75% and 50% thread tap drills, the closest hardware-store 1/64" bit with its real thread percentage, and clearance holes.
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Unified inch · ASME B1.1
1/4"-20UNC
0.250" / 6.35 mm major diameter
Tap drill · 75%
#7
0.201" · 5.11 mm
75.4% thread
Hardware-store bit
13/64"
0.2031"
72.2% thread
Wrench / hex key
7/16"nut3/16"Allen
Drills & threading
Major diameter
0.250"6.35 mm · nearest 1/4"
Pitch
20 TPI UNC0.0500" · 1.27 mm
Tap drill 75%
#7 · 75.4%0.2010" · 5.11 mm
Hardware-store bit
13/64" · 72.2%0.2031" from a 1/64" set
Form / roll tap 65%
#10.2280" · 5.79 mm
STI / Helicoil
H1/4"-20 STI tap · 0.2660"
Clearance · close / free
F / H0.2570" / 0.2660"
Counterbore
7/16"0.438" · 11.11 mm
Wrenches, keys & torque
Hex head
7/16"head 0.156" · 3.97 mm tall
Nut
7/16"0.219" · 5.56 mm thick · flange OD 0.590" · 14.99 mm
Socket cap key
3/16"head ⌀ 0.375" · 9.52 mm
Button / flat · set
5/32" · 1/8"
Flat washer SAE
0.281" / 0.625"USS: 0.312" / 0.734"
SAE Gr 2 · Gr 5
66 in-lb · 101 in-lb
SAE Grade 8
12 ft-lb
18-8 stainless
78 in-lbGalling risk: use anti-seize on stainless and tighten slowly.
Dry steel (K=0.20, ~75% proof load). Reduce by 20–25% if lubricated.
Full chart 70 rows
Sort any column, filter by series, print a shop sheet. Click a row to load it on the gauge.
Tap & clearance drill chart
70 rows
Tap & clearance drill chart · ScrewBoltChart
screwboltchart.com · inch-first · tap drills 75% (soft metals) · torque dry, K=0.20 · verify against the governing standard
3/64" · 0.0469" · 1.19 mm · 80.7%
#55 · 49.3%
3/64" → 80.8%
#52 (0.0635")
#50 (0.0700")
#53 · 0.0595" · 1.51 mm · 66.5%
1/16" · 51.7%
1/16" → 51.7%
#48 (0.0760")
#46 (0.0810")
#53 · 0.0595" · 1.51 mm · 74.8%
#52 · 52.7%
1/16" → 58.2%
#48 (0.0760")
#46 (0.0810")
#50 · 0.0700" · 1.78 mm · 69%
#49 · 56%
5/64" → 33.9%
#43 (0.0890")
#41 (0.0960")
#50 · 0.0700" · 1.78 mm · 78.8%
#48 · 49.3%
5/64" → 38.8%
#43 (0.0890")
#41 (0.0960")
#47 · 0.0785" · 1.99 mm · 75.7%
#44 · 48%
5/64" → 77.1%
#37 (0.1040")
#35 (0.1100")
#45 · 0.0820" · 2.08 mm · 73.3%
#44 · 56%
3/32" → 22.6%
#37 (0.1040")
#35 (0.1100")
#43 · 0.0890" · 2.26 mm · 70.8%
3/32" · 56.2%
3/32" → 56.2%
#32 (0.1160")
#30 (0.1285")
#42 · 0.0935" · 2.37 mm · 68.4%
#40 · 51.7%
3/32" → 67.4%
#32 (0.1160")
#30 (0.1285")
#38 · 0.1015" · 2.58 mm · 72.4%
#36 · 57%
7/64" → 48.1%
#30 (0.1285")
#29 (0.1360")
#37 · 0.1040" · 2.64 mm · 71.1%
7/64" · 52.9%
7/64" → 52.9%
#30 (0.1285")
#29 (0.1360")
#36 · 0.1065" · 2.71 mm · 77.6%
#32 · 54.2%
7/64" → 70.5%
#27 (0.1440")
#25 (0.1495")
#33 · 0.1130" · 2.87 mm · 77%
#31 · 55.4%
1/8" → 40%
#27 (0.1440")
#25 (0.1495")
#29 · 0.1360" · 3.45 mm · 69%
9/64" · 57.6%
9/64" → 57.6%
#18 (0.1695")
#16 (0.1770")
#29 · 0.1360" · 3.45 mm · 77.6%
#27 · 55.4%
9/64" → 64.8%
#18 (0.1695")
#16 (0.1770")
#25 · 0.1495" · 3.8 mm · 74.8%
#20 · 53.6%
5/32" → 62.4%
#9 (0.1960")
#7 (0.2010")
#21 · 0.1590" · 4.04 mm · 76.4%
#19 · 59.1%
5/32" → 83.1%
#9 (0.1960")
#7 (0.2010")
#16 · 0.1770" · 4.5 mm · 72.1%
#13 · 57.3%
11/64" → 81.5%
#2 (0.2210")
#1 (0.2280")
#14 · 0.1820" · 4.62 mm · 73.3%
#11 · 53.9%
3/16" → 61.4%
#2 (0.2210")
#1 (0.2280")
#7 · 0.2010" · 5.11 mm · 75.4%
#3 · 57%
13/64" → 72.2%
F (0.2570")
H (0.2660")
#3 · 0.2130" · 5.41 mm · 79.8%
#1 · 47.4%
7/32" → 67.4%
F (0.2570")
H (0.2660")
