Fraction ↔ Letter ↔ Number Drill Chart
Every fractional size lined up against its closest letter and number drill, for when you need to substitute one system for another.
| Fraction | Decimal (in) | Nearest letter | Nearest number |
|---|
Why you'd swap systems in the first place
Not every shop's drill index has every size in every system, and a tap drill or clearance hole calculation often lands closer to a letter or number size than a clean fraction — this table lets you go the other way too, starting from a fraction you know and finding what letter or number drill sits closest to it, useful when substituting for a missing size or standardizing a print's callouts to what's actually in the index.
How "nearest" is determined
Each row scans the full letter (A–Z) and number (#80–#1) drill index and picks whichever single size has the smallest decimal difference from that row's fraction — sizes don't line up on a shared grid, so the nearest match is sometimes a very close few ten-thousandths and sometimes several thousandths off. Check the decimal columns on the full Drill Bit Size Chart if you need to see how close a substitution really is.
Verify before cutting anything critical
These decimal equivalents follow standard ANSI B94.11M number, letter, and fractional drill sizes. Cross-check against your own drill index or a manufacturer's chart before committing to a tolerance-critical hole.