Metric Tap Drill Size Formula: Major Minus Pitch Tested Against 6H
The metric tap drill formula is major diameter minus pitch, and it holds up. We tested it against the class 6H limits on 17 coarse threads from M2 to M30, and the hole landed inside on all 17. It always sits 0.0825 times the pitch above the 6H minimum, which is why it never falls under, though the margin is only 0.033 mm on M2.
Major minus pitch against 6H, thread by thread
"Above min" and "below max" are how far the formula's hole sits from each 6H limit. "Drill" is the size the charts print, and the last column is the biggest 0.1 mm drill that stays inside, which leaves less thread but is easier on the tap. Percentages use the formula in the OSG chart.
| Thread | Pitch | Major minus pitch | 6H minor diameter | Above min | Below max | Drill | Biggest drill inside 6H |
|---|---|---|---|---|---|---|---|
| M2 | 0.4 mm | 1.60 mm | 1.567 to 1.679 mm | 0.033 mm | 0.079 mm | 1.6 mm 77% | 1.6 mm 77% |
| M2.5 | 0.45 mm | 2.05 mm | 2.013 to 2.138 mm | 0.037 mm | 0.088 mm | 2.05 mm 77% | 2.1 mm 68% |
| M3 | 0.5 mm | 2.50 mm | 2.459 to 2.599 mm | 0.041 mm | 0.099 mm | 2.5 mm 77% | 2.5 mm 77% |
| M3.5 | 0.6 mm | 2.90 mm | 2.850 to 3.010 mm | 0.050 mm | 0.110 mm | 2.9 mm 77% | 3.0 mm 64% |
| M4 | 0.7 mm | 3.30 mm | 3.242 to 3.422 mm | 0.058 mm | 0.122 mm | 3.3 mm 77% | 3.4 mm 66% |
| M5 | 0.8 mm | 4.20 mm | 4.134 to 4.334 mm | 0.066 mm | 0.134 mm | 4.2 mm 77% | 4.3 mm 67% |
| M6 | 1 mm | 5.00 mm | 4.917 to 5.153 mm | 0.083 mm | 0.153 mm | 5.0 mm 77% | 5.1 mm 69% |
| M8 | 1.25 mm | 6.75 mm | 6.647 to 6.912 mm | 0.103 mm | 0.162 mm | 6.8 mm 74% | 6.9 mm 68% |
| M10 | 1.5 mm | 8.50 mm | 8.376 to 8.676 mm | 0.124 mm | 0.176 mm | 8.5 mm 77% | 8.6 mm 72% |
| M12 | 1.75 mm | 10.25 mm | 10.106 to 10.441 mm | 0.144 mm | 0.191 mm | 10.2 mm 79% | 10.4 mm 70% |
| M14 | 2 mm | 12.00 mm | 11.835 to 12.210 mm | 0.165 mm | 0.210 mm | 12.0 mm 77% | 12.2 mm 69% |
| M16 | 2 mm | 14.00 mm | 13.835 to 14.210 mm | 0.165 mm | 0.210 mm | 14.0 mm 77% | 14.2 mm 69% |
| M18 | 2.5 mm | 15.50 mm | 15.294 to 15.744 mm | 0.206 mm | 0.244 mm | 15.5 mm 77% | 15.7 mm 71% |
| M20 | 2.5 mm | 17.50 mm | 17.294 to 17.744 mm | 0.206 mm | 0.244 mm | 17.5 mm 77% | 17.7 mm 71% |
| M22 | 2.5 mm | 19.50 mm | 19.294 to 19.744 mm | 0.206 mm | 0.244 mm | 19.5 mm 77% | 19.7 mm 71% |
| M24 | 3 mm | 21.00 mm | 20.752 to 21.252 mm | 0.248 mm | 0.252 mm | 21.0 mm 77% | 21.2 mm 72% |
| M30 | 3.5 mm | 26.50 mm | 26.211 to 26.771 mm | 0.289 mm | 0.271 mm | 26.5 mm 77% | 26.7 mm 73% |
Why the formula works
ISO metric threads put the minor diameter at the major diameter minus 1.0825 times the pitch (Wikipedia), and the 6H minimum in Emuge's chart matches that to within 0.001 mm on every size here. Major minus pitch is 1.0000 times the pitch below the major diameter, so it sits 0.0825 pitches above the minimum at every size. On M6 that is 0.083 mm, on M30 0.289 mm.
The tolerance band is much wider than that margin. On M6 it runs from 4.917 to 5.153 mm, so the hole sits in the lower part of the band. That leaves room for a drill that cuts a little oversize.
The formula leaves 77% thread rather than 75%, because a full thread is 1.299 pitches deep and 1 / 1.299 is 0.77. For exactly 75%, the sum is the major diameter minus 0.974 times the pitch. On M6 that is 5.03 mm, which is not a standard drill size. The method page covers the formula for inch threads too.
