What is the tap drill for 1/4-20?

The tap drill for 1/4"-20 UNC is #7 — 0.2010 in (5.105 mm) — leaving 75.4% thread engagement with a cut tap.

What thread engagement should a tap drill give?

75% is the general shop default and what most published charts assume. Dropping to 60–65% cuts tapping torque roughly in half and greatly reduces broken taps in tough material, while costing only a few percent of thread strength — the fastener almost always fails before a 60% thread strips. This chart lets you pick anything from 50% to 85% and shows the engagement the real drill actually produces.

Why does this chart list a different drill than my wall chart?

On 4 threads the drill nearest the computed 75% target differs from the traditional published chart, which lists a smaller drill and a heavier thread. Both values are shown, with the exact figures, so you can see what you are choosing between rather than trusting one chart over the other.

Does this tap drill chart cover metric and inch threads together?

Yes — 82 inch threads and 68 metric threads are in one table, interleaved by nominal diameter, with drill sizes given in both inches and millimetres on every row.

Chart · tap drills

Tap Drill Chart — Inch and Metric, Every Thread Engagement

https://shopmath.org/charts/tap-drill · data verified August 1, 2026

The tap drill for 1/4"-20 UNC is #7 (0.2010 in (5.105 mm)) at 75% thread engagement — and every other thread in this chart is below, inch and metric in the same table. 150 threads with selectable engagement from 50% to 85%, plus 34 taper pipe threads, each row linking to its full size page. Drill sizes are for cut taps; every value names the standard it came from.

Threads: 184
Inch: 82
Metric: 68
Engagements: 50–85%
Verified: August 1, 2026

Most looked-up row · 1/4-20 UNC at 75% thread

Drill Ø: 0.2010in
Drill Ø: 5.105mm
Actual thread: 75.4%

Cut tap, class 2B hole. Full 1/4"-20 UNC page →

Tap Drill Chart — Inch and Metric Threads, Interleaved by Size

One table, sorted by size, with metric sitting between the inch sizes it falls between. Change the engagement and every drill in the chart changes with it; print at whatever engagement you selected.