F · 0.2570" · 6.53 mm · 76.9%
I · 56.1%
17/64" → 65%
P (0.3230")
Q (0.3320")
I · 0.2720" · 6.91 mm · 74.8%
9/32" · 57.7%
9/32" → 57.7%
P (0.3230")
Q (0.3320")
5/16" · 0.3125" · 7.94 mm · 77%
Q · 53%
5/16" → 77%
W (0.3860")
X (0.3970")
Q · 0.3320" · 8.43 mm · 79.4%
11/32" · 57.7%
11/32" → 57.7%
W (0.3860")
X (0.3970")
U · 0.3680" · 9.35 mm · 74.9%
W · 55.5%
3/8" → 67.4%
29/64" (0.4531")
15/32" (0.4688")
25/64" · 0.3906" · 9.92 mm · 72.2%
Y · 51.6%
25/64" → 72.2%
29/64" (0.4531")
15/32" (0.4688")
27/64" · 0.4219" · 10.72 mm · 78.2%
29/64" · 46.9%
27/64" → 78.2%
33/64" (0.5156")
17/32" (0.5313")
29/64" · 0.4531" · 11.51 mm · 72.2%
15/32" · 48.1%
29/64" → 72.2%
33/64" (0.5156")
17/32" (0.5313")
31/64" · 0.4844" · 12.3 mm · 72.1%
1/2" · 57.7%
31/64" → 72.2%
37/64" (0.5781")
19/32" (0.5938")
33/64" · 0.5156" · 13.1 mm · 65%
17/32" · 43.3%
33/64" → 65%
37/64" (0.5781")
19/32" (0.5938")
17/32" · 0.5313" · 13.49 mm · 79.3%
9/16" · 52.9%
17/32" → 79.4%
41/64" (0.6406")
21/32" (0.6563")
37/64" · 0.5781" · 14.68 mm · 65%
19/32" · 43.3%
37/64" → 65%
41/64" (0.6406")
21/32" (0.6563")
21/32" · 0.6563" · 16.67 mm · 72.1%
11/16" · 48.1%
21/32" → 72.2%
49/64" (0.7656")
25/32" (0.7813")
11/16" · 0.6875" · 17.46 mm · 77%
45/64" · 57.7%
11/16" → 77%
49/64" (0.7656")
25/32" (0.7813")
49/64" · 0.7656" · 19.45 mm · 75.8%
51/64" · 54.1%
49/64" → 75.8%
57/64" (0.8906")
29/32" (0.9063")
13/16" · 0.8125" · 20.64 mm · 67.4%
53/64" · 50.5%
13/16" → 67.4%
57/64" (0.8906")
29/32" (0.9063")
7/8" · 0.8750" · 22.23 mm · 77%
29/32" · 57.7%
7/8" → 77%
1-1/64" (1.0156")
1-1/32" (1.0313")
59/64" · 0.9219" · 23.42 mm · 72.1%
15/16" · 57.7%
59/64" → 72.2%
1-1/64" (1.0156")
1-1/32" (1.0313")
1.25 mm · 77%
1.3 mm
3/64" → 90%
1.7 mm
1.8 mm
1.6 mm · 77%
1.7 mm
1/16" → 79.4%
2.2 mm
2.4 mm
2.05 mm · 77%
2.2 mm
5/64" → 88.2%
2.7 mm
2.9 mm
2.5 mm · 77%
2.6 mm
3/32" → 95.3%
3.2 mm
3.4 mm
2.9 mm · 77%
3.1 mm
7/64" → 92.6%
3.7 mm
3.9 mm
3.3 mm · 77%
3.5 mm
1/8" → 90.7%
4.3 mm
4.5 mm
3.5 mm · 77%
3.6 mm
9/64" → 65.9%
4.3 mm
4.5 mm
4.2 mm · 77%
4.4 mm
11/64" → 61%
5.3 mm
5.5 mm
4.5 mm · 77%
4.6 mm
11/64" → 97.7%
5.3 mm
5.5 mm
5 mm · 77%
5.3 mm
13/64" → 64.7%
6.4 mm
6.6 mm
5.25 mm · 77%
5.5 mm
13/64" → 86.3%
6.4 mm
6.6 mm
6.8 mm · 73.9%
7.1 mm
17/64" → 77.2%
8.4 mm
9 mm
7 mm · 77%
7.3 mm
9/32" → 65.9%
8.4 mm
9 mm
8.5 mm · 77%
8.9 mm
21/64" → 85.5%
10.5 mm
11 mm
8.8 mm · 73.9%
9.1 mm
11/32" → 78.1%
10.5 mm
11 mm
9 mm · 77%
9.3 mm
23/64" → 67.1%
10.5 mm
11 mm
10.2 mm · 79.2%
10.7 mm
13/32" → 74%
13 mm
13.5 mm
10.5 mm · 77%
10.9 mm
13/32" → 86.3%
13 mm
13.5 mm
10.8 mm · 73.9%
11.1 mm
27/64" → 79.1%
13 mm
13.5 mm
12 mm · 77%
12.6 mm
15/32" → 80.6%
15 mm
15.5 mm
12.5 mm · 77%
12.9 mm
31/64" → 87.1%
15 mm
15.5 mm
14 mm · 77%
14.6 mm
35/64" → 81.2%
17 mm
17.5 mm
14.5 mm · 77%
14.9 mm
37/64" → 67.5%
17 mm
17.5 mm
15.5 mm · 77%
16.2 mm
39/64" → 77.7%
19 mm
20 mm
16.5 mm · 77%
16.9 mm
21/32" → 68.3%
19 mm
20 mm
17.5 mm · 77%
18.2 mm
11/16" → 78.1%
21 mm
22 mm
18.5 mm · 77%
18.9 mm
47/64" → 69.1%
21 mm
22 mm
19.5 mm · 77%
20.2 mm
49/64" → 78.6%
23 mm
24 mm
20.5 mm · 77%
20.9 mm
13/16" → 69.9%
23 mm
24 mm
21 mm · 77%
21.9 mm
53/64" → 76.1%
25 mm
26 mm
22 mm · 77%
22.6 mm
55/64" → 83.6%
25 mm
26 mm
05 / Field guide
Drill once. Tap clean.
