The working formulas
Metric: tap drill = D - (percent thread x pitch) / 76.98.
Inch: tap drill = D - percent thread / (76.98 x TPI).
At 75 percent these collapse to D - 0.9743 x pitch and D - 0.9743 / TPI.
Workshop reference
Tap Drill Chart
181 tap drill sizes across metric coarse and fine, UNC, UNF, Whitworth, BSF, brass and BA, with the thread engagement math most charts leave out. Where the published charts genuinely disagree, both numbers are shown and the reason is explained rather than quietly picked.
The short answer
For metric threads, subtract the pitch from the diameter: an M10 x 1.5 takes an 8.5 mm drill. That gives about 77 percent thread engagement, not the 75 percent most charts claim. For inch threads, tap drill = D minus (percent thread / (76.98 x TPI)).
Why two charts give two different answers for the same thread
Because there are two different denominators in use and almost nobody says which one they are using. Full thread depth is 1.29904 x pitch. Maximum depth of engagement, the actual flank contact between bolt and nut, is 1.08253 x pitch. The tapping industry quotes percent of thread against the first number. So the same hole can honestly be called two different percentages.
| Hole size | On the 1.299 industry scale | On the 1.0825 engagement scale |
|---|---|---|
| D - 0.9743 P | 75.0 percent, the chart convention | 90.0 percent |
| D - P, the rule of thumb | 77.0 percent | 92.4 percent |
| D - 1.0825 P, basic minor diameter | 83.3 percent | 100 percent |
| D - 1.299 P | 100 percent | 120 percent, deeper than the form needs |
Read the bottom row again. On the industry scale, 100 percent thread is a hole smaller than the basic minor diameter, so the tap is cutting below the standard thread form for no strength gain at all. That is the real reason nobody targets it, and it is why the charts settled on 75 percent.
Whitworth and BA are different
The minus-the-pitch habit is a 60 degree rule. Whitworth is 55 degrees, so the 75 percent size is D - 0.960 x pitch. British Association is 47.5 degrees, so it is D - 0.900 x pitch.
Both were checked against the published BA chart row by row and reproduce it.
75 percent is not the target
Kennametal publishes that 100 percent thread is only 5 percent stronger than 75 percent and takes three times the power, and recommends 68 to 70 percent. Guhring recommends 60 to 70 percent.
Every manufacturer that publishes a figure recommends less than the chart convention.
Metric coarse tap drill sizes
All 37 sizes from M1 to M56. The tap drill column is the manufacturer chart figure. The next column is diameter minus pitch, so you can see exactly where the two disagree, which is only at M8 and M12 across this whole range.
| Thread | Pitch (mm) | Tap drill (mm) | D minus pitch (mm) | Minor dia (mm) | Note |