Thread %

Units

Sort

150 threads shown

Tap drill chart at 75% thread engagement — cut taps, inch and metric interleaved
Property	Property	Property	Tap drill at 75% thread				Property
Thread	Series	Pitch	Tap drill	in	mm	Actual %	Traditional chart
#0-80 UNF	Unified Fine (UNF)	80 TPI	3/64	0.0469	1.191	80.8	same
M1.6×0.2	ISO Metric Fine	0.200 mm	1.4 mm	0.0551	1.400	77.0	same
M1.6×0.35	ISO Metric Coarse	0.350 mm	1.25 mm	0.0492	1.250	77.0	same
M1.8×0.2	ISO Metric Fine	0.200 mm	1.6 mm	0.0630	1.600	77.0	same
M1.8×0.35	ISO Metric Coarse	0.350 mm	1.45 mm	0.0571	1.450	77.0	same
#1-64 UNC	Unified Coarse (UNC)	64 TPI	#53	0.0595	1.511	66.5	same
#1-72 UNF	Unified Fine (UNF)	72 TPI	#53	0.0595	1.511	74.8	same
M2×0.25	ISO Metric Fine	0.250 mm	1.75 mm	0.0689	1.750	77.0	same
M2×0.4	ISO Metric Coarse	0.400 mm	1.6 mm	0.0630	1.600	77.0	same
#2-56 UNC	Unified Coarse (UNC)	56 TPI	#50	0.0700	1.778	69.0	same
#2-64 UNF	Unified Fine (UNF)	64 TPI	#50	0.0700	1.778	78.8	same
M2.5×0.35	ISO Metric Fine	0.350 mm	2.15 mm	0.0846	2.150	77.0	same
M2.5×0.45	ISO Metric Coarse	0.450 mm	2.05 mm	0.0807	2.050	77.0	same
#3-48 UNC	Unified Coarse (UNC)	48 TPI	#47	0.0785	1.994	75.7	same
#3-56 UNF	Unified Fine (UNF)	56 TPI	#45	0.0820	2.083	73.3	same
#4-40 UNC	Unified Coarse (UNC)	40 TPI	#43	0.0890	2.261	70.8	same
#4-48 UNF	Unified Fine (UNF)	48 TPI	#42	0.0935	2.375	68.4	same
M3×0.35	ISO Metric Fine	0.350 mm	2.65 mm	0.1043	2.650	77.0	same
M3×0.5	ISO Metric Coarse	0.500 mm	2.5 mm	0.0984	2.500	77.0	same
#5-40 UNC	Unified Coarse (UNC)	40 TPI	#38	0.1015	2.578	72.4	same
#5-44 UNF	Unified Fine (UNF)	44 TPI	#37	0.1040	2.642	71.1	same
M3.5×0.35	ISO Metric Fine	0.350 mm	3.2 mm	0.1260	3.200	66.0	same
M3.5×0.6	ISO Metric Coarse	0.600 mm	2.9 mm	0.1142	2.900	77.0	same
#6-32 UNC	Unified Coarse (UNC)	32 TPI	#36	0.1065	2.705	77.6	same
#6-40 UNF	Unified Fine (UNF)	40 TPI	#33	0.1130	2.870	77.0	same
M4×0.5	ISO Metric Fine	0.500 mm	3.5 mm	0.1378	3.500	77.0	same
M4×0.7	ISO Metric Coarse	0.700 mm	3.3 mm	0.1299	3.300	77.0	same
#8-32 UNC	Unified Coarse (UNC)	32 TPI	#29	0.1360	3.454	69.0	same
#8-36 UNF	Unified Fine (UNF)	36 TPI	#29	0.1360	3.454	77.6	same
#10-24 UNC	Unified Coarse (UNC)	24 TPI	#25	0.1495	3.797	74.8	same
#10-32 UNF	Unified Fine (UNF)	32 TPI	#21	0.1590	4.039	76.4	same
M5×0.5	ISO Metric Fine	0.500 mm	4.5 mm	0.1772	4.500	77.0	same
M5×0.8	ISO Metric Coarse	0.800 mm	4.2 mm	0.1654	4.200	77.0	same
#12-24 UNC	Unified Coarse (UNC)	24 TPI	#16	0.1770	4.496	72.1	same
#12-28 UNF	Unified Fine (UNF)	28 TPI	#14	0.1820	4.623	73.3	same
#12-32 UNEF	Unified Extra Fine (UNEF)	32 TPI	#13	0.1850	4.699	76.4	3/16
M6×0.75	ISO Metric Fine	0.750 mm	5.3 mm	0.2087	5.300	71.8	same
M6×1	ISO Metric Coarse	1.000 mm	5 mm	0.1968	5.000	77.0	same
1/4"-20 UNC	Unified Coarse (UNC)	20 TPI	#7	0.2010	5.105	75.4	same
1/4"-28 UNF	Unified Fine (UNF)	28 TPI	#3	0.2130	5.410	79.8	same
1/4"-32 UNEF	Unified Extra Fine (UNEF)	32 TPI	7/32	0.2188	5.556	77.0	same
M7×0.75	ISO Metric Fine	0.750 mm	6.3 mm	0.2480	6.300	71.8	same
M7×1	ISO Metric Coarse	1.000 mm	6 mm	0.2362	6.000	77.0	same
5/16"-18 UNC	Unified Coarse (UNC)	18 TPI	F	0.2570	6.528	76.9	same
5/16"-24 UNF	Unified Fine (UNF)	24 TPI	I	0.2720	6.909	74.8	same
5/16"-32 UNEF	Unified Extra Fine (UNEF)	32 TPI	9/32	0.2813	7.144	77.0	same
M8×0.75	ISO Metric Fine	0.750 mm	7.3 mm	0.2874	7.300	71.8	same
M8×1	ISO Metric Fine	1.000 mm	7 mm	0.2756	7.000	77.0	same
M8×1.25	ISO Metric Coarse	1.250 mm	6.8 mm	0.2677	6.800	73.9	same
