 μin |
| Lapping / polishing | 1–4 μin |
Frequently Asked Questions
What is Ra and why does it matter?
Ra (roughness average) measures the average deviation of a surface's peaks and valleys from a flat mean line, usually in microinches (μin) or micrometres (μm). It's the standard way surface texture is specified on a machining drawing, alongside dimensional tolerances.
What's a typical Ra value for a ground surface?
A ground surface typically falls in the 8–32 μin Ra range — smoother than a milled or turned surface, but coarser than a honed or lapped finish.
What is the decimal equivalent of 5/16 inch on this chart?
5/16" equals 0.3125", which converts to 7.9375 mm.
Why combine decimal equivalents with surface finish and tolerances on one page?
A machinist moving between a fractional print, a metric print, and a surface-finish callout benefits from having all three reference types on one page instead of switching between separate charts mid-job.
What's the difference between Ra and RMS surface roughness?
Ra averages the absolute deviation from the mean line, while RMS (root-mean-square) squares each deviation before averaging and then takes the square root. RMS values run roughly 11% higher than Ra for a typical machined surface, which is why the two aren't directly interchangeable without a conversion factor.
Machinist & Workshop Tools
Every other reference in this cluster, one click away.