A tap drill sets how deep the thread will be. The charts aim for 75% of the full thread, which is nearly as strong as 100% and much easier to cut. Here is how to choose when you don’t have the exact drill.
Thread percentage, in one line
Thread percentage is how much of the full thread depth the tap actually cuts. 100% sounds ideal, but a 75% thread is only about 5% weaker in a steel nut and takes roughly a third of the torque to tap. That is why every tap chart is built around 75%.
In aluminium, brass, plastics and mild steel use the 75% drill. In stainless steel, tool steel or anything hard and gummy, step up to a drill that gives 50–60%. The thread is still stronger than the screw in most joints, and the tap is far less likely to break.
The hardware-store bit
Most people own a 1/64" fractional set, not number and letter drills. The chart lists the nearest fractional bit that doesn’t exceed 85% thread, with the percentage it actually gives. For a 1/4"-20 that is 13/64" at 72%, which is excellent. For an #8-32 it is 9/64" at 58%: usable in steel, weak in aluminium.
Tapping technique
Drill straight, chamfer the hole slightly so the tap starts square, and use cutting fluid. Turn the tap in a quarter to half a turn, then back it off until the chip breaks. In blind holes, drill deeper than the thread you need and switch from a taper tap to a bottoming tap for the last few threads. Most broken taps come from a hole that is too small, a tap that isn’t square, or chips packed in the flutes.
Questions
Answered plainly.
01What drill size for a 1/4-20 tap?+−
A #7 drill (0.201") for 75% thread. From a fractional set, 13/64" (0.2031") gives about 72%. The clearance hole for a 1/4" screw is F (close fit) or H (free fit).
02What drill size for an 8-32 tap?+−
A #29 drill (0.136") gives 69%. A 9/64" bit gives about 58%, fine in steel but on the weak side for aluminium.
03What is a clearance hole?+−
A hole the screw passes through without threading, in the part being clamped. Close fit locates parts accurately; free fit is easier to line up across several holes.
04What drill for a 3/8-16 tap?+−
A 5/16" drill gives about 77% thread. For a lighter thread in stainless, use the letter Q drill (0.332").
05Can I use a metric drill for an inch tap?+−
Yes, if it lands at a sensible thread percentage. Search the drill size in mm above and the reverse lookup shows which taps it suits.
06Why does the chart say 69% instead of 75%?+−
Drills come in fixed sizes, so the standard choice is the nearest drill to 75%, which can land anywhere from about 65% to 80%. The chart shows the real percentage for every drill.
07What drill size for an M5 tap?+−
A 4.2 mm drill gives about 77% thread in M5 × 0.8. In stainless, 4.4 mm gives a lighter, easier thread.
08What drill for a 1/2-13 tap?+−
A 27/64" drill (0.4219") for about 78% thread. The close-fit clearance hole for 1/2" is 33/64" and the free fit is 17/32".
09What drill for a 5/16-18 tap?+−
The letter F drill (0.257") for about 77% thread. From a fractional set, 1/4" gives about 87%, which is too much for a hand tap, so use 17/64" (65%) instead.
10Why did my tap break?+−
Usually the hole was too small, the tap entered at an angle, or chips packed the flutes. Check the drill against the chart, start square with a guide block, break the chip every half turn and use cutting fluid.