0.201 in / 5.11 mm for 1/4-20 UNC general tapping.
Tap Drill Size Converter
Convert a thread size into the drill bit you use before tapping. Choose common inch or metric threads, or enter your own diameter and pitch to get the calculated tap drill size and closest standard bits.
Find a tap drill size
Use a preset like 1/4-20 or M6 × 1.0, then compare the exact drill estimate with nearby drill bit sizes.
Closest fraction13/64 in
Closest number / letter#7
Thread detailsMajor Ø 0.250 in, 20 TPI
For 1/4-20 UNC, the simple drill estimate is 0.200 inches. The common tap drill is #7 at 0.201 inches.
How to use this tap drill converter
- Select inch, metric, or custom thread input.
- Choose a common thread size such as 1/4-20 UNC, M6 × 1.0, or M8 × 1.25.
- Use the recommended bit when it is listed, or choose the closest drill bit to the calculated size.
- For stainless, hard alloys, blind holes, production work, or critical fits, verify the final drill in a machinist reference.
Tap drill formula
For metric threads, the usual estimate is major diameter − pitch. M6 × 1.0 becomes about 5.0 mm.
For inch threads, the common estimate is major diameter − 1 ÷ TPI. A 1/4-20 thread becomes about 0.200 inches, which is why #7 is commonly used.
Common tap drill sizes
Quick reference for popular inch and metric taps. Values are general-purpose starting points.
| Thread | Tap drill | Decimal | Millimeters |
|---|
Tap drill size FAQ
Why does the calculator show a calculated size and a recommended bit?
The calculation gives the theoretical drill estimate. The recommended bit is the standard drill size people commonly use because real drill indexes come in number, letter, fractional, and metric steps.
Should I use a larger drill bit for easier tapping?
Sometimes, especially in tough material or when full thread strength is not required. A slightly larger hole can reduce tap breakage, while a smaller hole gives more thread engagement but takes more force to tap.
Can I use this for pipe taps or special thread forms?
This page is for common machine-screw and metric threads. Pipe taps, STI/helicoil taps, thread-forming taps, and specialty threads need their own drill recommendations.