Fasteners · Drilling

Tap Drill Size Chart

Every size below is calculated from the thread form, then matched to the nearest real drill in the ANSI set. The Actual % column shows what engagement that drill really gives you — the number most charts leave out.

For 75% thread engagement: 1/4-20 takes a #7 (0.2010″), 1/2-13 takes a 27/64″ (0.4219″), and M6×1.0 takes a 5.0 mm. Metric coarse threads follow the shop rule drill = D − pitch, which lands at 76.8% engagement.

Tap drill sizes for UNC, UNF and metric threads at 75% engagement, with the actual engagement each drill produces

#0-80	UNC	80	0.0600	0.0478	3/64"	0.0469	80.6	#43
#1-64	UNC	64	0.0730	0.0578	#53	0.0595	66.5	#37
#2-56	UNC	56	0.0860	0.0686	#50	0.0700	68.9	#32
#3-48	UNC	48	0.0990	0.0787	#47	0.0785	75.7	#30
#4-40	UNC	40	0.1120	0.0876	#43	0.0890	70.8	#27
#5-40	UNC	40	0.1250	0.1006	#38	0.1015	72.3	5/32"
#6-32	UNC	32	0.1380	0.1075	#36	0.1065	77.5	#18
#8-32	UNC	32	0.1640	0.1335	#29	0.1360	68.9	#9
#10-24	UNC	24	0.1900	0.1494	#25	0.1495	74.8	#2
#12-24	UNC	24	0.2160	0.1754	#16	0.1770	72.0	D
1/4-20	UNC	20	0.2500	0.2012	#7	0.2010	75.4	K
5/16-18	UNC	18	0.3125	0.2583	F	0.2570	76.8	11/32"
3/8-16	UNC	16	0.3750	0.3141	5/16"	0.3125	76.9	13/32"
7/16-14	UNC	14	0.4375	0.3679	U	0.3680	74.8	15/32"
1/2-13	UNC	13	0.5000	0.4250	27/64"	0.4219	78.1	17/32"
9/16-12	UNC	12	0.5625	0.4813	31/64"	0.4844	72.1	19/32"
5/8-11	UNC	11	0.6250	0.5364	17/32"	0.5312	79.4	21/32"
3/4-10	UNC	10	0.7500	0.6525	21/32"	0.6562	72.2	25/32"
7/8-9	UNC	9	0.8750	0.7667	49/64"	0.7656	75.7	29/32"
1-8	UNC	8	1.0000	0.8781	7/8"	0.8750	76.9	1-1/32"
1-1/8-7	UNC	7	1.1250	0.9857	63/64"	0.9844	75.7	1-5/32"
1-1/4-7	UNC	7	1.2500	1.1107	1-1/8"	1.1250	67.3	1-9/32"
1-3/8-6	UNC	6	1.3750	1.2125	1-7/32"	1.2188	72.1	1-13/32"
1-1/2-6	UNC	6	1.5000	1.3375	1-11/32"	1.3438	72.1	1-17/32"
#0-80	UNF	80	0.0600	0.0478	3/64"	0.0469	80.6	#43
#1-72	UNF	72	0.0730	0.0595	#53	0.0595	74.8	#37
#2-64	UNF	64	0.0860	0.0708	#50	0.0700	78.8	#32
#3-56	UNF	56	0.0990	0.0816	#45	0.0820	73.2	#30
#4-48	UNF	48	0.1120	0.0917	#42	0.0935	68.3	#27
#5-44	UNF	44	0.1250	0.1028	#37	0.1040	71.1	5/32"
#6-40	UNF	40	0.1380	0.1136	#33	0.1130	76.9	#18
#8-36	UNF	36	0.1640	0.1369	#29	0.1360	77.5	#9
#10-32	UNF	32	0.1900	0.1595	#21	0.1590	76.3	#2
#12-28	UNF	28	0.2160	0.1812	#14	0.1820	73.2	D
1/4-28	UNF	28	0.2500	0.2152	#3	0.2130	79.7	K
5/16-24	UNF	24	0.3125	0.2719	I	0.2720	74.8	11/32"
3/8-24	UNF	24	0.3750	0.3344	Q	0.3320	79.4	13/32"
7/16-20	UNF	20	0.4375	0.3887	25/64"	0.3906	72.2	15/32"
1/2-20	UNF	20	0.5000	0.4512	29/64"	0.4531	72.2	17/32"
9/16-18	UNF	18	0.5625	0.5083	33/64"	0.5156	64.9	19/32"
5/8-18	UNF	18	0.6250	0.5708	37/64"	0.5781	64.9	21/32"
