Tap Drill Calculator

Works both ways. Thread → drill: pick a thread, say what you are cutting it in and which drills you own, and it gives the closest drill you can actually use with the percent of thread it leaves. Drill → threads: pick a drill out of your index and see every thread it will tap. It covers all 124 threads in the chart plus any custom thread you type in, and whatever you set ends up in the address bar, so you can send someone the exact result.

Thread

Material

The general-purpose target

Tap type

Drills you have

Number (#1-#80)Letter (A-Z)FractionalMetric

Untick a set you do not own and the answer changes to a drill you actually have.

Best drill you have, cut tap, 70% target

5.2 mm0.2047 in69.7% thread

Theoretical hole 0.2045 in (5.2 mm)

One size down, deeper thread#6 0.204 in · 70.8%

One size up, easier tapping#5 0.2055 in · 68.5%

The published chart drill for 1/4-20 is #7 at 75.4% thread. Full page for 1/4-20 →

What the numbers mean

Theoretical hole is what the formula gives before rounding. It is almost never a drill that exists, which is the whole reason for the next line.

Best drill you have is the closest size in the sets you ticked, with the percent of thread it really leaves. That figure, not the target you asked for, is what you will get in the part.

One size up and down show the neighbouring drills and their percentages, because the decision is usually between two drills rather than in the abstract. A drill up means easier tapping and a weaker thread; a drill down means the opposite.

The formulas it uses

No hidden lookup tables. Every answer comes from these:

Inch percent of thread: % = 76.98 × TPI × (major − drill)
Metric percent of thread: % = (major − drill) ÷ (0.01299 × pitch)
Cut tap hole: drill = major − 0.01299 × % × pitch
Form tap hole: drill = major − 0.0068 × % × pitch

For inch threads, pitch is 1 ÷ TPI. Lengths stay in one unit throughout: inches for inch threads, millimetres for metric ones.

Working backwards from a drill

The Drill → threads tab answers the other question: you have a drill in your hand, or one size left in the index, and you want to know what it is good for. Pick it and the calculator lists every thread in the chart it taps between 50% and 85% of thread, deepest first, flagging any size where that drill is the published chart drill.

Threads outside that band are left out on purpose. Below 50% there is too little thread to hold, and above 85% the torque climbs far enough that the tap is at real risk. A drill that lists nothing at all is simply falling between sizes.

It is also the quickest way to see how little difference a drill size makes at a fine pitch, and how much it makes at a coarse one.

Sharing a result

Everything you set goes into the address bar as you set it, so the link in your browser is always the result you are looking at. Copy it out of the bar, or use the button at the top of the calculator. Open it on another machine and the calculator comes back exactly as you left it. Anything still at its default is left out of the link, which keeps it short.

Cut tap or form tap

If your tap has flutes and makes chips, it is a cutting tap. If it is smooth and lobed with no flutes, it is a form or roll tap, and it needs a noticeably larger hole because it pushes metal into the thread rather than cutting it out. Switching the toggle changes the formula, not just the label. Form-tap results are a starting point: check them against the chart for the tap you are holding, and see the form tap drill chart for the published sizes.

Every published tap drill

The full chart, in case you would rather look it up than calculate it. These are the standard published drills for each size; the calculator above is for when you want something other than the standard.