|---|---|---|---|---|---|
| M1 | 0.25 | 0.75 | 0.75 | 0.729 | |
| M1.1 | 0.25 | 0.85 | 0.85 | 0.829 | second choice size |
| M1.2 | 0.25 | 0.95 | 0.95 | 0.929 | |
| M1.4 | 0.3 | 1.1 | 1.1 | 1.075 | second choice size |
| M1.6 | 0.35 | 1.25 | 1.25 | 1.221 | |
| M1.8 | 0.35 | 1.45 | 1.45 | 1.421 | second choice size |
| M2 | 0.4 | 1.6 | 1.6 | 1.567 | |
| M2.2 | 0.45 | 1.75 | 1.75 | 1.713 | second choice size |
| M2.5 | 0.45 | 2.05 | 2.05 | 2.013 | some charts say 2.1 |
| M3 | 0.5 | 2.5 | 2.5 | 2.459 | |
| M3.5 | 0.6 | 2.9 | 2.9 | 2.850 | |
| M4 | 0.7 | 3.3 | 3.3 | 3.242 | |
| M4.5 | 0.75 | 3.7 | 3.75 | 3.688 | chart rounds down to a stocked drill |
| M5 | 0.8 | 4.2 | 4.2 | 4.134 | |
| M6 | 1 | 5 | 5 | 4.917 | |
| M7 | 1 | 6 | 6 | 5.917 | |
| M8 | 1.25 | 6.8 | 6.75 | 6.647 | charts disagree |
| M9 | 1.25 | 7.8 | 7.75 | 7.647 | second choice size |
| M10 | 1.5 | 8.5 | 8.5 | 8.376 | |
| M11 | 1.5 | 9.5 | 9.5 | 9.376 | second choice size |
| M12 | 1.75 | 10.2 | 10.25 | 10.106 | charts disagree |
| M14 | 2 | 12 | 12 | 11.835 | |
| M16 | 2 | 14 | 14 | 13.835 | |
| M18 | 2.5 | 15.5 | 15.5 | 15.294 | |
| M20 | 2.5 | 17.5 | 17.5 | 17.294 | |
| M22 | 2.5 | 19.5 | 19.5 | 19.294 | |
| M24 | 3 | 21 | 21 | 20.752 | |
| M27 | 3 | 24 | 24 | 23.752 | |
| M30 | 3.5 | 26.5 | 26.5 | 26.211 | |
| M33 | 3.5 | 29.5 | 29.5 | 29.211 | |
| M36 | 4 | 32 | 32 | 31.670 | |
| M39 | 4 | 35 | 35 | 34.670 | second choice size |
| M42 | 4.5 | 37.5 | 37.5 | 37.129 | |
| M45 | 4.5 | 40.5 | 40.5 | 40.129 | |
| M48 | 5 | 43 | 43 | 42.587 | |
| M52 | 5 | 47 | 47 | 46.587 | |
| M56 | 5.5 | 50.5 | 50.5 | 50.046 |
Minor diameter is the basic internal thread minor diameter, D - 1.0825 x pitch, per ISO 724. Second choice sizes are in the standard but are rarely stocked.
M8 and M12: why you see 6.8 and 6.75 for the same thread
These two rows cause more argument than the rest of the metric chart put together, and both answers are defensible. Diameter minus pitch gives 6.75 mm and 10.25 mm. Manufacturer charts give 6.8 mm and 10.2 mm, because those are the stocked drill sizes in the preferred number series that ISO 2306 lists for tapping.
In practice: 6.8 mm gives about 74 percent thread, 6.75 mm gives about 77 percent. Both are comfortably inside the working range and both will hold. If you have the 6.8 in the rack, use it. If you are tapping stainless, the 6.8 is the easier cut.
For anyone working in a shop with imperial drills only, 17/64 inch is 6.746 mm, which is within a hundredth of the calculated M8 size.
Metric fine tap drill sizes
All 33 fine pitch sizes. Worth knowing which is which: M10 x 1, M12 x 1.25, M14 x 1.25 and M18 x 1.5 are the spark plug threads, and M16 x 1.5 through M50 x 1.5 are the metric conduit threads. If you are chasing a thread on a head or a gland, that is usually what you have found.