3/8"-16 UNC	Unified Coarse (UNC)	16 TPI	5/16	0.3125	7.938	77.0	same
3/8"-24 UNF	Unified Fine (UNF)	24 TPI	Q	0.3320	8.433	79.4	same
3/8"-32 UNEF	Unified Extra Fine (UNEF)	32 TPI	11/32	0.3438	8.731	77.0	same
M10×1	ISO Metric Fine	1.000 mm	9 mm	0.3543	9.000	77.0	same
M10×1.25	ISO Metric Fine	1.250 mm	8.8 mm	0.3465	8.800	73.9	same
M10×1.5	ISO Metric Coarse	1.500 mm	8.5 mm	0.3346	8.500	77.0	same
7/16"-14 UNC	Unified Coarse (UNC)	14 TPI	U	0.3680	9.347	74.9	same
7/16"-20 UNF	Unified Fine (UNF)	20 TPI	25/64	0.3906	9.922	72.2	same
7/16"-28 UNEF	Unified Extra Fine (UNEF)	28 TPI	Y	0.4040	10.262	72.2	same
M12×1.25	ISO Metric Fine	1.250 mm	10.8 mm	0.4252	10.800	73.9	same
M12×1.5	ISO Metric Fine	1.500 mm	10.5 mm	0.4134	10.500	77.0	same
M12×1.75	ISO Metric Coarse	1.750 mm	10.3 mm	0.4055	10.300	74.8	same
1/2"-13 UNC	Unified Coarse (UNC)	13 TPI	27/64	0.4219	10.716	78.2	same
1/2"-20 UNF	Unified Fine (UNF)	20 TPI	29/64	0.4531	11.509	72.2	same
1/2"-28 UNEF	Unified Extra Fine (UNEF)	28 TPI	15/32	0.4688	11.906	67.4	same
M14×1.5	ISO Metric Fine	1.500 mm	12.5 mm	0.4921	12.500	77.0	same
M14×2	ISO Metric Coarse	2.000 mm	12.1 mm	0.4764	12.100	73.1	same
9/16"-12 UNC	Unified Coarse (UNC)	12 TPI	31/64	0.4844	12.303	72.2	same
9/16"-18 UNF	Unified Fine (UNF)	18 TPI	33/64	0.5156	13.097	65.0	1/2
9/16"-24 UNEF	Unified Extra Fine (UNEF)	24 TPI	33/64	0.5156	13.097	86.6	same
5/8"-11 UNC	Unified Coarse (UNC)	11 TPI	17/32	0.5313	13.494	79.4	same
5/8"-18 UNF	Unified Fine (UNF)	18 TPI	37/64	0.5781	14.684	65.0	9/16
5/8"-24 UNEF	Unified Extra Fine (UNEF)	24 TPI	37/64	0.5781	14.684	86.6	same
M16×1.5	ISO Metric Fine	1.500 mm	14.5 mm	0.5709	14.500	77.0	same
M16×2	ISO Metric Coarse	2.000 mm	14 mm	0.5512	14.000	77.0	same
11/16"-24 UNEF	Unified Extra Fine (UNEF)	24 TPI	41/64	0.6406	16.272	86.6	same
M18×1.5	ISO Metric Fine	1.500 mm	16.5 mm	0.6496	16.500	77.0	same
M18×2	ISO Metric Fine	2.000 mm	16 mm	0.6299	16.000	77.0	same
M18×2.5	ISO Metric Coarse	2.500 mm	15.5 mm	0.6102	15.500	77.0	same
3/4"-10 UNC	Unified Coarse (UNC)	10 TPI	21/32	0.6563	16.669	72.2	same
3/4"-16 UNF	Unified Fine (UNF)	16 TPI	11/16	0.6875	17.462	77.0	same
3/4"-20 UNEF	Unified Extra Fine (UNEF)	20 TPI	45/64	0.7031	17.859	72.2	same
M20×1.5	ISO Metric Fine	1.500 mm	18.5 mm	0.7283	18.500	77.0	same
M20×2	ISO Metric Fine	2.000 mm	18 mm	0.7087	18.000	77.0	same
M20×2.5	ISO Metric Coarse	2.500 mm	17.5 mm	0.6890	17.500	77.0	same
13/16"-20 UNEF	Unified Extra Fine (UNEF)	20 TPI	49/64	0.7656	19.447	72.2	same
M22×1.5	ISO Metric Fine	1.500 mm	20.5 mm	0.8071	20.500	77.0	same
M22×2	ISO Metric Fine	2.000 mm	20 mm	0.7874	20.000	77.0	same
M22×2.5	ISO Metric Coarse	2.500 mm	19.5 mm	0.7677	19.500	77.0	same
7/8"-14 UNF	Unified Fine (UNF)	14 TPI	13/16	0.8125	20.637	67.4	51/64
7/8"-20 UNEF	Unified Extra Fine (UNEF)	20 TPI	53/64	0.8281	21.034	72.2	same
7/8"-9 UNC	Unified Coarse (UNC)	9 TPI	49/64	0.7656	19.447	75.8	same
15/16"-20 UNEF	Unified Extra Fine (UNEF)	20 TPI	57/64	0.8906	22.622	72.2	same
M24×2	ISO Metric Fine	2.000 mm	22 mm	0.8661	22.000	77.0	same
M24×3	ISO Metric Coarse	3.000 mm	21 mm	0.8268	21.000	77.0	same
1"-12 UNF	Unified Fine (UNF)	12 TPI	59/64	0.9219	23.416	72.2	same
1"-20 UNEF	Unified Extra Fine (UNEF)	20 TPI	61/64	0.9531	24.209	72.2	same
1"-8 UNC	Unified Coarse (UNC)	8 TPI	7/8	0.8750	22.225	77.0	same
1 1/16"-18 UNEF	Unified Extra Fine (UNEF)	18 TPI	1	1.0000	25.400	86.6	same
M27×2	ISO Metric Fine	2.000 mm	25 mm	0.9843	25.000	77.0	same
