Tap Drill Size Calculator
Find the right drill for a thread, and how much thread you will get.
Calculator
Example Result
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The calculated result will appear here after you press Calculate.
Watch: M10 Tap Drill: 10 mm or 8.5 mm? Coarse vs Fine Pitch
YouTube Short · 0:29
An M10 hole is not drilled at 10 mm: the tap cuts the thread and the drill leaves the core. M10 × 1.5 coarse takes an 8.5 mm drill, M10 × 1.25 fine 8.8 mm and M10 × 1 fine 9.0 mm. Rule of thumb: drill ≈ major diameter − pitch, about 75% thread height. ISO coarse tap drills: M6 5.0, M8 6.8, M10 8.5, M12 10.2 mm.
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About the Tap Drill Size Calculator
A tap cuts its thread into a hole that is smaller than the screw. Choose the thread, enter the size and pitch, and set the percent of thread you want, usually 75%. You get the exact hole size, the nearest standard drill, and the share of load the thread can carry. Choose a forming tap if you are not cutting the thread.
How to Use the Tap Drill Size Calculator
- 1
Choose a cutting or a forming tap.
- 2
Choose the thread, or enter another size and pitch.
- 3
Enter the percent of thread, usually 75.
- 4
Add the thread depth if the hole is blind.
- 5
Press Calculate to see the drill, the thread you get and the load it can carry.
Tap Drill Size Calculator Examples
Frequently Asked Questions
How do you calculate tap drill size?
What is the tap drill for M12?
What is the tap drill for 1/2-13?
Why is 75% thread standard?
Is major diameter minus pitch accurate?
What size hole for a forming tap?
What happens if the hole is too large?
By O.E. — Partner, chairman of the board and CEO of a metal products manufacturer for 21 years; the company exports to 45 countries.
Drafted with AI assistance; calculations checked against primary sources and tested with the site's own code. About FlowUtil
Last reviewed
The formula, and a worked 1/2-13
Hole diameter = major diameter − (percent of thread × pitch) ÷ 76.98
For inch threads, pitch = 1 ÷ TPI.
1/2-13 UNC at 75%: 0.500 − (75 × 0.0769) ÷ 76.98 = 0.4251 in. The nearest fractional drill is 27/64 (0.4219 in), which gives 78% thread.
M12 × 1.75 at 75%: 12 − (75 × 1.75) ÷ 76.98 = 10.30 mm. The standard drill is 10.2 mm, which gives 79%.
The metric rule of thumb, major diameter minus pitch, comes straight out of this formula at about 77% thread. M12 gives 12 − 1.75 = 10.25 mm.
| Thread | Exact, 75% | Common drill | Thread you get |
|---|---|---|---|
| M6 × 1 | 5.03 mm | 5.0 mm | 77% |
| M8 × 1.25 | 6.78 mm | 6.8 mm | 74% |
| M10 × 1.5 | 8.54 mm | 8.5 mm | 77% |
| M12 × 1.75 | 10.30 mm | 10.2 mm | 79% |
| 1/4-20 | 0.2013 in | #7 (0.201 in) | 75% |
| 3/8-16 | 0.3141 in | 5/16 (0.3125 in) | 77% |
| 1/2-13 | 0.4251 in | 27/64 (0.4219 in) | 78% |
| 1/2-20 | 0.4513 in | 29/64 (0.4531 in) | 72% |
The full lists are in the tap drill size chart. The calculator keeps its drill inside the thread's limits. For 1/2-13 UNC class 2B, the minor diameter must lie between 0.417 and 0.434 in, so a 7/16 (0.4375 in) drill is too large. For M12 class 6H, it must lie between 10.106 and 10.441 mm.
What thread percent actually buys you
Thread percent is the height of the thread compared with a full thread. More is not always better. Lew Merrick, PE, gives this estimate of the load a cut thread can carry compared with a 100% thread:
Load % = 100 × [1 − (1 − T)²]², with T the thread percent as a decimal.
| Thread | Load carried |
|---|---|
| 60% | 70.6% |
| 65% | 77.0% |
| 70% | 82.8% |
| 75% | 87.9% |
| 85% | 95.6% |
| 100% | 100% |
Dropping from 100% to 75% gives up about 12% of the load, in return for a much lower tapping torque and about twice the tap life. That is why 75% is the usual choice, and why deep holes and tough alloys often drop to 60–65%.
Strength also depends on how long the thread is. The thread engagement calculator covers depth, and a longer engagement can more than make up for a lower thread percent.
Forming taps need a bigger hole
A forming tap pushes metal into shape and removes no chips. The metal flows inward, so the hole starts larger:
Hole diameter = major diameter − (percent × pitch) ÷ 147.1
At 65%, an M12 × 1.75 needs about 11.2 mm, against 10.2 mm for a cutting tap. Use the tap maker's figure, since the hole size is less forgiving here. A cutting-tap hole will overload a forming tap and can break it. These taps need ductile material, so they do not suit cast iron or very hard steels.
How deep to drill a blind hole
The drill point and the tap's lead-in do not make full threads, so the hole must go deeper than the thread. From Merrick's guide, add these to the thread depth:
| Tap type | Add |
|---|---|
| Plug tap | 6 × pitch + minor diameter |
| Bottoming tap | 3 × pitch + minor diameter |
Add more for chips: up to 40% of the diameter for each diameter of depth in stringy materials such as mild steel.
For M10 × 1.5, 15 mm deep with a plug tap: 15 + 9 + 8.4 = 32.4 mm before the chip allowance. The bolt must also fit, so check it with the screw length calculator.
Reading the size from a thread
If you are matching an existing part, measure the major diameter and the pitch. The metric thread size chart and the UNC thread chart list the standard pitches. An M12 can be coarse (1.75) or fine (1.5 or 1.25), and each needs a different hole.
Sources
- L. Merrick, Internal Threading: Pilot Hole Considerations, Revision A, 2010, Little Machine Shop.
- ASME, ASME B1.1, Unified Inch Screw Threads, minor diameter limits as published in thread tables.
- ISO, ISO 2306, Drills for use prior to tapping screw threads.
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