| Thread | Pitch (mm) | Tap drill (mm) | D minus pitch (mm) | Minor dia (mm) | Note |
|---|---|---|---|---|---|
| M1.6 x 0.2 | 0.2 | 1.4 | 1.4 | - | |
| M2 x 0.25 | 0.25 | 1.75 | 1.75 | - | |
| M2.5 x 0.35 | 0.35 | 2.15 | 2.15 | - | |
| M3 x 0.35 | 0.35 | 2.65 | 2.65 | - | |
| M4 x 0.5 | 0.5 | 3.5 | 3.5 | - | |
| M5 x 0.5 | 0.5 | 4.5 | 4.5 | - | |
| M6 x 0.75 | 0.75 | 5.3 | 5.25 | - | charts disagree |
| M8 x 1 | 1 | 7 | 7 | - | spark plug thread |
| M8 x 0.75 | 0.75 | 7.25 | 7.25 | - | |
| M10 x 1.25 | 1.25 | 8.8 | 8.75 | - | charts disagree |
| M10 x 1 | 1 | 9 | 9 | - | |
| M10 x 0.75 | 0.75 | 9.25 | 9.25 | - | |
| M12 x 1.5 | 1.5 | 10.5 | 10.5 | - | |
| M12 x 1.25 | 1.25 | 10.8 | 10.75 | - | spark plug thread |
| M12 x 1 | 1 | 11 | 11 | - | |
| M14 x 1.5 | 1.5 | 12.5 | 12.5 | - | |
| M14 x 1.25 | 1.25 | 12.8 | 12.75 | - | spark plug thread |
| M16 x 1.5 | 1.5 | 14.5 | 14.5 | - | metric conduit thread |
| M18 x 1.5 | 1.5 | 16.5 | 16.5 | - | spark plug thread |
| M20 x 2 | 2 | 18 | 18 | - | |
| M20 x 1.5 | 1.5 | 18.5 | 18.5 | - | metric conduit thread |
| M22 x 1.5 | 1.5 | 20.5 | 20.5 | - | |
| M24 x 2 | 2 | 22 | 22 | - | |
| M24 x 1.5 | 1.5 | 22.5 | 22.5 | - | |
| M25 x 1.5 | 1.5 | 23.5 | 23.5 | - | metric conduit thread |
| M27 x 2 | 2 | 25 | 25 | - | |
| M30 x 2 | 2 | 28 | 28 | - | |
| M30 x 1.5 | 1.5 | 28.5 | 28.5 | - | |
| M32 x 1.5 | 1.5 | 30.5 | 30.5 | - | metric conduit thread |
| M36 x 3 | 3 | 33 | 33 | - | |
| M36 x 2 | 2 | 34 | 34 | - | |
| M40 x 1.5 | 1.5 | 38.5 | 38.5 | - | metric conduit thread |
| M50 x 1.5 | 1.5 | 48.5 | 48.5 | - | metric conduit thread |
UNC tap drill sizes
Unified National Coarse, 25 sizes from #1-64 to 2-4.5, with the exact millimeter conversion of each drill and the metric drill a shop holding metric only would reach for instead.
| Thread | TPI | Tap drill | Decimal (in) | Exact (mm) | Metric substitute (mm) |
|---|---|---|---|---|---|
| #1-64 | 64 | #53 | 0.0595 | 1.511 | - |
| #2-56 | 56 | #50 | 0.0700 | 1.778 | 1.85 |
| #3-48 | 48 | #47 | 0.0785 | 1.994 | 2.1 |
| #4-40 | 40 | #43 | 0.0890 | 2.261 | 2.3 |
| #5-40 | 40 | #38 | 0.1015 | 2.578 | 2.6 |
| #6-32 | 32 | #36 | 0.1065 | 2.705 | 2.8 |
| #8-32 | 32 | #29 | 0.1360 | 3.454 | 3.4 |
| #10-24 | 24 | #25 | 0.1495 | 3.797 | 3.8 |
| #12-24 | 24 | #16 | 0.1770 | 4.496 | 4.5 |
| 1/4-20 | 20 | #7 | 0.2010 | 5.105 | 5.1 |
| 5/16-18 | 18 | F | 0.2570 | 6.528 | 6.6 |
| 3/8-16 | 16 | 5/16 | 0.3125 | 7.938 | 8 |
| 7/16-14 | 14 | U | 0.3680 | 9.347 | 9.4 |
| 1/2-13 | 13 | 27/64 | 0.4219 | 10.716 | 10.8 |
| 9/16-12 | 12 | 31/64 | 0.4844 | 12.303 | 12.2 |
| 5/8-11 | 11 | 17/32 | 0.5312 | 13.494 | 13.5 |
| 3/4-10 | 10 | 21/32 | 0.6562 | 16.669 | 16.5 |
| 7/8-9 | 9 | 49/64 | 0.7656 | 19.447 | 19.5 |
| 1-8 | 8 | 7/8 | 0.8750 | 22.225 | 22.2 |
| 1 1/8-7 | 7 | 63/64 | 0.9844 | 25.003 | 25 |
| 1 1/4-7 | 7 | 1 7/64 | 1.1094 | 28.178 | 28 |
| 1 3/8-6 | 6 | 1 7/32 | 1.2188 | 30.957 | 31 |
| 1 1/2-6 | 6 | 1 11/32 | 1.3438 | 34.132 | 34 |
| 1 3/4-5 | 5 | 1 9/16 | 1.5625 | 39.688 | 39.5 |
| 2-4.5 | 4.5 | 1 25/32 | 1.7812 | 45.244 | 45 |
The five largest rows are computed from the 75 percent formula and matched to the nearest fractional drill, then checked against a manufacturer metric chart that agrees within 0.2 mm on every one. They are marked as computed rather than presented as double-source published.
