How to Read This Chart
Find your thread in the first column. Tap drill is the hole you drill before tapping; it leaves enough material for the tap to cut about 75% of a full thread. Close fit and free fit are clearance holes for a screw of that size to pass through: close fit locates the part precisely, free fit forgives a little misalignment between parts. Metric clearance holes follow ISO 273’s close, normal and free series.
Number drills (#1–#80) and letter drills (A–Z) fill the gaps between fractional sizes, which is why most inch tap drills are a number or letter rather than a fraction.
Worked Example: 1/4-20
The tap drill for a given percentage of thread is the major diameter minus that share of the full thread depth:
tap drill = major − (0.01299 × % thread) ÷ TPI
= .2500 − (0.01299 × 75) ÷ 20
= .2013″ → nearest drill: #7 (.2010″)
Any drill near that size works; it just changes how much thread you get. For 1/4-20:
| Drill | Decimal | Thread | Use |
|---|---|---|---|
| #8 | .1990 | 79% | Soft materials, maximum strength |
| #7 | .2010 | 75% | The standard choice |
| 13/64 | .2031 | 72% | Common substitute |
| #5 | .2055 | 69% | Steel, easier tapping |
| #4 | .2090 | 63% | Stainless and tough alloys |
| #3 | .2130 | 57% | Hard materials |
| 7/32 | .2188 | 48% | Thin sheet, hand tapping |
For metric threads the rule of thumb is simpler: tap drill = major diameter − pitch. An M6 × 1.0 thread takes a 5 mm drill, which leaves about 77% thread.