Engineering DisclaimerEngineering calculations are provided for preliminary design and educational/reference purposes only. Users must verify all results according to applicable engineering standards, supplier data, manufacturing requirements, and professional engineering judgment.
Category: drill sizes
Reference Dataset
Standard Drill Sizes Reference (Number, Letter & Fractional) Table
Complete diameter lookup matching number series, letter series, and fractional inch drill bits to decimal sizes.
Overview & Scope
Standard drill bit sizes represent fractional, decimal, number, and letter gauge drills. Knowing the exact diameters is essential for drafting clearances, tap drills, and dowel fits.
Technical Sizing Values
The table below maps fractional, letter, and number drill designations to their decimal equivalents in millimeters and inches.
| Drill Size Designation | Diameter (mm) | Diameter (in) | Series Type |
|---|---|---|---|
| No. 80 | 0.343 | 0.0135 | Number |
| No. 60 | 1.016 | 0.04 | Number |
| 1/16 in | 1.588 | 0.0625 | Fractional |
| No. 40 | 2.489 | 0.098 | Number |
| No. 30 | 3.264 | 0.1285 | Number |
| 1/8 in | 3.175 | 0.125 | Fractional |
| No. 1 | 5.791 | 0.228 | Number |
| Letter A | 5.944 | 0.234 | Letter |
| 1/4 in | 6.35 | 0.25 | Fractional |
| Letter M | 7.493 | 0.295 | Letter |
| Letter Z | 10.49 | 0.413 | Letter |
| 1/2 in | 12.7 | 0.5 | Fractional |
Engineering Notes & Best Practices
- •Number drill series ranges from No. 1 (largest) to No. 80 (smallest).
- •Letter drill series ranges from A (smallest) to Z (largest).
- •Tap drill sizes should be selected carefully to achieve 65% to 75% thread engagement.
Dataset Citations & References
Standard Machinist Drill Tables
Public domain source and standard catalog reference parameters.