Tap Drill Size Calculator
Find the correct tap drill size for any metric or imperial thread. Pick a thread from the ISO metric coarse/fine and UNC/UNF tables (or enter a custom major diameter and pitch), set the thread engagement you want, and get the theoretical hole size plus the nearest real drill — metric, fractional, number or letter — with the engagement it actually delivers.
Input
Thread
Fit
75% is the shop standard. Drop to 60–65% in tough or deep material to cut tapping torque; go higher only in soft, thin material.
Which drill set to recommend from. Auto follows the thread standard.
Output
| Property | Value |
|---|---|
| No data yet | |
| Drill | Ø (mm) | Ø (in) | Engagement | Note |
|---|---|---|---|---|
| No data yet | ||||
A larger drill lowers engagement and tapping torque; a smaller one raises both. Strength gains above ~75% are marginal.
Guides
Before you can cut an internal thread you have to drill the hole it lives in — and that hole is not the thread's nominal size. Drill it too big and the thread is weak; too small and the tap binds and snaps off inside the part. This calculator gives you the right hole for any metric or imperial thread, at whatever percentage of thread engagement you are aiming for, and then names a real drill you can actually pull out of the index.
How to use it
- Pick a thread standard: ISO metric (coarse and fine, M1.6 to M36), imperial UNC/UNF (#0 to 1"), or Custom thread if you have a diameter and pitch that is not on either list.
- Choose the thread from the list, or type the major diameter and pitch (mm) / threads per inch.
- Set the thread engagement you want. 75% is the shop default and what published tap-drill charts assume.
- Choose which drill set you have. Auto recommends metric drills for metric threads and fractional/number/letter drills for imperial ones — but you can force either, which is exactly what you need when tapping a metric thread in an imperial shop or vice versa.
The result names the recommended drill, its diameter in both mm and inches, and the engagement that drill actually produces — plus the three sizes either side of it so you can trade engagement against tapping torque yourself.
The formula
Both the metric and the inch shop formulas come from the same geometry — a 60° thread whose double depth is 1.299 × pitch:
tap drill Ø = major Ø − (engagement% / 100) × 1.299 × pitch
In inches, with pitch = 1/TPI, that is the familiar D − (%thread × 0.01299)/TPI. In millimetres it is the equally familiar D − (%thread × pitch)/76.98. Worked examples: M8 × 1.25 at 75% gives 6.78 mm → a 6.8 mm drill; 1/4-20 UNC at 75% gives 0.2013" → a #7 drill. Both match the standard charts.
What thread engagement should I use?
75% for general work. The strength gain above 75% is small — roughly 5% more thread strength for double the tapping torque — while the risk of breaking a tap climbs steeply. Drop to 60–65% for stainless, titanium, hardened steel, deep blind holes, or any hole you would rather not have to salvage. Go above 80% only in soft material with a very short engagement length.
Why doesn't the calculator match my shop chart exactly?
Published charts round to the drill someone decided was closest, and different charts round differently — some list 10.2 mm for M12 × 1.75, others 10.3 mm. This tool shows you the theoretical size and the engagement each nearby drill gives, so you can see the trade-off the chart hid rather than trusting its rounding.
Does it cover pipe threads?
No. NPT, BSPT and other tapered pipe threads are drilled to a completely different scheme that depends on the taper and how far the tap is driven in — the 60° straight-thread formula above does not apply.
Privacy
The whole calculation and every thread and drill table run in your browser. Nothing is uploaded.
Related tools
Once the hole is tapped, the Bolt Torque Calculator gives you a tightening torque for the fastener that goes into it. For the drilling and tapping operation itself, the Machining Speed and Feed Calculator works out spindle RPM and feed rate, and the Unit Converter handles any leftover mm-to-inch conversions.