M27×3	ISO Metric Coarse	3.000 mm	24 mm	0.9449	24.000	77.0	same
1 1/8"-12 UNF	Unified Fine (UNF)	12 TPI	1	1.0000	25.400	115.5	same
1 1/8"-18 UNEF	Unified Extra Fine (UNEF)	18 TPI	1	1.0000	25.400	173.2	same
1 1/8"-7 UNC	Unified Coarse (UNC)	7 TPI	63/64	0.9844	25.003	75.8	same
M30×2	ISO Metric Fine	2.000 mm	25 mm	0.9843	25.000	192.4	same
M30×3.5	ISO Metric Coarse	3.500 mm	25 mm	0.9843	25.000	110.0	same
1 3/16"-18 UNEF	Unified Extra Fine (UNEF)	18 TPI	1	1.0000	25.400	259.8	same
1 1/4"-12 UNF	Unified Fine (UNF)	12 TPI	1	1.0000	25.400	230.9	same
1 1/4"-18 UNEF	Unified Extra Fine (UNEF)	18 TPI	1	1.0000	25.400	346.4	same
1 1/4"-7 UNC	Unified Coarse (UNC)	7 TPI	1	1.0000	25.400	134.7	same
M33×2	ISO Metric Fine	2.000 mm	25 mm	0.9843	25.000	307.9	same
M33×3.5	ISO Metric Coarse	3.500 mm	25 mm	0.9843	25.000	176.0	same
1 5/16"-18 UNEF	Unified Extra Fine (UNEF)	18 TPI	1	1.0000	25.400	433.0	same
1 3/8"-12 UNF	Unified Fine (UNF)	12 TPI	1	1.0000	25.400	346.4	same
1 3/8"-18 UNEF	Unified Extra Fine (UNEF)	18 TPI	1	1.0000	25.400	519.6	same
1 3/8"-6 UNC	Unified Coarse (UNC)	6 TPI	1	1.0000	25.400	173.2	same
M36×3	ISO Metric Fine	3.000 mm	25 mm	0.9843	25.000	282.3	same
M36×4	ISO Metric Coarse	4.000 mm	25 mm	0.9843	25.000	211.7	same
1 7/16"-18 UNEF	Unified Extra Fine (UNEF)	18 TPI	1	1.0000	25.400	606.2	same
1 1/2"-12 UNF	Unified Fine (UNF)	12 TPI	1	1.0000	25.400	461.9	same
1 1/2"-18 UNEF	Unified Extra Fine (UNEF)	18 TPI	1	1.0000	25.400	692.8	same
1 1/2"-6 UNC	Unified Coarse (UNC)	6 TPI	1	1.0000	25.400	230.9	same
M39×3	ISO Metric Fine	3.000 mm	25 mm	0.9843	25.000	359.2	same
M39×4	ISO Metric Coarse	4.000 mm	25 mm	0.9843	25.000	269.4	same
1 9/16"-18 UNEF	Unified Extra Fine (UNEF)	18 TPI	1	1.0000	25.400	779.4	same
1 5/8"-18 UNEF	Unified Extra Fine (UNEF)	18 TPI	1	1.0000	25.400	866.0	same
M42×3	ISO Metric Fine	3.000 mm	25 mm	0.9843	25.000	436.2	same
M42×4.5	ISO Metric Coarse	4.500 mm	25 mm	0.9843	25.000	290.8	same
1 11/16"-18 UNEF	Unified Extra Fine (UNEF)	18 TPI	1	1.0000	25.400	952.6	same
1 3/4"-5 UNC	Unified Coarse (UNC)	5 TPI	1	1.0000	25.400	288.7	same
M45×3	ISO Metric Fine	3.000 mm	25 mm	0.9843	25.000	513.2	same
M45×4.5	ISO Metric Coarse	4.500 mm	25 mm	0.9843	25.000	342.1	same
M48×3	ISO Metric Fine	3.000 mm	25 mm	0.9843	25.000	590.2	same
M48×5	ISO Metric Coarse	5.000 mm	25 mm	0.9843	25.000	354.1	same
2"-4 UNC	Unified Coarse (UNC)	4 TPI	1	1.0000	25.400	307.9	same
M52×4	ISO Metric Fine	4.000 mm	25 mm	0.9843	25.000	519.6	same
M52×5	ISO Metric Coarse	5.000 mm	25 mm	0.9843	25.000	415.7	same
M56×4	ISO Metric Fine	4.000 mm	25 mm	0.9843	25.000	596.6	same
M56×5.5	ISO Metric Coarse	5.500 mm	25 mm	0.9843	25.000	433.9	same
2 1/4"-4 UNC	Unified Coarse (UNC)	4 TPI	1	1.0000	25.400	384.9	same
M60×4	ISO Metric Fine	4.000 mm	25 mm	0.9843	25.000	673.6	same
M60×5.5	ISO Metric Coarse	5.500 mm	25 mm	0.9843	25.000	489.9	same
2 1/2"-4 UNC	Unified Coarse (UNC)	4 TPI	1	1.0000	25.400	461.9	same
M64×4	ISO Metric Fine	4.000 mm	25 mm	0.9843	25.000	750.6	same
M64×6	ISO Metric Coarse	6.000 mm	25 mm	0.9843	25.000	500.4	same
2 3/4"-4 UNC	Unified Coarse (UNC)	4 TPI	1	1.0000	25.400	538.9	same
3"-4 UNC	Unified Coarse (UNC)	4 TPI	1	1.0000	25.400	615.8	same
3 1/4"-4 UNC	Unified Coarse (UNC)	4 TPI	1	1.0000	25.400	692.8	same
3 1/2"-4 UNC	Unified Coarse (UNC)	4 TPI	1	1.0000	25.400	769.8	same
3 3/4"-4 UNC	Unified Coarse (UNC)	4 TPI	1	1.0000	25.400	846.8	same
4"-4 UNC	Unified Coarse (UNC)	4 TPI	1	1.0000	25.400	923.8	same