UNF tap drill sizes
Unified National Fine, 20 sizes. Three things on this table are worth reading before you use it, because they are wrong on several of the charts that rank for this search.
| Thread | TPI | Tap drill | Decimal (in) | Exact (mm) | Metric substitute (mm) |
|---|---|---|---|---|---|
| #0-80 | 80 | 3/64 | 0.0469 | 1.191 | - |
| #1-72 | 72 | #53 | 0.0595 | 1.511 | 1.7 |
| #2-64 | 64 | #50 | 0.0700 | 1.778 | - |
| #3-56 | 56 | #45 | 0.0820 | 2.083 | 2.1 |
| #4-48 | 48 | #42 | 0.0935 | 2.375 | 2.35 |
| #5-44 | 44 | #37 | 0.1040 | 2.642 | 2.65 |
| #6-40 | 40 | #33 | 0.1130 | 2.870 | 2.9 |
| #8-36 | 36 | #29 | 0.1360 | 3.454 | 3.5 |
| #10-32 | 32 | #21 | 0.1590 | 4.039 | 4.1 |
| #12-28 | 28 | #14 | 0.1820 | 4.623 | 4.6 |
| 1/4-28 | 28 | #3 | 0.2130 | 5.410 | 5.5 |
| 5/16-24 | 24 | I | 0.2720 | 6.909 | 6.9 |
| 3/8-24 | 24 | Q | 0.3320 | 8.433 | 8.5 |
| 7/16-20 | 20 | 25/64 | 0.3906 | 9.922 | 9.8 |
| 1/2-20 | 20 | 29/64 | 0.4531 | 11.509 | 11.5 |
| 9/16-18 | 18 | 33/64 | 0.5156 | 13.097 | 12.8 |
| 5/8-18 | 18 | 37/64 | 0.5781 | 14.684 | 14.5 |
| 3/4-16 | 16 | 11/16 | 0.6875 | 17.463 | 17.5 |
| 7/8-14 | 14 | 13/16 | 0.8125 | 20.638 | 20.5 |
| 1-12 | 12 | 59/64 | 0.9219 | 23.416 | 23.5 |
1 inch UNF is 1-12, not 1-14
Several widely copied charts list a 1-14 row under UNF. The UNF series at 1 inch nominal is 1-12. 1-14 is a UNS special thread, and tapping one when the drawing says UNF gives you a hole nothing fits.
9/16-18 and 5/8-18 run lean
The standard fractional drills, 33/64 and 37/64, actually give 68 and 63 percent thread, not 75. That is fine for most work and it is why the manufacturer metric substitutes of 12.8 and 14.5 mm sit a step tighter. Pick deliberately rather than by accident.
Watch the decimal column
At least one popular chart lists the 1 inch UNF drill as 59/64 but prints the decimal as 0.9375, which is 15/16. 59/64 is 0.9219 and is the correct drill. If a decimal and a fraction disagree on any chart, trust neither until you have checked the arithmetic.
BSW Whitworth tap drill sizes, and why the big ones feel so heavy
Whitworth is a 55 degree form, so the metric habit does not transfer. The percent thread column here is computed from the verified Whitworth depth of 0.640327 x pitch, and it shows something the standard charts never mention: the published BSW tapping drills run at 77 to 89 percent thread, not 75. That is exactly why a 1 inch BSW tap fights you.