124 sizes

All published tap drills: size, tap drill, decimal inch, millimetres, percent of thread, hard-material drill, form-tap drill, basic minor diameter and pitch.
					tough metal		roll tap
M1 x 0.25	0.75 mm	0.0295	0.75	77.0	0.8 mm	61.6	0.9 mm	0.0287	0.25 mm
M1.2 x 0.25	0.95 mm	0.0374	0.95	77.0	1 mm	61.6	1.1 mm	0.0366	0.25 mm
M1.4 x 0.3	1.1 mm	0.0433	1.10	77.0	1.15 mm	64.2	1.25 mm	0.0423	0.3 mm
#0-80	3/64″	0.0469	1.19	80.7	#55	49.3	#54	0.0465	80 TPI
M1.6 x 0.35	1.25 mm	0.0492	1.25	77.0	1.35 mm	55.0	1.45 mm	0.0481	0.35 mm
#1-64	#53	0.0595	1.51	66.5	#53	66.5	#51	0.0561	64 TPI
#1-72	#53	0.0595	1.51	74.8	1/16″	58.2	#51	0.058	72 TPI
M2 x 0.4	1.6 mm	0.063	1.60	77.0	1.7 mm	57.7	1.8 mm	0.0617	0.4 mm
#2-56	#50	0.070	1.78	69.0	#49	56.0	5/64″	0.0667	56 TPI
#2-64	#50	0.070	1.78	78.8	#49	64.0	#47	0.0691	64 TPI
M2.5 x 0.45	2.05 mm	0.0807	2.05	77.0	2.15 mm	59.9	2.3 mm	0.0793	0.45 mm
#3-48	#47	0.0785	1.99	75.7	#45	62.8	#43	0.0764	48 TPI
#3-56	#45	0.082	2.08	73.3	#44	56.0	#43	0.0797	56 TPI
#4-40	#43	0.089	2.26	70.8	#42	57.0	#38	0.0849	40 TPI
#4-48	#42	0.0935	2.38	68.4	#41	59.1	#37	0.0894	48 TPI
M3 x 0.5	2.5 mm	0.0984	2.50	77.0	2.6 mm	61.6	2.8 mm	0.0968	0.5 mm
#5-40	#38	0.1015	2.58	72.4	#36	57.0	#33	0.0979	40 TPI
#5-44	#37	0.104	2.64	71.1	#36	62.7	#32	0.1004	44 TPI
M3.5 x 0.6	2.9 mm	0.1142	2.90	77.0	3 mm	64.2	3.2 mm	0.1122	0.6 mm
#6-32	#36	0.1065	2.71	77.6	#33	61.6	1/8″	0.1042	32 TPI
#6-40	#33	0.113	2.87	77.0	#31	55.4	#30	0.1109	40 TPI
M4 x 0.7	3.3 mm	0.1299	3.30	77.0	3.5 mm	55.0	3.7 mm	0.1276	0.7 mm
#8-32	#29	0.136	3.45	69.0	#28	57.9	#25	0.1302	32 TPI
#8-36	#29	0.136	3.45	77.6	#27	55.4	#24	0.1339	36 TPI
#10-24	#25	0.1495	3.80	74.8	#22	61.0	11/64″	0.1449	24 TPI
#10-32	#21	0.159	4.04	76.4	#19	59.1	#16	0.1562	32 TPI
M5 x 0.8	4.2 mm	0.1654	4.20	77.0	4.4 mm	57.7	4.6 mm	0.1628	0.8 mm
#12-24	#16	0.177	4.50	72.1	#13	57.3	#8	0.1709	24 TPI
#12-28	#14	0.182	4.62	73.3	3/16″	61.4	#7	0.1773	28 TPI
#12-32	#13	0.185	4.70	76.4	#11	61.6	13/64″	0.1822	32 TPI
M6 x 1	5 mm	0.1969	5.00	77.0	5.2 mm	61.6	5.6 mm	0.1936	1 mm
1/4-20	#7	0.201	5.11	75.4	#3	57.0	#1	0.1959	20 TPI
1/4-28	#3	0.213	5.41	79.8	#2	62.5	15/64″	0.2113	28 TPI
1/4-32	7/32″	0.2188	5.56	76.9	#1	54.2	15/64″	0.2162	32 TPI
M7 x 1	6 mm	0.2362	6.00	77.0	6.2 mm	61.6	6.6 mm	0.233	1 mm
5/16-18	F	0.257	6.53	76.9	I	56.1	L	0.2524	18 TPI
5/16-24	I	0.272	6.91	74.8	K	58.2	M	0.2674	24 TPI
5/16-32	9/32″	0.2812	7.14	77.1	L	55.4	19/64″	0.2787	32 TPI
M8 x 1.25	6.8 mm	0.2677	6.80	73.9	7 mm	61.6	7.4 mm	0.2617	1.25 mm
M8 x 1	7 mm	0.2756	7.00	77.0	7.2 mm	61.6	7.6 mm	0.2723	1 mm
M8 x 0.75	7.3 mm	0.2874	7.30	71.9	7.4 mm	61.6	7.7 mm	0.283	0.75 mm
3/8-16	5/16″	0.3125	7.94	77.0	21/64″	57.8	S	0.3073	16 TPI