Sizes are interleaved by nominal diameter, so a metric thread sits between the inch sizes it falls between. Actual % is the engagement the listed real drill produces, which is not always the engagement asked for — that gap is the whole reason this column exists. Rows flagged in the Traditional chart column are the ones where the classic published chart lists a different drill; both are shown, and neither overrides the other.

Where This Tap Drill Chart Differs From the Traditional Chart

On 4 of 150 threads, the drill nearest the computed 75% target is not the drill the classic published chart lists. Both are shown here and on the size page; neither overrides the other.

Threads where the computed tap drill and the traditional published chart disagree
Thread	Computed here	Traditional chart	Why they differ
#12-32 UNEF	#13	3/16	Computed 75% target 0.1856 in. Nearest standard drill is #13 (0.185 in, off by 0.0006 in); the traditional published chart lists 3/16 (0.188 in, off by 0.0024 in, 69% thread). Both are shown; neither overrides the other.
9/16-18 UNF	33/64	1/2	Computed 75% target 0.5084 in. Nearest standard drill is 33/64 (0.515625 in, off by 0.0073 in); the traditional published chart lists 1/2 (0.5 in, off by 0.0084 in, 86.6% thread). Both are shown; neither overrides the other.
5/8-18 UNF	37/64	9/16	Computed 75% target 0.5709 in. Nearest standard drill is 37/64 (0.578125 in, off by 0.0073 in); the traditional published chart lists 9/16 (0.563 in, off by 0.0079 in, 85.9% thread). Both are shown; neither overrides the other.
7/8-14 UNF	13/16	51/64	Computed 75% target 0.8054 in. Nearest standard drill is 13/16 (0.8125 in, off by 0.0071 in); the traditional published chart lists 51/64 (0.797 in, off by 0.0084 in, 84.1% thread). Both are shown; neither overrides the other.

