Tap drill size chart (inch and metric)

Pick a thread to get the drill to use before tapping. Sizes give about 75% thread, the standard for most tapping in steel and aluminum.

Tap drill–

Drill diameter–

Thread not listed? Calculate it

System Major diameter (in) Threads per inch

Inch threads (UNC / UNF)

Thread	Tap drill	Decimal (in)

Metric threads

Thread	Tap drill (mm)	Nearest inch drill

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Questions & answers

What size drill for a 1/4-20 tap?

Use a #7 drill (0.201 in.). For 1/4-28 use a #3 (0.213 in.).

What size drill for an M6 tap?

For M6 × 1.0 use a 5.0 mm drill. For metric threads the tap drill is the thread size minus the pitch: 6 − 1.0 = 5.0 mm.

How do you work out a tap drill size?

For inch threads, subtract 1 divided by the threads per inch from the major diameter (for 1/2-13: 0.500 − 1/13 = 0.423 in., so a 27/64 drill). For metric, subtract the pitch from the diameter. That gives about 75% thread.

Should I use a bigger drill in hard material?

In hard or tough metals a slightly larger drill (about 60–65% thread) makes tapping easier and cuts tap breakage, with little loss of strength. Follow the tap maker's chart when it differs.