3/8-24	Q	0.332	8.43	79.4	11/32″	57.6	T	0.3299	24 TPI
3/8-32	11/32″	0.3438	8.73	76.9	S	66.5	23/64″	0.3412	32 TPI
M10 x 1.5	8.5 mm	0.3346	8.50	77.0	8.8 mm	61.6	9.3 mm	0.3298	1.5 mm
M10 x 1.25	8.8 mm	0.3465	8.80	73.9	9 mm	61.6	9.4 mm	0.3404	1.25 mm
M10 x 1	9 mm	0.3543	9.00	77.0	9.2 mm	61.6	9.6 mm	0.3511	1 mm
M10 x 0.75	9.3 mm	0.3661	9.30	71.9	9.4 mm	61.6	9.7 mm	0.3617	0.75 mm
7/16-14	U	0.368	9.35	74.9	W	55.5	13/32″	0.3602	14 TPI
7/16-20	25/64″	0.3906	9.92	72.2	X	62.4	Z	0.3834	20 TPI
7/16-28	Y	0.404	10.26	72.2	Z	52.8	27/64″	0.3988	28 TPI
M12 x 1.75	10.2 mm	0.4016	10.20	79.2	10.6 mm	61.6	11.2 mm	0.3979	1.75 mm
M12 x 1.5	10.5 mm	0.4134	10.50	77.0	10.8 mm	61.6	11.3 mm	0.4085	1.5 mm
M12 x 1.25	10.8 mm	0.4252	10.80	73.9	11 mm	61.6	11.4 mm	0.4192	1.25 mm
M12 x 1	11 mm	0.4331	11.00	77.0	11.2 mm	61.6	11.6 mm	0.4298	1 mm
1/2-13	27/64″	0.4219	10.72	78.2	7/16″	62.5	15/32″	0.4167	13 TPI
1/2-20	29/64″	0.4531	11.51	72.2	15/32″	48.0	31/64″	0.4459	20 TPI
1/2-28	15/32″	0.4688	11.91	67.2	15/32″	67.2	31/64″	0.4613	28 TPI
M14 x 2	12 mm	0.4724	12.00	77.0	12.4 mm	61.6	13.1 mm	0.4659	2 mm
M14 x 1.5	12.5 mm	0.4921	12.50	77.0	12.8 mm	61.6	13.3 mm	0.4872	1.5 mm
M14 x 1.25	12.8 mm	0.5039	12.80	73.9	13 mm	61.6	13.4 mm	0.4979	1.25 mm
9/16-12	31/64″	0.4844	12.30	72.1	1/2″	57.7	17/32″	0.4723	12 TPI
9/16-18	33/64″	0.5156	13.10	65.0	33/64″	65.0	17/32″	0.5024	18 TPI
9/16-24	33/64″	0.5156	13.10	86.6	17/32″	57.8	35/64″	0.5174	24 TPI
5/8-11	17/32″	0.5312	13.49	79.4	35/64″	66.1	37/64″	0.5266	11 TPI
5/8-18	37/64″	0.5781	14.68	65.0	37/64″	65.0	19/32″	0.5649	18 TPI
5/8-24	37/64″	0.5781	14.68	86.6	19/32″	57.6	39/64″	0.5799	24 TPI
M16 x 2	14 mm	0.5512	14.00	77.0	14.4 mm	61.6	15.1 mm	0.5447	2 mm
M16 x 1.5	14.5 mm	0.5709	14.50	77.0	14.8 mm	61.6	15.3 mm	0.566	1.5 mm
M16 x 1	15 mm	0.5906	15.00	77.0	15.2 mm	61.6	15.6 mm	0.5873	1 mm
M18 x 2.5	15.5 mm	0.6102	15.50	77.0	16.1 mm	58.5	16.9 mm	0.6021	2.5 mm
M18 x 2	16 mm	0.6299	16.00	77.0	16.4 mm	61.6	17.1 mm	0.6234	2 mm
M18 x 1.5	16.5 mm	0.6496	16.50	77.0	16.8 mm	61.6	17.3 mm	0.6447	1.5 mm
M18 x 1	17 mm	0.6693	17.00	77.0	17.2 mm	61.6	17.6 mm	0.666	1 mm
3/4-10	21/32″	0.6562	16.67	72.2	43/64″	60.1	45/64″	0.6417	10 TPI
3/4-16	11/16″	0.6875	17.46	77.0	45/64″	57.8	23/32″	0.6823	16 TPI
3/4-20	45/64″	0.7031	17.86	72.2	23/32″	48.0	47/64″	0.6959	20 TPI
M20 x 2.5	17.5 mm	0.689	17.50	77.0	18.1 mm	58.5	18.9 mm	0.6809	2.5 mm
M20 x 2	18 mm	0.7087	18.00	77.0	18.4 mm	61.6	19.1 mm	0.7022	2 mm
M20 x 1.5	18.5 mm	0.7283	18.50	77.0	18.8 mm	61.6	19.3 mm	0.7235	1.5 mm
M20 x 1	19 mm	0.748	19.00	77.0	19.2 mm	61.6	19.6 mm	0.7448	1 mm
M22 x 2.5	19.5 mm	0.7677	19.50	77.0	20 mm	61.6	21 mm	0.7596	2.5 mm
M22 x 2	20 mm	0.7874	20.00	77.0	20.5 mm	57.7	21 mm	0.7809	2 mm