If you are tapping BSW in stainless or into a blind hole, going up one drill step is not sloppy. It brings you back toward the engagement the metric charts would have given you in the first place.
| Thread | TPI | Tap drill (mm) | D - 0.960P (mm) | Actual thread |
|---|---|---|---|---|
| 1/16 | 60 | 1.2 | 1.29 | 89 percent |
| 3/32 | 48 | 1.85 | 1.88 | 80 percent |
| 1/8 | 40 | 2.55 | 2.57 | 78 percent |
| 5/32 | 32 | 3.2 | 3.2 | 75 percent |
| 3/16 | 24 | 3.7 | 3.75 | 81 percent |
| 7/32 | 24 | 4.5 | 4.54 | 79 percent |
| 1/4 | 20 | 5.1 | 5.13 | 77 percent |
| 5/16 | 18 | 6.5 | 6.58 | 81 percent |
| 3/8 | 16 | 7.9 | 8 | 78 percent |
| 7/16 | 14 | 9.3 | 9.37 | 78 percent |
| 1/2 | 12 | 10.5 | 10.67 | 81 percent |
| 9/16 | 12 | 12.1 | 12.26 | 80 percent |
| 5/8 | 11 | 13.5 | 13.66 | 80 percent |
| 11/16 | 11 | 15 | 15.06 | 77 percent |
| 3/4 | 10 | 16.25 | 16.61 | 86 percent |
| 7/8 | 9 | 19.25 | 19.51 | 84 percent |
| 1 | 8 | 22 | 22.35 | 84 percent |
| 1 1/8 | 7 | 24.75 | 25.11 | 82 percent |
| 1 1/4 | 7 | 28 | 28.28 | 79 percent |
| 1 1/2 | 6 | 33.5 | 33.83 | 82 percent |
| 1 3/4 | 5 | 39 | 39.57 | 83 percent |
| 2 | 4.5 | 44.5 | 45.02 | 82 percent |
BSF, brass and BA tap drill sizes
The threads you meet on older machinery and in model engineering. BSF is shown with two independent sources side by side; they agree everywhere except two rows, and those differ by a tenth of a millimeter.
BSF, British Standard Fine
| Thread | TPI | Source A (mm) | Source B (mm) |
|---|---|---|---|
| 3/16 | 32 | 4 | 4 |
| 7/32 | 28 | 4.6 | not listed |
| 1/4 | 26 | 5.3 | 5.3 |
| 5/16 | 22 | 6.8 | 6.8 |
| 3/8 | 20 | 8.3 | 8.3 |
| 7/16 | 18 | 9.8 | 9.7 |
| 1/2 | 16 | 11 | 11.1 |
| 9/16 | 16 | 12.7 | 12.7 |
| 5/8 | 14 | 14 | 14 |
| 11/16 | 14 | 15.5 | not listed |
| 3/4 | 12 | 16.75 | 16.75 |
| 7/8 | 11 | 19.75 | 19.75 |
| 1 | 10 | 22.75 | 22.75 |
| 1 1/8 | 9 | 25.5 | not listed |
| 1 1/4 | 9 | 28.5 | not listed |
| 1 1/2 | 8 | 34.5 | not listed |
| 1 3/4 | 7 | 41 | not listed |
BSB, British Standard Brass, 26 TPI throughout
| Thread | TPI | Tap drill (mm) |
|---|---|---|
| 1/4 | 26 | 5.2 |
| 5/16 | 26 | 6.8 |
| 3/8 | 26 | 8.4 |
| 7/16 | 26 | 10 |
| 1/2 | 26 | 11.6 |
| 9/16 | 26 | 13.2 |
| 5/8 | 26 | 14.8 |
| 3/4 | 26 | 18 |
| 7/8 | 26 | 20.8 |
| 1 | 26 | 24.3 |
BA, British Association
| BA | OD (mm) | Pitch (mm) | Tap drill (mm) | Minor dia (mm) | Thread |
|---|---|---|---|---|---|
| 0BA | 6 | 1 | 5.1 | 4.8 | 75 percent |
| 1BA | 5.3 | 0.9 | 4.5 | 4.22 | 74 percent |
| 2BA | 4.7 | 0.81 | 4 | 3.73 | 72 percent |
| 3BA | 4.1 | 0.73 | 3.4 | 3.22 | 80 percent |
| 4BA | 3.6 | 0.66 | 3 | 2.81 | 76 percent |
| 5BA | 3.2 | 0.59 | 2.65 | 2.49 | 78 percent |
| 6BA | 2.8 | 0.53 | 2.3 | 2.16 | 78 percent |
| 7BA | 2.5 | 0.48 | 2.05 | 1.92 | 78 percent |
| 8BA | 2.2 | 0.43 | 1.8 | 1.68 | 77 percent |
| 9BA | 1.9 | 0.39 | 1.55 | 1.43 | 75 percent |
