Convert drill bit (inch) to drill bit (mm) instantly. Enter a value below, or use the formula and reference table further down the page.
In practice
Drill bit sizes in inches are given as fractions (1/8, 5/16, 1/2), number sizes or letters, and converting them to millimetres shows how close the metric and imperial sets really are. Fractional sizes convert exactly with inches times 25.4, so a 1/4 inch bit is 6.35 mm, but there is no whole-millimetre bit that matches it — a 6 mm or 6.5 mm bit is the nearest substitute. For clearance holes that is fine; for a tight press fit or tapped thread, use the exact size the thread chart specifies.
How to convert drill bit (inch) to drill bit (mm)
Multiply the value by 25.4.
mm = in × 25.4Example: 10 in = 254 mm.
Drill Bit (inch) to Drill Bit (mm) conversion table
| Drill Bit (inch) (in) | Drill Bit (mm) (mm) |
|---|---|
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 3 in | 76.2 mm |
| 4 in | 101.6 mm |
| 5 in | 127 mm |
| 10 in | 254 mm |
| 15 in | 381 mm |
| 20 in | 508 mm |
| 25 in | 635 mm |
| 50 in | 1270 mm |
| 100 in | 2540 mm |
Frequently asked questions
How many drill bit (mm) is 1 drill bit inch?
1 drill bit inch equals 25.4 drill bit (mm).
What is the formula to convert in to mm?
To convert in to mm, multiply the value by 25.4. The full formula is: mm = in × 25.4.
How do I convert drill bit (mm) back to drill bit (inch)?
Use the same calculator above and type your value into the drill bit (mm) field instead — it converts in both directions. Or use the reverse formula: in = mm ÷ 25.4.
Is the in to mm conversion exact?
The conversion factor used here (25.4) is the standard internationally defined value. Results on this page are rounded to four decimal places for readability.