M22 x 1.5	20.5 mm	0.8071	20.50	77.0	21 mm	51.3	21.5 mm	0.8022	1.5 mm
7/8-9	49/64″	0.7656	19.45	75.8	25/32″	65.0	53/64″	0.7547	9 TPI
7/8-14	13/16″	0.8125	20.64	67.4	13/16″	67.4	27/32″	0.7977	14 TPI
7/8-20	53/64″	0.8281	21.03	72.2	27/32″	48.0	55/64″	0.8209	20 TPI
M24 x 3	21 mm	0.8268	21.00	77.0	21.5 mm	64.2	22.5 mm	0.817	3 mm
M24 x 2	22 mm	0.8661	22.00	77.0	22.5 mm	57.7	23 mm	0.8596	2 mm
M24 x 1.5	22.5 mm	0.8858	22.50	77.0	23 mm	51.3	23.5 mm	0.8809	1.5 mm
1-8	7/8″	0.875	22.23	77.0	29/32″	57.8	15/16″	0.8647	8 TPI
1-12	59/64″	0.9219	23.42	72.1	15/16″	57.7	31/32″	0.9098	12 TPI
1-20	61/64″	0.9531	24.21	72.2	31/32″	48.0	63/64″	0.9459	20 TPI
M27 x 3	24 mm	0.9449	24.00	77.0	24.5 mm	64.2	25.5 mm	0.9351	3 mm
M27 x 2	25 mm	0.9843	25.00	77.0	25.5 mm	57.7	26 mm	0.9778	2 mm
M27 x 1.5	25.5 mm	1.0039	25.50	77.0	26 mm	51.3	26.5 mm	0.9991	1.5 mm
1-1/8-7	63/64″	0.9844	25.00	75.8	1-1/64″	59.0	1-1/16″	0.9704	7 TPI
1-1/8-12	1-3/64″	1.0469	26.59	72.1	1-1/16″	57.7	1-3/32″	1.0348	12 TPI
M30 x 3.5	26.5 mm	1.0433	26.50	77.0	27.5 mm	55.0	28.5 mm	1.0319	3.5 mm
M30 x 2	28 mm	1.1024	28.00	77.0	28.5 mm	57.7	29 mm	1.0959	2 mm
M30 x 1.5	28.5 mm	1.122	28.50	77.0	29 mm	51.3	29.5 mm	1.1172	1.5 mm
1-1/4-7	1-7/64″	1.1094	28.18	75.8	1-9/64″	59.0	1-3/16″	1.0954	7 TPI
1-1/4-12	1-11/64″	1.1719	29.77	72.1	1-3/16″	57.7	1-7/32″	1.1598	12 TPI
M33 x 3.5	29.5 mm	1.1614	29.50	77.0	30.5 mm	55.0	31.5 mm	1.150	3.5 mm
M33 x 2	31 mm	1.2205	31.00	77.0	31.5 mm	57.7	32 mm	1.214	2 mm
1-3/8-6	1-7/32″	1.2188	30.96	72.1	1-1/4″	57.7	1-19/64″	1.1946	6 TPI
1-3/8-12	1-19/64″	1.2969	32.94	72.1	1-5/16″	57.7	1-11/32″	1.2848	12 TPI
M36 x 4	32 mm	1.2598	32.00	77.0	33 mm	57.7	34 mm	1.2469	4 mm
M36 x 3	33 mm	1.2992	33.00	77.0	33.5 mm	64.2	34.5 mm	1.2894	3 mm
M36 x 2	34 mm	1.3386	34.00	77.0	34.5 mm	57.7	35 mm	1.3321	2 mm
1-1/2-6	1-11/32″	1.3438	34.13	72.1	1-3/8″	57.7	1-27/64″	1.3196	6 TPI
1-1/2-12	1-27/64″	1.4219	36.12	72.1	1-7/16″	57.7	1-15/32″	1.4098	12 TPI
M39 x 4	35 mm	1.378	35.00	77.0	36 mm	57.7	37 mm	1.365	4 mm
M42 x 4.5	37.5 mm	1.4764	37.50	77.0	38.5 mm	59.9	40 mm	1.4618	4.5 mm
M42 x 3	39 mm	1.5354	39.00	77.0	39.5 mm	64.2	40.5 mm	1.5257	3 mm
M42 x 2	40 mm	1.5748	40.00	77.0	40.5 mm	57.7	41 mm	1.5683	2 mm
M45 x 4.5	40.5 mm	1.5945	40.50	77.0	41.5 mm	59.9	43 mm	1.5799	4.5 mm
M48 x 5	43 mm	1.6929	43.00	77.0	44 mm	61.6	46 mm	1.6767	5 mm
M48 x 3	45 mm	1.7717	45.00	77.0	45.5 mm	64.2	46.5 mm	1.7619	3 mm
M48 x 2	46 mm	1.811	46.00	77.0	46.5 mm	57.7	47 mm	1.8045	2 mm
M52 x 5	47 mm	1.8504	47.00	77.0	48 mm	61.6	50 mm	1.8341	5 mm
M56 x 5.5	50.5 mm	1.9882	50.50	77.0	51.5 mm	63.0	53.5 mm	1.9703	5.5 mm
M60 x 5.5	54.5 mm	2.1457	54.50	77.0	55.5 mm	63.0	57.5 mm	2.1278	5.5 mm
M64 x 6	58 mm	2.2835	58.00	77.0	59.5 mm	57.7	60 mm	2.264	6 mm