| 10BA | 1.7 | 0.35 | 1.4 | 1.28 | 71 percent |
| 11BA | 1.5 | 0.31 | 1.2 | 1.13 | 81 percent |
| 12BA | 1.3 | 0.28 | 1.05 | 0.96 | 74 percent |
| 13BA | 1.2 | 0.25 | 0.98 | 0.9 | 73 percent |
| 14BA | 1 | 0.23 | 0.8 | 0.72 | 72 percent |
| 15BA | 0.9 | 0.21 | 0.7 | 0.65 | 79 percent |
| 16BA | 0.79 | 0.19 | 0.6 | 0.56 | 82 percent |
Every BA row above is also reproduced by the D - 0.900 x pitch rule, which is a third, geometric check on the published chart. 16BA outside diameter is derived from the BA geometric series rather than published directly, so treat that one row with more care. In model engineering you will also see 5BA tapped 2.7 mm and 2BA tapped 3.9 mm; both are legitimate lower engagement choices for brass, not the standard figures.
Clearance holes, and the check that catches bad charts
A clearance hole is always larger than the bolt. It sounds too obvious to say, until you notice how many published charts fail it. Clearance columns get transcribed row-shifted, and the result is a chart telling you to drill a hole a 5/16 bolt physically cannot pass through.
Two figures matter and most charts only give one. Close fit is for location, where you want the hole to control position. Free fit is for assembly, where you want the parts to go together on site.
| Bolt | Bolt dia (in) | Close fit (in) | Close fit drill | Free fit (in) | Free fit drill |
|---|---|---|---|---|---|
| 1/4 | 0.2500 | 0.2570 | F | 0.2660 | H |
| 5/16 | 0.3125 | 0.3230 | P | 0.3320 | Q |
| 3/8 | 0.3750 | 0.3860 | W | 0.3970 | X |
| 7/16 | 0.4375 | 0.4531 | 29/64 | 0.4687 | 15/32 |
| 1/2 | 0.5000 | 0.5156 | 33/64 | 0.5312 | 17/32 |
Deliberately a short table. These are the rows that could be confirmed against a live independent source and that pass the arithmetic check, so these are the rows published. A padded clearance table is exactly how the errors spread in the first place.
How much thread you actually want, by material
Published manufacturer guidance, not shop folklore. The pattern is consistent: the tougher the material, the less thread you want, because torque climbs far faster than strength does.
| Material group | Deep holes | Average work | Thin sheet |
|---|---|---|---|
| Hard and tough: stainless, Monel, nickel steel, heat treated alloys | 55 to 65 percent | 60 to 70 percent | not published |
| Free cutting: aluminum, brass, bronze, cast iron, mild steel, tool steel | 60 to 70 percent | 65 to 75 percent | 75 to 85 percent |
The defensible default: 65 percent for general work, drop to 55 to 60 percent in stainless and anything that work hardens, and only push toward 75 to 85 percent in thin sheet where you have very little engaged length to play with. In a fabrication shop, tapping a boss on a tank or a plate lug usually gives you one and a half diameters of engagement or more, so 65 percent is plenty and the extra couple of tenths on the drill is what saves the tap.
