ENGINEERING TOOL
M18×2.5 Tap Drill Size: 15.5 mm
Answer for this thread
Tap drill, rule value
15.5 mm
major diameter − pitch = 18 − 2.5 = 15.5 mm
Percent of thread at the rule value
77.0%
the same figure for every size in this chart
Nearest drill in this set
15.5 mm
(18 − 15.5) ÷ (1.299 × 2.5) × 100 = 77.0% · ±0 mm
This rule value is itself an entry in the metric drill set, so the nearest drill and the rule value are the same diameter here.
Basic minor diameter 15.294 mm · Percent of thread ceiling 83.3%
One size in this chart sits exactly halfway between two entries of the set: see the gap note below before taking the nearest one.
Use the rule value for the hole, then take whichever drill is nearest to it. The difference is small in the small sizes and widens as the pitches get coarse, so the two figures above are given side by side rather than as one number.
Below: the three entries of the metric drill set closest to this rule value, each with its own percent of thread, and the two gaps in the set that decide what can be made from off-the-shelf metric drills.
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Entries of the metric drill set nearest to this rule value
Ranked by distance from the rule value, larger diameter first when the two are equally close, because a larger hole means less tap torque and less risk of breaking the tap.
| Drill entry (mm) | Percent of thread | Difference from rule value (mm) | Status |
|---|---|---|---|
| 15.5 mm | 77.0% | ±0 mm | within the thread form |
| 16 mm | 61.6% | +0.5 mm | within the thread form |
| 15 mm | 92.4% | -0.5 mm | below the basic minor diameter — cuts into the thread form |
Percent of thread here is that of the entry, not that of the rule value. A flagged entry is smaller than the basic minor diameter: the tap would remove metal that the thread profile needs.
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). Example: the rule value for M12×1.75, 10.25 mm, sits exactly halfway between 10.5 mm and 10 mm — 10.5 mm gives 66.0% of thread and is usable; 10 mm gives 88.0%, past the 83.3% ceiling and below the basic minor diameter 10.106 mm, so it would cut into the thread form. That is why this chart answers with the rule value.
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 110.0% 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. M18×2.5 Tap Drill Size: 15.5 mm. https://www.yaxiio.com/en/tools/tap-drill-chart/metric-coarse/m18 (accessed 2026-10-29).
Free to cite and embed — attribution required: “YAXIIO” + a link to this page.
Other metric coarse threads
Related References
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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.
Stud Bolt Torque Chart
Tightening torque for stud bolts across three nut factors and six property classes, with the target preload stated.
Wrench Size & Socket Chart
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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
What tap drill does M18×2.5 take?
The rule value is 15.5 mm: the nominal diameter minus the pitch, M18 − 2.5 mm. That gives 77.0% of thread, the same percent as every other size in this chart. The nearest entry in the metric drill set is 15.5 mm, which gives 77.0% and sits ±0 mm from the rule value.
Is the nearest drill the right one to use?
It is the right conversion, not a second answer. It is also not what a printed tap drill table would give you: this column follows the rule value and takes the closest entry of the drill set, larger first when the two are equally close, whereas published tables often print the theoretical figure d − P itself. Use the rule value as the target and the nearest entry as the drill in your hand, and re-measure if the thread is critical.
What are the limits of this figure?
It is a reference value for a cutting tap in a through hole, not a design value: a drill cuts oversize, bore limits loosen as the length of engagement grows, and a soft material can be squeezed so the finished minor diameter is smaller than the hole drilled. The percent of thread ceiling is 83.3%, at the basic minor diameter 15.294 mm for this thread; below that the tap cuts into the form. Adjust for the material and the tap you are using, and drill a test piece first.
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.