When major minus pitch is not a drill you can buy
Three of the 17 sizes fall between 0.1 mm drills. M8 works out at 6.75 mm, and both 6.7 mm (80%) and 6.8 mm (74%) are inside 6H. Published charts split on this one, as the charts compared page shows. M12 works out at 10.25 mm, and 10.2 and 10.3 mm are both inside.
M2.5 is the one to watch. It works out at 2.05 mm, a size charts print but a set in 0.1 mm steps does not have. Rounding down to 2.0 mm puts the hole 0.013 mm under the 6H minimum, so round up to 2.1 mm (68%). On M3, one step up to 2.6 mm misses the maximum by 0.001 mm, so stay on 2.5 mm where a drawing calls for 6H.
Not every published chart stays inside. Bolt Depot's metric table gives drills that cut over the 6H maximum on M6, M10 and M12, even though the page says its drills are for 75% thread.
Fine pitches
The rule holds on fine pitches too. Using the 6H limits in the AmesWeb chart, which cites ASME B1.13M, the rule and the drill it rounds to are inside on 12 of 12 fine threads. M4 x 0.5, M5 x 0.5, M6 x 0.75 and M20 x 2 are not in that chart, so we left them out.
| Thread | Major minus pitch | 6H minor diameter | Drill | Thread left |
|---|---|---|---|---|
| M8 x 1 | 7.00 mm | 6.917 to 7.153 mm | 7.0 mm | 77% |
| M10 x 1.25 | 8.75 mm | 8.647 to 8.912 mm | 8.8 mm | 74% |
| M10 x 1 | 9.00 mm | 8.917 to 9.153 mm | 9.0 mm | 77% |
| M12 x 1.5 | 10.50 mm | 10.376 to 10.676 mm | 10.5 mm | 77% |
| M12 x 1.25 | 10.75 mm | 10.647 to 10.912 mm | 10.8 mm | 74% |
| M14 x 1.5 | 12.50 mm | 12.376 to 12.676 mm | 12.5 mm | 77% |
| M16 x 1.5 | 14.50 mm | 14.376 to 14.676 mm | 14.5 mm | 77% |
| M18 x 1.5 | 16.50 mm | 16.376 to 16.676 mm | 16.5 mm | 77% |
| M20 x 1.5 | 18.50 mm | 18.376 to 18.676 mm | 18.5 mm | 77% |
| M22 x 1.5 | 20.50 mm | 20.376 to 20.676 mm | 20.5 mm | 77% |
| M24 x 2 | 22.00 mm | 21.835 to 22.210 mm | 22.0 mm | 77% |
| M30 x 2 | 28.00 mm | 27.835 to 28.210 mm | 28.0 mm | 77% |
What this test does not show
This is arithmetic against published limits. We did not drill or gauge any holes. A real hole can run over the drill's nominal size, and how much depends on the drill, the machine and the material, so check the hole if the thread class matters. The formula is for cut taps: form taps need a bigger hole. Minor diameter is also only one of the class limits, and a tap sets the pitch diameter.
FAQ
What is the formula for a metric tap drill size?
Major diameter minus pitch. M6 x 1 takes 5.0 mm and M10 x 1.5 takes 8.5 mm. The hole leaves about 77% thread, because the pitch is 77% of the 1.299 pitches a full thread would take.
Does major minus pitch always stay inside 6H?
It did on all 17 coarse sizes here, M2 to M30, using the limits Emuge prints. The hole sits 0.0825 times the pitch above the 6H minimum, and every 6H band is wider than that. The check uses nominal drill sizes.
What if major minus pitch is not a drill size?
Use the next drill either side if it stays inside 6H. M8 works out at 6.75 mm, and both 6.7 and 6.8 mm are inside. M12 works out at 10.25 mm, and both 10.2 and 10.3 mm are inside. M2.5 is different: 2.0 mm is under the minimum, so use 2.05 or 2.1 mm.
Can I drill bigger than major minus pitch?
On most sizes, yes. A bigger drill leaves less thread and makes the tap easier to turn. The last column of the table gives the biggest 0.1 mm drill inside 6H. On M2 and M3, no 0.1 mm step up stays inside.
Cite or reuse this data
Please quote these figures with a link to this page: TapDrillSize, "Metric tap drill size formula tested against class 6H", https://tapdrillsize.com/metric-tap-drill-formula/, published November 13, 2026. Every row is in the CSV download. If a number is wrong, write to contact@tapdrillsize.com and we will correct it.
Class 6H limits from the Emuge chart. Thread percentages use the formula in the OSG tap drill chart, Vol 3 (2024). Fine pitch limits from AmesWeb. Minor diameter formula from Wikipedia: ISO metric screw thread. Published on November 13, 2026.