Tap drill FAQ
The questions people actually ask at the pillar drill
Short answers, with the reasoning underneath so you can tell when the rule of thumb stops applying.
How do you work out tap drill size?
Subtract the pitch from the major diameter for a quick metric answer: an M10 x 1.5 needs an 8.5 mm drill. The exact version is tap drill = D - (percent thread x pitch) / 76.98 for metric, and D - percent thread / (76.98 x TPI) for inch threads. The rule of thumb and the formula agree at about 77 percent thread engagement, which is why the shortcut works.
What drill size for an M8 tap?
6.8 mm on every tap manufacturer chart, and 6.75 mm if you apply the diameter minus pitch rule. Both are correct. 6.8 mm is the stocked drill size specified in ISO 2306 and gives about 74 percent thread; 6.75 mm gives about 77 percent. Either will hold. If you only have imperial drills, 17/64 inch is 6.746 mm and is a valid substitute.
Does diameter minus pitch give 75 percent thread?
No, it gives about 77 percent. The industry percent-of-thread formula divides by 1.29904 x pitch, so a hole at D minus pitch works out at 100 / 1.29904, which is 76.98 percent. Most charts round that to 75 percent and never explain the difference. It is close enough to work and worth knowing when you are deciding whether to go up a drill size.
What is 75 percent thread engagement, and is it the right target?
It is the depth of thread the hole leaves relative to the full theoretical thread form, and it is a legacy convention rather than an optimum. Kennametal publishes that a 100 percent thread is only 5 percent stronger than 75 percent but takes three times the power to tap, and recommends 68 to 70 percent. Guhring recommends 60 to 70 percent. For most fabrication work 65 percent is plenty and saves broken taps.
Why do tap drill charts disagree with each other?
Two reasons. First, charts target different thread percentages without saying so, and a hole for 50 percent thread is genuinely different from one for 75 percent. Second, some charts publish the nearest stocked drill from the preferred number series rather than the calculated size, which is why M8 appears as both 6.8 and 6.75 mm. Neither is wrong; they answer slightly different questions.
What tap drill do I use in stainless steel?
Go larger, not smaller. Stainless work hardens and the torque climbs fast, so drop to about 55 to 60 percent thread rather than 75. On an M10 x 1.5 that moves you from 8.5 mm to roughly 8.7 mm. You lose almost no joint strength because the failure mode is the bolt, not the tapped hole, provided you have at least one diameter of thread engagement.
Is the tapping drill rule different for Whitworth and BA threads?
Yes, and this is where the minus-the-pitch habit goes wrong. Whitworth is a 55 degree form, so the 75 percent rule is D - 0.960 x pitch. British Association is 47.5 degrees, so it is D - 0.900 x pitch. Applying the metric rule to a BSW thread gives a hole that is too small, which is part of why large Whitworth taps feel so heavy.
What is the difference between a tap drill and a clearance hole?
A tap drill is smaller than the bolt, because it leaves material for the thread to be cut into. A clearance hole is larger than the bolt so the bolt passes straight through. If a chart ever gives you a clearance hole smaller than the bolt diameter, the chart is wrong; that is the fastest sanity check there is.
Do roll form taps use a different drill size?
Yes, a noticeably larger one, because forming taps displace metal rather than cut it and the material has to flow somewhere. Manufacturer form-tap charts commonly target around 65 percent thread. Do not use a cutting tap chart for a forming tap; the tap will jam or snap.
Where these numbers come from
The anchor is a tap manufacturer's own published tapping drill chart, cross-checked against independent trade suppliers and reference tables. Every figure here is either present in two or more independent sources that agree, present in a manufacturer chart and corroborated, or computed from verified thread geometry and shown to reproduce the published values.
Where sources genuinely disagree, this page shows both numbers and explains the reason instead of picking one silently. That is the difference between a chart you can use and a chart that has been copied from another chart. Always confirm against your own tap manufacturer's data for critical work.