The traditional chart values are not errors — they are long-standing shop practice, and in each case they leave a heavier thread. Knowing which one you are holding is the point.

Tap Drill Sizes for NPT and NPTF Taper Pipe Threads

Taper pipe threads are cut with a taper tap into a straight drilled hole, so they have one published drill size rather than a range of thread engagements.

NPT and NPTF taper pipe thread tap drill sizes
Property	Property	Property	Tap drill			Property
Thread	Series	TPI	Tap drill	in	mm	Pipe OD in
1/16-27 NPT	NPT Taper Pipe Thread	27	E	0.2500	6.350	0.3125
1/8-27 NPT	NPT Taper Pipe Thread	27	11/32	0.3438	8.731	0.4050
1/4-18 NPT	NPT Taper Pipe Thread	18	7/16	0.4375	11.113	0.5400
3/8-18 NPT	NPT Taper Pipe Thread	18	37/64	0.5781	14.684	0.6750
1/2-14 NPT	NPT Taper Pipe Thread	14	23/32	0.7188	18.256	0.8400
3/4-14 NPT	NPT Taper Pipe Thread	14	59/64	0.9219	23.416	1.0500
1-11.5 NPT	NPT Taper Pipe Thread	12	1	1.0000	25.400	1.3150
1 1/4-11.5 NPT	NPT Taper Pipe Thread	12	1	1.0000	25.400	1.6600
1 1/2-11.5 NPT	NPT Taper Pipe Thread	12	1	1.0000	25.400	1.9000
2-11.5 NPT	NPT Taper Pipe Thread	12	1	1.0000	25.400	2.3750
2 1/2-8 NPT	NPT Taper Pipe Thread	8	1	1.0000	25.400	2.8750
3-8 NPT	NPT Taper Pipe Thread	8	1	1.0000	25.400	3.5000
3 1/2-8 NPT	NPT Taper Pipe Thread	8	1	1.0000	25.400	4.0000
4-8 NPT	NPT Taper Pipe Thread	8	1	1.0000	25.400	4.5000
5-8 NPT	NPT Taper Pipe Thread	8	1	1.0000	25.400	5.5630
6-8 NPT	NPT Taper Pipe Thread	8	1	1.0000	25.400	6.6250
8-8 NPT	NPT Taper Pipe Thread	8	1	1.0000	25.400	8.6250
10-8 NPT	NPT Taper Pipe Thread	8	1	1.0000	25.400	10.7500
12-8 NPT	NPT Taper Pipe Thread	8	1	1.0000	25.400	12.7500
14-8 NPT	NPT Taper Pipe Thread	8	1	1.0000	25.400	14.0000
16-8 NPT	NPT Taper Pipe Thread	8	1	1.0000	25.400	16.0000
18-8 NPT	NPT Taper Pipe Thread	8	1	1.0000	25.400	18.0000
20-8 NPT	NPT Taper Pipe Thread	8	1	1.0000	25.400	20.0000
24-8 NPT	NPT Taper Pipe Thread	8	1	1.0000	25.400	24.0000
1/16-27 NPTF	NPTF Dryseal Pipe Thread	27	E	0.2500	6.350	0.3125
1/8-27 NPTF	NPTF Dryseal Pipe Thread	27	11/32	0.3438	8.731	0.4050
1/4-18 NPTF	NPTF Dryseal Pipe Thread	18	7/16	0.4375	11.113	0.5400
3/8-18 NPTF	NPTF Dryseal Pipe Thread	18	37/64	0.5781	14.684	0.6750
1/2-14 NPTF	NPTF Dryseal Pipe Thread	14	23/32	0.7188	18.256	0.8400
3/4-14 NPTF	NPTF Dryseal Pipe Thread	14	59/64	0.9219	23.416	1.0500
1-11.5 NPTF	NPTF Dryseal Pipe Thread	12	1	1.0000	25.400	1.3150
1 1/4-11.5 NPTF	NPTF Dryseal Pipe Thread	12	1	1.0000	25.400	1.6600
1 1/2-11.5 NPTF	NPTF Dryseal Pipe Thread	12	1	1.0000	25.400	1.9000
2-11.5 NPTF	NPTF Dryseal Pipe Thread	12	1	1.0000	25.400	2.3750

Pipe OD is the actual outside diameter of the pipe, not the nominal pipe size in the designation — 1/2 NPT goes on 0.840 in pipe.