3/4-16	UNF	16	0.7500	0.6891	11/16"	0.6875	76.9	25/32"
7/8-14	UNF	14	0.8750	0.8054	13/16"	0.8125	67.3	29/32"
1-12	UNF	12	1.0000	0.9187	59/64"	0.9219	72.1	1-1/32"
1-1/8-12	UNF	12	1.1250	1.0437	1-1/32"	1.0312	86.6	1-5/32"
1-1/4-12	UNF	12	1.2500	1.1687	1-5/32"	1.1562	86.6	1-9/32"
1-3/8-12	UNF	12	1.3750	1.2937	1-9/32"	1.2812	86.6	1-13/32"
1-1/2-12	UNF	12	1.5000	1.4187	1-13/32"	1.4062	86.6	1-17/32"
M1.6×0.35	Metric Coarse	0.35 mm	0.0630	0.0495	1.3 mm	0.0512	65.8	2.4 mm
M2×0.4	Metric Coarse	0.4 mm	0.0787	0.0633	1.6 mm	0.0630	76.8	2.8 mm
M2.5×0.45	Metric Coarse	0.45 mm	0.0984	0.0811	2.1 mm	0.0827	68.3	3.3 mm
M3×0.5	Metric Coarse	0.5 mm	0.1181	0.0989	2.5 mm	0.0984	76.8	3.8 mm
M3.5×0.6	Metric Coarse	0.6 mm	0.1378	0.1147	2.9 mm	0.1142	76.8	4.3 mm
M4×0.7	Metric Coarse	0.7 mm	0.1575	0.1306	3.3 mm	0.1299	76.8	4.8 mm
M5×0.8	Metric Coarse	0.8 mm	0.1969	0.1661	4.2 mm	0.1654	76.8	5.8 mm
M6×1	Metric Coarse	1 mm	0.2362	0.1978	5.0 mm	0.1969	76.8	6.8 mm
M8×1.25	Metric Coarse	1.25 mm	0.3150	0.2669	6.8 mm	0.2677	73.7	8.8 mm
M10×1.5	Metric Coarse	1.5 mm	0.3937	0.3360	8.5 mm	0.3346	76.8	10.8 mm
M12×1.75	Metric Coarse	1.75 mm	0.4724	0.4052	10.3 mm	0.4055	74.6	12.8 mm
M14×2	Metric Coarse	2 mm	0.5512	0.4743	12.0 mm	0.4724	76.8	14.8 mm
M16×2	Metric Coarse	2 mm	0.6299	0.5530	14.0 mm	0.5512	76.8	16.8 mm
M18×2.5	Metric Coarse	2.5 mm	0.7087	0.6126	15.6 mm	0.6142	73.7	18.8 mm
M20×2.5	Metric Coarse	2.5 mm	0.7874	0.6913	17.6 mm	0.6929	73.7	20.8 mm
M22×2.5	Metric Coarse	2.5 mm	0.8661	0.7700	19.6 mm	0.7717	73.7	22.8 mm
M24×3	Metric Coarse	3 mm	0.9449	0.8295	21.1 mm	0.8307	74.2	24.8 mm
M27×3	Metric Coarse	3 mm	1.0630	0.9476	24.1 mm	0.9488	74.2	27.8 mm
M30×3.5	Metric Coarse	3.5 mm	1.1811	1.0465	26.6 mm	1.0472	74.6	30.0 mm
M8×1	Metric Fine	1 mm	0.3150	0.2765	7.0 mm	0.2756	76.8	8.8 mm
M10×1.25	Metric Fine	1.25 mm	0.3937	0.3456	8.8 mm	0.3465	73.7	10.8 mm
M10×1	Metric Fine	1 mm	0.3937	0.3552	9.0 mm	0.3543	76.8	10.8 mm
M12×1.5	Metric Fine	1.5 mm	0.4724	0.4148	10.5 mm	0.4134	76.8	12.8 mm
M12×1.25	Metric Fine	1.25 mm	0.4724	0.4244	10.8 mm	0.4252	73.7	12.8 mm
M14×1.5	Metric Fine	1.5 mm	0.5512	0.4935	12.5 mm	0.4921	76.8	14.8 mm
M16×1.5	Metric Fine	1.5 mm	0.6299	0.5722	14.5 mm	0.5709	76.8	16.8 mm
M18×1.5	Metric Fine	1.5 mm	0.7087	0.6510	16.5 mm	0.6496	76.8	18.8 mm
M20×1.5	Metric Fine	1.5 mm	0.7874	0.7297	18.5 mm	0.7283	76.8	20.8 mm
M22×1.5	Metric Fine	1.5 mm	0.8661	0.8085	20.5 mm	0.8071	76.8	22.8 mm
M24×2	Metric Fine	2 mm	0.9449	0.8680	22.0 mm	0.8661	76.8	24.8 mm
M27×2	Metric Fine	2 mm	1.0630	0.9861	25.0 mm	0.9843	76.8	27.8 mm
M30×2	Metric Fine	2 mm	1.1811	1.1042	28.0 mm	1.1024	76.8	30.0 mm

