ENGINEERING TOOL
UNS Tap Drill Chart: 1/4 to 2 7/16, 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 8 UNS threads from 1/4 to 2 7/16: the rule value, the nearest drill this drill set actually holds, the percent of thread at each, and where the set runs out. Every figure is computed from one module rather than copied from a table.
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UNS 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 inch drill 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 | Threads per inch (TPI) | Pitch (in) | Tap drill, rule value (in) | Nearest drill in the set (in) | Percent of thread there | Basic minor diameter (in) |
|---|---|---|---|---|---|---|
| 1/4 | 36 | 0.0278 | 0.2222 | 0.2210 (5.613 mm) | 80.4% | 0.2199 |
| 5/16 | 36 | 0.0278 | 0.2847 | 0.2812 (7.143 mm)(below minor dia.) | 86.7% | 0.2824 |
| 3/8 | 36 | 0.0278 | 0.3472 | 0.3480 (8.839 mm) | 74.8% | 0.3449 |
| 1/2 | 12 | 0.0833 | 0.4167 | 0.4130 (10.49 mm) | 80.4% | 0.4098 |
| 2 1/16 | 16 | 0.0625 | 2.0000 | 0.9844 (25.004 mm)(below minor dia.) | 1327.9% | 1.9948 |
| 2 3/16 | 16 | 0.0625 | 2.1250 | 0.9844 (25.004 mm)(below minor dia.) | 1481.8% | 2.1198 |
| 2 5/16 | 16 | 0.0625 | 2.2500 | 0.9844 (25.004 mm)(below minor dia.) | 1635.8% | 2.2448 |
| 2 7/16 | 16 | 0.0625 | 2.3750 | 0.9844 (25.004 mm)(below minor dia.) | 1789.8% | 2.3698 |
A row whose nearest entry is flagged is a size the inch 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 is taken from the 169 inch drill sizes (number, letter and fractional drills, 0.0135–0.9844 in, i.e. 0.343–25.004 mm; steps 0.0000–0.0157 in); 5 rows take a nearest entry smaller than the basic minor diameter and are flagged in the table rather than offered as usable drills.
The inch drill set stops at 0.9844 in: 4 threads have no drill that can make their tap drill hole (for 2 1/16, the nearest entry 0.9844 in is 1.0156 in below the rule value 2.0000 in and would give 1327.9% 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.
The inch thread list (diameter – threads per inch – series) is taken from the appendix of FED-STD-H28/2B, a US Government publication; the tap drill, millimetre pitch, percent of thread and basic minor diameter are all computed from the thread geometry. The same standard notes that this appendix is not a mandatory part of the standard and is intended for guidance only (§10).
Cite this chart
YAXIIO. UNS Tap Drill Chart: 1/4 to 2 7/16, Rule Value plus Nearest Drill. https://www.yaxiio.com/en/tools/tap-drill-chart/unified-uns (accessed 2026-10-29).
Free to cite and embed — attribution required: “YAXIIO” + a link to this page.
Every size in this chart
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.
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
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 an inch tap drill size?
Subtract the pitch from the nominal diameter, both in inches: D = d − P. The pitch is one divided by the threads per inch, so the subtraction uses the TPI column in the table above. The result is the rule value, and it lands at 77.0% of thread for every size in the chart, whatever the diameter — 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 the drills on the shelf. Many published inch tap drill tables print the theoretical figure d − P itself, so this nearest-entry column can differ from a printed chart by a drill size or two, and it differs for a reason: it names a drill that exists. Where the nearest entry falls below the thread's basic minor diameter it is flagged, because that drill would cut into the thread form.
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.
Do the largest inch sizes have a usable drill?
The inch drill set used here stops at 0.9844 in, and 4 threads in this chart sit above that. Above the top of the set 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. Those 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.