Common Questions About Tap Drill Sizes

What is the tap drill for 1/4-20?: The tap drill for 1/4"-20 UNC is #7 — 0.2010 in (5.105 mm) — leaving 75.4% thread engagement with a cut tap.
What thread engagement should a tap drill give?: 75% is the general shop default and what most published charts assume. Dropping to 60–65% cuts tapping torque roughly in half and greatly reduces broken taps in tough material, while costing only a few percent of thread strength — the fastener almost always fails before a 60% thread strips. This chart lets you pick anything from 50% to 85% and shows the engagement the real drill actually produces.
Why does this chart list a different drill than my wall chart?: On 4 threads the drill nearest the computed 75% target differs from the traditional published chart, which lists a smaller drill and a heavier thread. Both values are shown, with the exact figures, so you can see what you are choosing between rather than trusting one chart over the other.
Does this tap drill chart cover metric and inch threads together?: Yes — 82 inch threads and 68 metric threads are in one table, interleaved by nominal diameter, with drill sizes given in both inches and millimetres on every row.

Sources and Method for This Tap Drill Chart

Inch thread dimensionscomputed

NBS Handbook H28 (1969) Part I §§6-7 formula system (public domain, 17 U.S.C. 105)

Verified against threads-unified: 1064 checks passed across 3 harness(es)

Computed from the standard's formulas, then checked against published tables.

Metric thread dimensionscomputed+compiled

ISO 68-1 basic profile computed; ISO 261/262 diameter-pitch pairs and 6H class limits compiled

Verified against threads-metric: 215 checks passed across 3 harness(es)

Part computed from the standard's formulas, part compiled from agreeing public sources.

Pipe thread dimensionscomputed+compiled

ASME B1.20.1 taper formulas computed; pipe OD, tpi, L1 and L4 compiled

Verified against threads-pipe: 98 checks passed across 1 harness(es)

Part computed from the standard's formulas, part compiled from agreeing public sources.

Standard drill indexcomputed+compiled

bd_warehouse drill_sizes.csv (Apache-2.0) + Wikipedia Drill bit sizes (CC BY-SA 4.0); fractional and metric computed from ANSI B94.11M / BS 328 increment patterns

Verified against drills: 31 checks passed across 1 harness(es)

Part computed from the standard's formulas, part compiled from agreeing public sources.

Formula

Tap drill hole diameter — inch threads

hole Ø  =  D − (%thread ÷ 100) × (1.29904 ÷ n)

  D = basic major diameter (in)
  n = threads per inch

Tap drill hole diameter — metric threads

hole Ø  =  D − (%thread ÷ 100) × 1.29904 × P

  D = basic major diameter (mm)
  P = pitch (mm)

1.29904 = 2 × 0.64952, twice the engaged thread height per side on the 60° profile shared by the Unified and ISO systems.

Engagement the chosen drill actually produces

%thread  =  (D − drill Ø) × n ÷ 1.29904 × 100

The drill column rounds the target to a real drill from the number, letter, fractional and metric index, then this reports what that drill leaves.

Assumptions

Every drill in this chart is for a cut tap.
Drill diameters are nominal. A worn drill, run-out or a soft material that closes in behind the drill all move the finished hole, and every one of them moves it toward a heavier thread than the table says.
The chart selects the nearest standard drill to the computed target, in either direction — not the next size up. The Actual % column is what that choice costs or gains.
Inch dataset scope: Le = 1D for UNC/UNF, 9P for UNEF — determined empirically against H28 Table 2.21, resolving data-sourcing.md open item 5. EXCLUDED: form (roll) tap drills, class 1A/1B limits, class 1AR allowances, constant-pitch UN series.
Metric dataset scope: EXCLUDED: ISO 965-1 tolerance-grade formulas (not located — data-sourcing.md open item 2), 6g external limits (single-sourced), form tap drills, M68-M100 (verified demand cliff at M64).

Data last verified

August 1, 2026

How this data is built and checked →

How to Cite This Tap Drill Chart

Citation

ShopMath. "Tap Drill Chart — Inch and Metric, Every Thread Engagement." ShopMath, verified August 1, 2026, https://shopmath.org/charts/tap-drill

BibTeX

@misc{shopmath-charts-tap-drill,
  title        = {Tap Drill Chart — Inch and Metric, Every Thread Engagement},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/charts/tap-drill}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/charts/tap-drill

Every thread series · All reference tables