CNC / machinist calculator
Tap Drill Size Calculator and Chart
Drill the right hole before you tap. This calculator gives the tap drill diameter for any inch or metric thread at the percent of full thread you choose, defaulting to the general-purpose 75 percent most shops use. Enter a major diameter and pitch, or read a common size straight off the chart below. It also works in reverse, telling you how much thread a drill you already have will leave, which saves a trip to the tool crib when the exact size is missing.
How it works
A tapped hole is never cut to a perfectly sharp 100 percent thread. The last few percent of thread add almost no strength but demand far more tapping torque and break taps, so the standard practice is to drill slightly oversize and leave a chosen percent of full thread, commonly 75 percent.
For a 60 degree thread the math is simple. The tap drill diameter equals the major diameter minus the thread percent times a geometry constant of 1.299 times the pitch. For inch threads the pitch is one divided by the threads per inch, so the drill equals the major diameter minus the percent times 1.299 divided by the TPI. The same formula in millimetres handles metric threads directly.
As a check, a quarter-inch twenty-TPI thread at 75 percent works out to 0.201 inch, which is the number seven drill every chart lists for 1/4-20. Dropping to 50 percent thread leaves a larger hole and an easier tap, useful in hard or gummy material where torque is the limit.
Worked example
A 1/4-20 thread at 75 percent: 0.25 - 0.75 x 1.299 / 20 = 0.2013 in, which rounds to the number 7 drill (0.201 in) that every tap drill chart lists.
Inch tap drill chart (75% thread)
| Thread | Tap drill | Decimal (in) |
|---|---|---|
| #4-40 UNC | #43 | 0.0890 |
| #6-32 UNC | #36 | 0.1065 |
| #8-32 UNC | #29 | 0.1360 |
| #10-24 UNC | #25 | 0.1495 |
| #10-32 UNF | #21 | 0.1590 |
| 1/4-20 UNC | #7 | 0.2010 |
| 1/4-28 UNF | #3 | 0.2130 |
| 5/16-18 UNC | F | 0.2570 |
| 3/8-16 UNC | 5/16 | 0.3125 |
| 1/2-13 UNC | 27/64 | 0.4219 |
Metric coarse tap drill chart (75% thread)
| Thread | Tap drill |
|---|---|
| M3 x 0.5 | 2.5 mm |
| M4 x 0.7 | 3.3 mm |
| M5 x 0.8 | 4.2 mm |
| M6 x 1.0 | 5.0 mm |
| M8 x 1.25 | 6.8 mm |
| M10 x 1.5 | 8.5 mm |
| M12 x 1.75 | 10.2 mm |
Chart values are the standard 75 percent-thread tap drills. The calculator above handles any size, pitch or thread percent, and matches these on the common threads.
NPT pipe thread tap drill chart
| Pipe size | Tap drill |
|---|---|
| 1/8-27 NPT | R (0.339") |
| 1/4-18 NPT | 7/16" (0.4375") |
| 3/8-18 NPT | 37/64" (0.5781") |
| 1/2-14 NPT | 23/32" (0.7188") |
| 3/4-14 NPT | 59/64" (0.9219") |
| 1-11.5 NPT | 1-5/32" (1.1563") |
NPT threads are tapered, so the calculator's straight-thread formula does not apply; these are the standard NPT tap drills (no reaming). The number after the size is threads per inch.
Frequently asked questions
How do I calculate tap drill size?
Subtract the thread percent times 1.299 times the pitch from the major diameter. For inch threads the pitch is one over the TPI, so a quarter-inch 20-TPI thread at 75 percent needs a 0.201 inch hole.
Why is 75 percent thread the standard?
A 75 percent thread keeps almost all the strength of a full thread while cutting tapping torque and tap breakage sharply. Going to 100 percent roughly doubles the torque for only a few percent more holding power.
What tap drill does 1/4-20 need?
A quarter-inch coarse thread at 75 percent engagement calls for a 0.201 inch hole, which is a number seven drill. That is the value the calculator returns and the one printed on standard tap drill charts.
Can I use a slightly different drill if I do not have the exact size?
Often yes. A nearby drill simply changes the percent of thread. Use the reverse mode to see what percent a given drill leaves; anywhere from about 60 to 75 percent is usually fine for general work.
Does this work for metric threads?
Yes. Switch to metric and enter the M diameter and the pitch in millimetres. An M6 by 1.0 thread at 75 percent comes out near 5.0 millimetres, the standard metric tap drill for that size.
Related calculators
Sources
Every formula on this page is shown and sourced. See how we verify.
These calculators are for planning and as a starting point. Recommended speeds and feeds are published starting values that vary with your specific tool, coating, machine rigidity, workholding and coolant. Always start conservative, listen to the cut, and follow your tool maker data sheet.