Tap Drill Size Chart
Complete tap drill size chart for UNC, UNF, Metric, and Metric Fine threads. Searchable reference table with drill sizes in inch and mm.
Introduction
The Tap Drill Chart lists the standard drill size for every common thread — UNC and UNF from #1-64 through the fractional sizes, and metric coarse and fine from M1.6 upward — with each drill given by its designation (number, letter, fractional, or millimeter) plus exact decimal-inch and millimeter equivalents. A wrong tap drill is the classic shop mistake with two failure modes: drill too small and the tap seizes and snaps in the hole; too large and the threads engage so little they strip under load. The chart pins the correct size for roughly 75% thread engagement, the sweet spot between those failures.
How it Works
Each row pairs a thread with its standard tap drill. For metric threads the underlying rule is visible in the data: tap drill ≈ major diameter − pitch, so M6×1.0 takes a 5.0 mm drill and M8×1.25 takes 6.8 mm. Inch threads follow the same geometry expressed through the 75%-engagement formula — drill ≈ major − 0.974/TPI — which is why 1/4-20 lands on a #7 drill (0.201 in). Decimal equivalents in both unit systems let you substitute the nearest drill you actually own and judge how far engagement drifts.
Usage Scenarios
- Tapping M8×1.25 holes in a fixture plate and confirming the 6.8 mm drill instead of trusting a half-remembered rule of thumb.
- Selecting the #7 drill for 1/4-20 holes instead of guessing between 13/64 and 7/32 fractional sizes.
- Substituting drills using the decimal columns when the exact number drill is missing from the index, and judging how much engagement shifts.
- Programming a CNC drill-and-tap sequence and pulling exact decimal diameters for the spot, drill, and chamfer tools.
- Tapping 6061 versus 304: choosing to keep full engagement in soft aluminum, or open the hole a size in tough stainless to cut tapping torque.
FAQ
Why 75% thread engagement instead of 100%?
Strength testing shows engagement beyond about 75% adds almost no stripping resistance while tapping torque climbs steeply — full-depth threads mostly increase the chance of breaking the tap, not the joint strength.
What is the quick rule for metric tap drills?
Tap drill = major diameter − pitch. M6×1.0 → 5.0 mm, M10×1.5 → 8.5 mm. It works because subtracting one pitch yields very close to 75% engagement for ISO threads.
Can I deviate from the chart for hard materials?
Yes — in tough materials like stainless or titanium, machinists often drill one size over to drop engagement toward 65%, trading a little static strength for dramatically lower tapping torque and tap survival.
Do form (roll) taps use the same drill sizes?
No. Form taps displace metal instead of cutting it, so they need a larger hole — roughly major − pitch/2 for metric sizes. Using a cutting-tap drill with a form tap overloads it immediately.