Inch Tap Drill Chart (1/4 to 7/8, UNC and UNF)
Fractional inch threads from 1/4 to 7/8, coarse (UNC) and fine (UNF), plus the extra-fine (UNEF) 1/2-28 and 5/8-24. A fine thread takes a larger drill than the coarse thread of the same diameter.
| Thread | Series | Major diameter | Pitch | Aluminum, brass (75%) | Mild steel (60%) | Stainless, titanium (50%) |
|---|---|---|---|---|---|---|
| 1/4-20 | UNC | 0.2500 in (6.35 mm) | 20 TPI (1.270 mm) | #7 75% | #4 63%, over 2B | 7/32 in 48%, over 2B |
| 5/16-18 | UNC | 0.3125 in (7.94 mm) | 18 TPI (1.411 mm) | letter F 77% | letter H 64%, over 2B | letter J 49%, over 2B |
| 3/8-16 | UNC | 0.3750 in (9.52 mm) | 16 TPI (1.587 mm) | 5/16 in 77% | letter P 64%, over 2B | letter Q 53%, over 2B |
| 7/16-14 | UNC | 0.4375 in (11.11 mm) | 14 TPI (1.814 mm) | letter U 75% | letter V 65%, over 2B | 25/64 in 51%, over 2B |
| 1/2-13 | UNC | 0.5000 in (12.70 mm) | 13 TPI (1.954 mm) | 27/64 in 78% | 7/16 in 63%, over 2B | 29/64 in 47%, over 2B |
| 9/16-12 | UNC | 0.5625 in (14.29 mm) | 12 TPI (2.117 mm) | 31/64 in 72% | 1/2 in 58%, over 2B | 1/2 in 58%, over 2B |
| 5/8-11 | UNC | 0.6250 in (15.88 mm) | 11 TPI (2.309 mm) | 17/32 in 79% | 35/64 in 66%, over 2B | 9/16 in 53%, over 2B |
| 3/4-10 | UNC | 0.7500 in (19.05 mm) | 10 TPI (2.540 mm) | 21/32 in 72% | 43/64 in 60%, over 2B | 11/16 in 48%, over 2B |
| 7/8-9 | UNC | 0.8750 in (22.22 mm) | 9 TPI (2.822 mm) | 49/64 in 76% | 25/32 in 65%, over 2B | 51/64 in 54%, over 2B |
| 1/4-28 | UNF | 0.2500 in (6.35 mm) | 28 TPI (0.907 mm) | #3 80% | #2 63%, over 2B | #1 47%, over 2B |
| 5/16-24 | UNF | 0.3125 in (7.94 mm) | 24 TPI (1.058 mm) | letter I 75% | letter K 58%, over 2B | 9/32 in 58%, over 2B |
| 3/8-24 | UNF | 0.3750 in (9.52 mm) | 24 TPI (1.058 mm) | letter Q 79% | letter R 67% | letter S 50%, over 2B |
| 7/16-20 | UNF | 0.4375 in (11.11 mm) | 20 TPI (1.270 mm) | 25/64 in 72% | letter X 62%, over 2B | 13/32 in 48%, over 2B |
| 1/2-20 | UNF | 0.5000 in (12.70 mm) | 20 TPI (1.270 mm) | 29/64 in 72% | 29/64 in 72% | 15/32 in 48%, over 2B |
| 9/16-18 | UNF | 0.5625 in (14.29 mm) | 18 TPI (1.411 mm) | 33/64 in 65%, over 2B | 33/64 in 65%, over 2B | 33/64 in 65%, over 2B |
| 5/8-18 | UNF | 0.6250 in (15.88 mm) | 18 TPI (1.411 mm) | 37/64 in 65%, over 2B | 37/64 in 65%, over 2B | 37/64 in 65%, over 2B |
| 3/4-16 | UNF | 0.7500 in (19.05 mm) | 16 TPI (1.587 mm) | 11/16 in 77% | 45/64 in 58%, over 2B | 45/64 in 58%, over 2B |
| 7/8-14 | UNF | 0.8750 in (22.22 mm) | 14 TPI (1.814 mm) | 13/16 in 67% | 13/16 in 67% | 53/64 in 51%, over 2B |
| 1/2-28 | UNEF | 0.5000 in (12.70 mm) | 28 TPI (0.907 mm) | 15/32 in 67% | 15/32 in 67% | 15/32 in 67% |
| 5/8-24 | UNEF | 0.6250 in (15.88 mm) | 24 TPI (1.058 mm) | 37/64 in 87%, under 2B | 19/32 in 58%, over 2B | 19/32 in 58%, over 2B |
Small percentages show the thread engagement each drill leaves. "Over 2B" (6H for metric) marks a hole bigger than the maximum minor diameter of the usual thread class, and "under" one smaller than the minimum. Such a drill is out of tolerance for that class, so check the class on your drawing first. Click a thread for its full page, which names a drill inside the limits.
