Interactive Thread Cheat Sheet
Filter ISO, UNC, and UNF series, reference drill diameters, and copy callouts for drawings without opening a handbook.
Open utility
What it does​
Search ISO, UNC, and UNF threads, then copy callouts or drill diameters straight into your drawing. The tables stay filterable in-browser so you can bounce between coarse, fine, or custom pitch values without flipping through a PDF handbook.
When to use it (and when not to)​
Use it when:
- You need a fast tap drill or clearance drill reference for a drawing or setup sheet.
- You want to copy a properly formatted callout into CAD without retyping.
- You are comparing metric and imperial series to pick a substitute size.
Avoid it when:
- You need tapered pipe threads, trapezoidal threads, or non-standard forms.
- You require tolerance class calculations beyond the provided reference values.
Inputs and outputs​
Inputs​
| Input | Description |
|---|---|
| Thread series | Pick ISO, UNC, or UNF to filter the table. |
| Nominal size | Select the diameter or gauge you need to drill or tap. |
| Pitch or TPI | Choose coarse, fine, or a specific pitch within the series. |
| Tolerance class | Reference fit such as 6H/6g or class 2; use to mirror drawing intent. |
| Drill preference | Toggle between tap drill and clearance drill references. |
Outputs​
| Output | Format | Notes |
|---|---|---|
| Tap drill diameter | Numeric | Recommended drill size for the selected thread. |
| Major/minor/pitch diameters | Numeric | Reference dimensions for QA and callouts. |
| Formatted callout | Text | Copy-ready string to paste into CAD or travelers. |
How to use​
- Pick the thread series and filter for the nominal size you need.
- Select the pitch and tolerance class.
- Switch to tap or clearance drill to see the right diameter.
- Copy the callout or drill size straight into your drawing.
- Optional: flip between metric and imperial lists to compare substitutes.
Example dataset: Thread: M12 x 1.75, Tolerance: 6H, Drill: tap drill.
- Expected output: Returns a tap drill recommendation (10.2 mm) and a callout string formatted as M12x1.75-6H.
How a tapping drill size is derived​
The number most people come here for is the tapping drill diameter, and it is worth knowing where it comes from — because the "standard" value in a handbook is a choice, not a constant.
A thread's depth is set by its pitch. For an ISO metric thread the theoretical
triangle height is H = 0.866 × P, and the actual thread engages 5/8 of that.
Working it through gives the classic form:
tapping drill ≈ D − P
where D is the nominal (major) diameter and P the pitch. M10 × 1.5 →
10 − 1.5 = 8.5 mm. M6 × 1.0 → 5.0 mm. That simple subtraction is the
formula behind most of the table.
What it hides is that D − P targets roughly 100 % thread engagement, and
full engagement is rarely what you want. The relationship is not proportional:
| Thread engagement | Holding strength | Tapping torque |
|---|---|---|
| 100 % | reference | reference |
| 75 % | ~95 % of full | ~50 % of full |
| 60 % | ~90 % | ~35 % |
Dropping to 75 % engagement costs about 5 % of thread strength and halves the
torque on the tap — which is why 75 % is the shop-floor default and why taps
break far more often in holes drilled "correctly" to D − P. In ductile
materials the joint almost always fails by bolt tension long before the threads
strip, so the last 25 % of engagement buys nothing.
The engagement-adjusted drill is:
drill = D − (P × engagement × 1.0825)
For M10 × 1.5 at 75 %: 10 − (1.5 × 0.75 × 1.0825) = 8.78 mm — in practice an 8.8 mm drill, noticeably larger than the 8.5 mm the simple formula gives.
Worked example — choosing a drill for M12 × 1.75 in steel​
Simple formula: 12 − 1.75 = 10.25 mm (100 % engagement).
At 75 % engagement: 12 − (1.75 × 0.75 × 1.0825) = 10.58 mm, so a 10.5 mm drill from the standard set.
The 10.5 mm drill is the handbook value for M12, and now the reason is visible: it is not the geometric minor diameter, it is the 75 %-engagement compromise rounded to a stocked drill size.
When to prefer the tighter 10.25 mm: thin sections where there are only one or two threads of engagement, or soft materials (aluminium, brass) where thread strength is the limiting factor rather than tap survival.
When to go larger still: deep blind holes in tough alloys, where tap breakage is the real risk and the extra engagement is not needed.
Common pitfalls​
- Assuming coarse pitch. M10 is 1.5 mm coarse but 1.25 or 1.0 fine — and the drill changes with it. Always state the pitch on the drawing for anything that is not coarse.
- Reading a UNC/UNF table as metric. Unified threads are specified in
threads per inch, so pitch is
1 / TPI, and the drill sizes are letter or number gauges rather than millimetres. - Ignoring the material. The same thread wants a slightly different hole in cast iron than in stainless; the table is a starting point, not a spec.
- Forgetting clearance holes are a different table. The tapping drill goes in the tapped part; the part being clamped needs a clearance hole, which is larger than the bolt's nominal diameter.
- Using the tapping drill for a through-hole in a nut plate. Roll-formed and cut threads have different starting diameters; follow the tap manufacturer's figure when one is given.
Accuracy and verification​
- Values follow common ISO and UNC/UNF reference tables; verify against your governing standard.
- Does not cover tapered pipe threads or specialty forms.
- Always validate drill sizes against tool availability and material before cutting.
- For critical fits, confirm tolerances with the official standard or gauge blocks.
FAQ​
- Can I add custom rows? Not yet. Use the provided series; paste results where needed.
- Are pipe threads included? No. The current set covers ISO, UNC, and UNF only.
- Does it run offline? Yes. The tables load locally in your browser.
- Can I copy values into CAD? Yes. Copy the callout or drill size directly from the table.
- Does it show both metric and imperial? Yes. Switch series to move between ISO and UNC/UNF lists.
Related tools​
Changelog​
- Initial documentation.
Feedback / bug report​
- Open a GitHub issue
- Email or DM with the slug
interactive-threadso we can reproduce the issue