ENGINEERING TOOL
Metric Fine Tap Drill Chart: M8 to M36, Rule Value plus Nearest Drill
The rule this whole table follows
Tap drill = major diameter − pitch (D = d − P)
The rule value is the answer for every row: at D = d − P the percent of thread is always 77.0%, whatever the size, because the numerator is the pitch and the denominator is a multiple of the pitch. The nearest drill column is a conversion to what is on the shelf, not a second opinion about the rule.
Ceiling check: the highest percent of thread a Unified thread profile can reach is 83.3%, at the basic minor diameter. A drill below that diameter cuts into the thread form, and the rows above the top of this drill set say so instead of naming a drill.
Tap drill sizes for the 13 metric fine threads from M8 to M36: the rule value, the nearest drill this drill set actually holds, the percent of thread at each, and the point where the set runs out. Every number is computed from one module rather than copied from a table.
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Metric fine tap drills, all sizes
Ordered by nominal diameter. The rule value column is the answer; the nearest drill column is the closest entry in the metric set, with the percent of thread that entry gives. The last column is the basic minor diameter, the diameter at which the percent of thread reaches its ceiling.
| Thread | Pitch (mm) | Tap drill, rule value (mm) | Nearest drill in the set (mm) | Percent of thread there | Basic minor diameter (mm) |
|---|---|---|---|---|---|
| M8 | 1 | 7 | 7 | 77.0% | 6.917 |
| M10 | 1 | 9 | 9 | 77.0% | 8.917 |
| M12 | 1.5 | 10.5 | 10.5 | 77.0% | 10.376 |
| M14 | 1.5 | 12.5 | 12.5 | 77.0% | 12.376 |
| M16 | 1.5 | 14.5 | 14.5 | 77.0% | 14.376 |
| M18 | 1.5 | 16.5 | 16.5 | 77.0% | 16.376 |
| M20 | 1.5 | 18.5 | 18.5 | 77.0% | 18.376 |
| M22 | 1.5 | 20.5 | 20.5 | 77.0% | 20.376 |
| M24 | 2 | 22 | 22 | 77.0% | 21.835 |
| M27 | 2 | 25 | 25 | 77.0% | 24.835 |
| M30 | 2 | 28 | 25(below minor dia.) | 192.5% | 27.835 |
| M33 | 2 | 31 | 25(below minor dia.) | 307.9% | 30.835 |
| M36 | 3 | 33 | 25(below minor dia.) | 282.3% | 32.752 |
A row whose nearest entry is flagged is a size the metric drill set cannot make: the closest drill is smaller than the thread's basic minor diameter, so it is shown for completeness rather than as a drill to use.
Basis & references
How to read this chart
Percent of thread = actual thread height ÷ 0.75H, where H is the height of the fundamental triangle of a 60° thread form. A Unified thread's basic height is 0.625H, so the ceiling is 83.3%; the basic minor diameter is d − 1.082532·P. (Definition: FED-STD-H28/2B §20.2.3.)
The nearest entry converts the rule value into a drill you can actually buy (the metric set has 169 sizes, stepped 0.01–0.5 mm, not evenly spaced); no rule value in this series lands exactly halfway between two sizes, so the nearest entry is never ambiguous.
The metric drill set stops at 25 mm: M30, M33, M36 — 3 threads — have no drill that can make their tap drill hole (for M30, the nearest entry 25 mm would give 192.5% of thread, far past the ceiling). These rows still print the rule value, marked.
Why the drilled hole differs from the chart
- A drill typically cuts a hole larger than its stamped diameter, so bore tolerance should allow for the probable average enlargement; the largest sizes are usually reamed instead.
- Use a bore near the minimum for short engagement; open the bore up for long engagement to lower the risk of breaking the tap.
- In soft materials (especially with fine pitches) metal is squeezed toward the tap root, so the finished minor diameter can end up smaller than the drilled hole.
From FED-STD-H28/2B §20.2.2, §30.1 and §40.3.
Nature of the values
The figures are engineering guidance, not mandatory values, given for a cutting tap in a through hole; adjust for material, tap type, length of engagement and through vs blind holes. Of the appendix carrying figures of this kind, FED-STD-H28/2B says it is not a mandatory part of the standard and is intended for guidance only (§10).
Every figure in the chart is computed from the thread geometry by a single calculation module — one consistent basis, reproducible value by value.
References
- Number, letter and fractional drill size series constants
- Metric coarse and fine pitch series held in this site's own single source of thread dimensions
- FED-STD-H28/2B Unified Inch Screw Threads, US Federal Standard (percent-of-thread definition, guidance-only clause) (1984)
- TC 9-524 Fundamentals of Machine Tools, US Army (commonly obtainable metric drill sizes)
Cite this chart
YAXIIO. Metric Fine Tap Drill Chart: M8 to M36, Rule Value plus Nearest Drill. https://www.yaxiio.com/en/tools/tap-drill-chart/metric-fine (accessed 2026-10-29).
Free to cite and embed — attribution required: “YAXIIO” + a link to this page.
Every size in this chart
Related References
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Every drill size this site carries: number, letter, fractional and metric series, with the diameter in both systems.
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Tightening torque by property class for the same thread sizes, with the friction basis stated.
Bolt Size Chart
Head dimensions across the preferred and non-preferred metric sizes, both tolerance grades side by side.
Threaded Rod Weight Chart
Unit weight by diameter on the same disclosed basis, for costing a threaded order.
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Tightening torque for stud bolts across three nut factors and six property classes, with the target preload stated.
Wrench Size & Socket Chart
Which wrench or socket fits each hex bolt: width across flats for the bolt, the hex nut and the flange nut side by side, with the standard named row by row.
Hex Key Size Chart
Which hex key fits a socket head cap screw: the socket width across flats plus the key long and short arm lengths, over the same metric sizes as this page.
FAQ
How do I work out a metric tap drill size?
Subtract the pitch from the nominal diameter: D = d − P. For a coarse metric thread the pitch is the standard one for that diameter, so the subtraction uses the pitch column in the table above. The result is the rule value in the third column, and it lands at 77.0% of thread for every size in the chart, which is why the percent falls out of the arithmetic instead of having to be looked up.
Should I use the rule value or the nearest drill?
The rule value is the answer and the nearest drill is a conversion to what is on the shelf. In the small sizes the two often coincide; as the diameters grow the metric set starts stepping by 0.5 mm at 10 mm, so the nearest entry can be a noticeable distance from the rule value. Where that distance pushes the hole below the basic minor diameter the entry is flagged, because it would cut into the thread form — the row is the clearest case in this chart.
Why is the percent of thread never a full profile?
Because the definition divides by 0.75H rather than by the basic thread height of a Unified thread, which is 0.625H. The ceiling that follows is 83.3%: a thread filled any further would have no clearance left at the crest and would be cut by the tap rather than formed by it. A figure near a full profile is a sign the hole is undersized, not a sign of a better thread.
Why do the largest sizes show no usable drill?
The metric drill set used here stops at 25 mm. Above that, the nearest entry is not merely far from the rule value, it is smaller than the thread's basic minor diameter, so the tap would have to remove metal the profile needs. 3 of the 13 threads in this chart are in that position (M30, M33, M36); their rows print the rule value and mark the nearest entry instead of naming a drill that would ruin the thread.
Disclaimer: the figures on this page are computed reference values and text rules, not standard values, design values or an acceptance basis. Tap drill size depends on the material, the tap and the length of engagement; for work that carries load, follow the thread standard for the joint and drill a test piece before running the job.