Click any row to pin it — useful when comparing two sizes. Bars are drawn to scale.

How these numbers are calculated

Thread engagement is set by how much smaller the hole is than the thread's major diameter. Both formulas below come from the basic 60° thread form, so the whole table is reproducible — you can check any row by hand.

Inch:    drill = D − 0.0130 × %thread × (1 / TPI)
Metric:  drill = D − (%thread / 76.8) × pitch

The metric formula is written against 76.8 rather than 100 for a reason: at exactly 76.8% it reduces to drill = D − pitch, which is the rule most machinists already carry in their head. That is why M6×1.0 gets a 5.0 mm drill and not the 5.02 mm that a literal 75% would ask for.

Why the "Actual %" column matters

Drills come in fixed sizes, so you almost never hit the engagement you asked for. Ask for 75% on a 1/2-13 and the nearest drill (27/64″) actually delivers 78.1% — a stiffer tap, more torque, more heat. Ask for 75% on a 1/4-20 and a #7 gives you 75.4%, which is close enough to ignore.

Charts that only print the drill size hide this. Knowing the real number tells you when to expect a hard tap and when you have room to go one size up in tough material.

Choosing an engagement percentage

50–60% — hard or gummy material, deep holes, hand tapping. Much lower breakage risk.
75% — the default for steel and aluminum in most shops.
85%+ — thin material or soft alloys where you need every bit of thread you can get. Expect heavy torque.

Going from 75% to 50% engagement costs far less strength than it sounds like, while cutting tapping torque substantially — which is why 50% is common in production work. Verify the exact strength trade-off against your own joint requirement rather than treating a rule of thumb as a spec.

Clearance holes

The last column is a normal-fit clearance hole — the size to drill in the part that the fastener passes through, not the part being tapped. Confusing the two is the most common mistake with these charts.

Before you cut: tap drill size is a starting point, not a specification. Material, tap type (form vs cut), coolant, and hole depth all shift the right answer. Test on scrap when the part matters.

Data source and method

Thread data: ASME B1.1 (Unified inch screw threads) and ISO 261 / ISO 965 (metric) nominal diameters and pitches — factual values, not a reproduced table.
Drill set: ANSI fractional, number, letter and metric drill sizes.
Calculation: drill = D − 0.0130 × %thread × (1/TPI) (inch), drill = D − (%thread / 76.8) × pitch (metric). Engagement percentages are back-calculated from the selected real drill.
Drill selection: Nearest available size to the theoretical value. An earlier "never go under" rule was rejected because it produced sizes one step large on 1/4-20, 1/2-13 and M6 — all three disagree with standard shop practice.
Verification: Six known pairs checked: 1/4-20→#7, #10-24→#25, 1/2-13→27/64″, M6×1.0→5.0 mm, M8×1.25→6.8 mm, M10×1.5→8.5 mm. All six match.
Rows: 80 thread sizes (48 inch, 32 metric).

Related charts

Drill bit size chart — all four size systems side by side, with equivalence groups
Thread size chart — pitch diameter, minor diameter and tensile stress area