Inch Tap Drill Chart (1/4 to 7/8, UNC and UNF) | tapdrillsize.com/inch-tap-drill-chart/ | Last reviewed October 12, 2026 | Check your tap maker's chart before cutting.
UNC and UNF side by side
Every diameter here comes as a coarse (UNC) and a fine (UNF) thread, with a different drill for each.
| Diameter | Coarse thread | Coarse drill | Fine thread | Fine drill |
|---|---|---|---|---|
| 1/4 in (6.35 mm) | 1/4-20 | #7 | 1/4-28 | #3 |
| 5/16 in (7.94 mm) | 5/16-18 | letter F | 5/16-24 | letter I |
| 3/8 in (9.52 mm) | 3/8-16 | 5/16 in | 3/8-24 | letter Q |
| 7/16 in (11.11 mm) | 7/16-14 | letter U | 7/16-20 | 25/64 in |
| 1/2 in (12.70 mm) | 1/2-13 | 27/64 in | 1/2-20 | 29/64 in |
| 9/16 in (14.29 mm) | 9/16-12 | 31/64 in | 9/16-18 | 33/64 in |
| 5/8 in (15.88 mm) | 5/8-11 | 17/32 in | 5/8-18 | 37/64 in |
| 3/4 in (19.05 mm) | 3/4-10 | 21/32 in | 3/4-16 | 11/16 in |
| 7/8 in (22.22 mm) | 7/8-9 | 49/64 in | 7/8-14 | 13/16 in |
Extra-fine threads (UNEF)
Two of these diameters also come in the extra-fine series, with more threads per inch than UNF. 1/2-28 takes 15/32 in, a bigger drill than the 29/64 in for 1/2-20. 5/8-24 takes 37/64 in, the same drill as 5/8-18, so it leaves about 87% thread against 65% on 5/8-18.
Why a finer thread needs a bigger drill
A full thread is 0.0325 in deep on each side at 20 threads per inch (1/4-20) and 0.0232 in deep at 28 threads per inch (1/4-28). The fine thread cuts less into the wall, so its drill is closer to the screw diameter: #7 for 1/4-20 and #3 for 1/4-28.
Fractions to decimals
1/4 in is 0.2500 in and 6.35 mm. Each diameter in the table above is shown in inches and millimetres, so you can match it to a metric drill set. Thread depth per side is 0.6495 times the pitch.
FAQ
What is the tap drill size for 3/8-16?
5/16 in (0.3125 in, 7.94 mm) leaves about 77% thread. That is the standard drill for the coarse 3/8 thread. The fine 3/8-24 takes letter Q.
What is the tap drill size for 1/2-13?
27/64 in (0.4219 in, 10.72 mm) leaves about 78% thread. The fine 1/2-20 takes 29/64 in.
What is the difference between UNC and UNF?
UNC is the coarse series and UNF is the fine series of the same Unified Thread Standard (Wikipedia). At one diameter the fine thread has more threads per inch, so it is shallower and takes a bigger drill.
What is 5/16 inch in millimetres?
5/16 in is 0.3125 in, which is 7.94 mm. The table above lists every diameter in both units.
Chart drills match the American Fastener tap and drill chart and the Fabworks tap drill chart. Thread percentages use the formula printed in the OSG tap drill chart, Vol 3 (2024). The American Fastener chart has no extra-fine (UNEF) rows, so those are checked against the Fabworks chart and the Microform Precision tap drill chart. Last reviewed October 12, 2026. Method: how the numbers are calculated.