Tap Drill Size Calculator
This tap drill size calculator finds the right drill for a tapped hole from the thread diameter, the pitch or threads per inch, and the thread percentage you want. It works for metric and inch threads, returns the exact tap drill size, the nearest standard drill, and the thread engagement that drill gives, so you can drill, tap, and get strong threads the first time.
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The Tap Drill Size Calculator is part of our Machinery collection. Browse more machinery to size, convert, and check your work alongside this tool.
What the tap drill size calculator does
A tap cuts threads into the wall of a drilled hole, so the hole must be smaller than the thread by the right amount. Too large and the thread is shallow and weak; too small and the tap binds or snaps. The tap drill size calculator applies the standard thread percentage formula, then rounds to the nearest 0.1 mm metric drill or 1/64 in fractional drill and shows how much thread that drill leaves.
Tap drill formula
For 60° unified and ISO metric threads, the full thread depth on the diameter is 1.299 times the pitch. The drill removes all but the chosen percentage:
At 77% thread the metric formula reduces to the familiar shortcut Drill = D − P, which is why an M10 × 1.5 tap uses an 8.5 mm drill.
Tap drill size calculator example
An M10 × 1.5 thread at 75%:
The nearest standard drill is 8.5 mm, which gives 77% thread engagement. For a 1/4-20 UNC thread at 75%, the tap drill size calculator returns 0.2013 in; the nearest fractional drill is 13/64 in (0.2031), good for 72.2% thread, and the standard chart choice is a #7 drill at 0.201 in.
How to use the tap drill size calculator
Pick the thread system. For metric, enter the major diameter and the pitch in millimeters, such as 10 and 1.5 for M10 × 1.5. For inch threads, enter the major diameter in inches and the threads per inch, such as 0.25 and 20 for 1/4-20. Enter the thread percentage, 75% by default, and press Calculate. The nearest standard drill and its thread engagement appear under the exact size.
Reading the tap drill size
The exact size is the ideal hole for your thread percentage; the nearest drill is what you can buy. Check the engagement line: 65 to 75% suits most work in steel, and 50 to 60% is common in hard or tough materials and for deep holes, where it eases tapping torque. A higher percentage adds little strength but much more torque, which raises the risk of a broken tap. Tapping soft materials such as aluminum and brass can run closer to 75 to 80%.
When to use a tap drill calculator
- Picking a drill for a thread missing from your chart
- Choosing a smaller thread percentage for stainless or titanium
- Checking the drill for fine pitch metric threads
- Converting a metric tap drill to inches and back
- Planning threaded holes on a drawing
Common tap drill size calculator mistakes
- Entering TPI for a metric thread. Metric needs pitch in mm; inch needs threads per inch.
- Using a cutting tap chart for a forming tap. Forming taps need a larger hole.
- Choosing 100% thread. It adds little strength and breaks taps.
- Ignoring hole depth. Drill deeper than the thread to leave room for the tap point.
Tips for drilling the right tap drill size
- Use cutting fluid matched to the material.
- Reverse the tap a half turn every turn or two to break chips by hand.
- Chamfer the hole entry so the tap starts square.
Metric tap drill size and thread percentage
Thread percentage compares the depth of the thread you cut with the full theoretical depth. A 100% thread would leave sharp crests that tear and need huge tapping torque. Tests show that a 75% thread carries close to the full strength of the fastener in steel, because the bolt usually breaks before the thread strips once engagement is long enough. That is why metric tap drill charts use D − P, about 77%, and why inch charts settle near 75%. For roll form taps, which push metal instead of cutting it, the hole must be larger, close to D − P ÷ 2, so never use a cutting tap chart for a forming tap. When the drill size falls between standard drills, pick the larger drill for hard materials and the smaller for soft ones. Once the hole is drilled, the surface speed calculator helps you set the RPM for the drill itself.
UNC tap drill size and number drills
Inch tap drill charts mix fractional, number, and letter drills because a 1/64 in step is too coarse for small threads. The 1/4-20 thread is a good example: 13/64 in gives 72% thread, while the #7 drill, 0.201 in, sits almost exactly at 75%. Number drills run from #80 at 0.0135 in up to #1 at 0.228 in, and letter drills from A at 0.234 in to Z at 0.413 in. This calculator shows the fractional choice and its engagement; check a number or letter drill chart when you need to land closer to the exact value. Fine threads such as 1/4-28 UNF need a larger drill than coarse threads of the same diameter because the thread is shallower.
Tap drill size chart
Common coarse threads with the standard chart drill and the 75% formula value:
| Thread | Standard tap drill | 75% formula |
|---|---|---|
| M3 × 0.5 | 2.5 mm | 2.513 mm |
| M4 × 0.7 | 3.3 mm | 3.318 mm |
| M5 × 0.8 | 4.2 mm | 4.221 mm |
| M6 × 1.0 | 5.0 mm | 5.026 mm |
| M8 × 1.25 | 6.8 mm | 6.782 mm |
| M10 × 1.5 | 8.5 mm | 8.539 mm |
| M12 × 1.75 | 10.2 mm | 10.295 mm |
| #10-24 UNC | #25 (0.1495 in) | 0.1494 in |
| 1/4-20 UNC | #7 (0.201 in) | 0.2013 in |
| 5/16-18 UNC | F (0.257 in) | 0.2584 in |
| 3/8-16 UNC | 5/16 (0.3125 in) | 0.3141 in |
| 1/2-13 UNC | 27/64 (0.4219 in) | 0.4251 in |
Tap Drill Size Calculator FAQ
How do you calculate tap drill size?
For metric threads, subtract the pitch from the diameter for about 77% thread, or use D minus thread percent ÷ 100 × 1.299 × P for an exact percentage. For inch threads, use D minus 0.01299 × percent ÷ TPI.
What size drill for an M8 tap?
An M8 × 1.25 coarse thread uses a 6.8 mm drill. The formula gives 6.782 mm at 75% thread.
What size drill for a 1/4-20 tap?
The standard choice is a #7 drill at 0.201 in. A 13/64 in drill also works and gives about 72% thread.
What thread percentage should I use?
75% is the general standard. Drop to 60 to 65% for stainless steel, deep holes, or hand tapping, since the extra thread adds little strength.