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Tap Drill Size Chart for UNC, UNF and Metric Threads

Every machinist and CNC programmer needs the tap drill for a given thread many times a week, and the chart is usually on a wall poster or buried in a catalog. This searchable chart covers the common Unified (UNC and UNF) and metric coarse and fine sizes with pitch and drill.

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Thread	System	Pitch / TPI	Tap drill	Note
M3 x 0.5	Metric coarse	0.5 mm	2.5 mm	Cutting tap, about 75% thread
M4 x 0.7	Metric coarse	0.7 mm	3.3 mm	Cutting tap
M5 x 0.8	Metric coarse	0.8 mm	4.2 mm	Cutting tap
M6 x 1.0	Metric coarse	1.0 mm	5.0 mm	Cutting tap
M8 x 1.25	Metric coarse	1.25 mm	6.8 mm	Cutting tap
M10 x 1.5	Metric coarse	1.5 mm	8.5 mm	Cutting tap
M12 x 1.75	Metric coarse	1.75 mm	10.2 mm	Cutting tap
M14 x 2.0	Metric coarse	2.0 mm	12.0 mm	Cutting tap
M16 x 2.0	Metric coarse	2.0 mm	14.0 mm	Cutting tap
M20 x 2.5	Metric coarse	2.5 mm	17.5 mm	Cutting tap
M24 x 3.0	Metric coarse	3.0 mm	21.0 mm	Cutting tap
M8 x 1.0	Metric fine	1.0 mm	7.0 mm	Cutting tap
M10 x 1.25	Metric fine	1.25 mm	8.8 mm	Cutting tap
M12 x 1.25	Metric fine	1.25 mm	10.8 mm	Cutting tap
M12 x 1.5	Metric fine	1.5 mm	10.5 mm	Cutting tap
M16 x 1.5	Metric fine	1.5 mm	14.5 mm	Cutting tap
M20 x 1.5	Metric fine	1.5 mm	18.5 mm	Cutting tap
#4-40 UNC	Unified coarse	40 TPI	#43	Number drill
#6-32 UNC	Unified coarse	32 TPI	#36	Number drill
#8-32 UNC	Unified coarse	32 TPI	#29	Number drill
#10-24 UNC	Unified coarse	24 TPI	#25	Number drill
#10-32 UNF	Unified fine	32 TPI	#21	Number drill
1/4-20 UNC	Unified coarse	20 TPI	#7	Number drill
1/4-28 UNF	Unified fine	28 TPI	#3	Number drill
5/16-18 UNC	Unified coarse	18 TPI	F	Letter drill
5/16-24 UNF	Unified fine	24 TPI	I	Letter drill
3/8-16 UNC	Unified coarse	16 TPI	5/16 in	Fractional drill
3/8-24 UNF	Unified fine	24 TPI	Q	Letter drill
7/16-14 UNC	Unified coarse	14 TPI	U	Letter drill
7/16-20 UNF	Unified fine	20 TPI	25/64 in	Fractional drill
1/2-13 UNC	Unified coarse	13 TPI	27/64 in	Fractional drill
1/2-20 UNF	Unified fine	20 TPI	29/64 in	Fractional drill
9/16-12 UNC	Unified coarse	12 TPI	31/64 in	Fractional drill
5/8-11 UNC	Unified coarse	11 TPI	17/32 in	Fractional drill
3/4-10 UNC	Unified coarse	10 TPI	21/32 in	Fractional drill
7/8-9 UNC	Unified coarse	9 TPI	49/64 in	Fractional drill
1-8 UNC	Unified coarse	8 TPI	7/8 in	Fractional drill

Drills listed are the conventional tap drills for roughly 75% thread engagement with a cutting tap. Forming (roll) taps need a larger drill, so use the tap maker's chart for those. Always confirm against your tap supplier's catalog for hard or tough materials.

Tooling data inside your ERP

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How to use the chart

Type a thread such as M8 or 1/4-20 into the search box to filter the rows. Read across to the tap drill column and use that drill for the pilot hole before tapping. Export the table to PDF, Excel or CSV if you want a printed copy at the machine or a lookup list inside your tooling database.

Search by thread, system or drill size
Number and letter drills are inch series sizes
Metric drills are in millimetres
Export to keep a copy by the machine

Where the numbers come from

A tap drill is chosen so the pilot hole leaves enough material to form roughly 75% of a full thread. For metric threads the common rule of thumb is tap drill equals nominal diameter minus pitch, which is why M8 x 1.25 uses 6.8 mm and M10 x 1.5 uses 8.5 mm. Inch threads use the nearest standard number, letter or fractional drill to the same percentage of thread.

Choosing thread engagement

Seventy five percent is the default, but it is not always right. In tough materials such as stainless steel or titanium, a larger drill giving 60 to 65 percent engagement reduces tap torque and breakage with very little loss of strength. In soft aluminum, the full 75 percent is normal. Blind holes also need drill depth beyond the thread depth to give the chips room.

Common mistakes

The most frequent errors are mixing up M10 x 1.5 with M10 x 1.25, using a cutting tap drill for a forming tap, and forgetting that fine threads need a different drill than coarse threads of the same diameter. Record the correct drill in your tool list so the setup sheet, the program and the ERP tool record all agree.

Frequently Asked Questions

What is the tap drill for M8 x 1.25?

The standard tap drill for M8 x 1.25 coarse is 6.8 mm, which gives roughly 75% thread engagement with a cutting tap. For M8 x 1.0 fine pitch the drill is 7.0 mm. Always check the pitch before drilling.

What drill do I use for a 1/4-20 tap?

Use a number 7 drill, which is 0.201 inch, for a 1/4-20 UNC cutting tap. For 1/4-28 UNF the conventional drill is number 3. Number drills are part of the inch drill series, not metric.

Does a forming tap use the same drill?

No. Forming taps displace metal instead of cutting it, so the pilot hole is larger than for a cutting tap, for example about 7.4 mm for M8 x 1.25. Use the chart supplied by the tap manufacturer for forming taps.

Can I use the chart for pipe threads?

This chart covers Unified and ISO metric straight threads only. Pipe threads such as NPT, BSPP and BSPT have their own tap drills and are tapered or use different forms, so use a dedicated pipe thread chart for those.

If your tool list lives in a spreadsheet and your ERP routing does not know it, we can show how tooling data from the shop floor can be queried from the ERP in plain language.

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