Tap a column heading to sort. Every row links to that size's full page.

Tap drill calculator: common questions

How does this calculator work out the drill size?

It takes the thread's major diameter and pitch, applies drill = major minus 0.01299 times percent of thread times pitch for a cutting tap (or 0.0068 for a form tap), then finds the closest real drill in the sets you said you own and reports the percent of thread that drill actually leaves.

Why does the answer differ from the chart drill?

Because the chart drill targets about 75% of thread and you may have asked for something else. Pick the mild steel or stainless preset and you will get a larger hole than the chart, on purpose: less thread depth, much less tapping torque.

What percent of thread should I aim for?

About 75% in aluminium, brass and plastic; around 70% in mild steel; 60 to 65% in stainless and tool steel, and in any deep hole. Going above 80% buys very little strength and a great deal of torque.

What does the drill sets option do?

It limits the answer to drills you actually have. Untick metric if you own no metric drills and the calculator will only offer number, letter and fractional sizes, with the percent of thread the closest one gives.

Can I use it for a thread that is not in the list?

Yes. Choose "Custom thread" at the bottom of the picker and enter a major diameter with either threads per inch or a metric pitch. The same formulas apply.

How do I find out what a drill I already have is good for?

Switch to the "Drill to threads" tab and pick the drill. It lists every thread in the chart that drill taps between 50% and 85% of thread, deepest first, and marks the sizes where it is the published chart drill.

Can I share a result?

Yes. The settings are written into the page address as you change them, so the link in your browser is always the result on screen. Copy it from the address bar, or use the copy button above the calculator.

Does it work without JavaScript?

The calculator itself needs JavaScript, but the full chart of all published tap drills is on this page below it as plain HTML, so the page still answers the question.