ENGINEERING TOOL
Tap Drill Chart: 7 Thread Series, Metric M3–M36 and Unified Inch #0–3 3/4
The rule, in one line
Tap drill = major diameter − pitch (D = d − P)
Every table in this family answers with the rule value d − P, which lands at 77.0% of thread whatever the size, and then gives the nearest drill that the relevant drill set actually holds, as a conversion to what is on the shelf. The conversion is not a second opinion about the rule.
Pick the series first, then the size: the two metric series share one drill set, while the unified inch series draw on the number, letter and fractional drill series.
The 7 series in this family
| Series | Thread sizes | Sizes with their own page | Nominal diameter range |
|---|---|---|---|
| Metric Coarse | 17 | 17 | M3–M36 |
| Metric Fine | 13 | 13 | M8–M36 |
| UNC | 28 | 9 | #1–3 3/4 |
| UNF | 24 | 1 | #0–1 1/2 |
| UNEF | 26 | 0 | #12–1 11/16 |
| UN | 155 | 0 | 5/16–3 |
| UNS | 8 | 0 | 1/4–2 7/16 |
This is the family page for tap drill sizes: 271 thread sizes in 7 series, 30 metric and 241 unified inch, each series with its own table. The rule is the same throughout — tap drill = major diameter − pitch — and every figure in those tables is computed from the thread geometry rather than copied from a printed chart.
Full PDF
Download the full PDF (M3–M36 / #0–3 3/4, all sizes)
Printable chart covering every size in this family — enter your email to get it.
The inch sizes searched for most
Each of these opens its own page: the rule value for that thread, the percent of thread it gives, and the nearest drill in the inch series with the difference stated.
Basis & how to use
How to read these charts
Why the ceiling is 83.3%: percent of thread = actual thread height ÷ three quarters of the fundamental triangle of a 60° thread form, while a Unified thread's basic height is five eighths of that triangle. A reading near a full profile means the hole is undersized and the tap would have to cut its way in.
Treat the rule value as the target and the nearest entry as the drill in your hand, and measure the difference before tapping a critical thread. Rows marked "smaller than the basic minor diameter" print only the rule value — that drill would cut into the thread form.
Nature & sources of the values
The figures in this family 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. The standard whose appendix carries figures of this kind states the appendix is not a mandatory part of the standard and is intended for guidance only.
Metric rows are computed from thread geometry and the pitch series in this site's own dimension source; the inch thread list is taken from a US Government publication (the diameter – TPI – series columns of the FED-STD-H28/2B appendix). Every tap drill, percent of thread and basic minor diameter in the family is computed by one module, 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)
The series table above prints on one page. Each series page has its own print button for the full table of that series.
Cite this chart
YAXIIO. Tap Drill Chart: 7 Thread Series, Metric M3–M36 and Unified Inch #0–3 3/4. https://www.yaxiio.com/en/tools/tap-drill-chart (accessed 2026-10-28).
Free to cite and embed — attribution required: “YAXIIO” + a link to this page.
FAQ
Which tap drill chart do I need?
Start from the thread system on your drawing. Metric threads use the metric coarse or metric fine table; unified inch threads use the series the callout names — UNC for coarse, UNF for fine, UNEF for extra fine, UN for the constant-pitch series and UNS for special pitches. A callout such as M8×1 or #0-80 UNF names its series outright, so the table follows from the callout.
Why does the same diameter appear in more than one table?
Because the pitch decides the tap drill, not the diameter on its own. One nominal diameter can be coarse in one series and fine in another, and each pitch gives a different rule value, so rows are kept in their own series rather than merged into a single row per diameter. The series overview above shows how many thread sizes sit in each one.
Should I use the rule value or the nearest drill?
The rule value is the target and the nearest drill is the drill in your hand. Published tap drill tables often print the theoretical figure d − P itself, whereas the nearest-entry column in these tables names a drill that exists, so the two can differ by a drill size. Where the nearest entry would cut below the thread's basic minor diameter the row is flagged rather than presented as a drill to use.
Why is the percent of thread never a full profile?
Because the definition divides by three quarters of the fundamental triangle height, while a Unified thread's basic thread height is five eighths of it. 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.
Related References
Drill Bit Size Chart
Every drill size this site carries: number, letter, fractional and metric series, with the diameter in both systems.
Bolt Torque Chart
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.
Disclaimer: the figures on these pages 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.