Thread pitch reference: metric and unified charts you can actually use.
A working thread pitch reference for the shop floor and the design desk: metric coarse and fine pitch, UNC and UNF threads per inch, tap drill sizes, and a quick method to identify a thread you are holding in your hand.
Pick up an unmarked bolt and the first question is always the same: what thread is this? Get the pitch wrong and the tap drill is wrong, the nut will not run, and a ₹5 fastener can stall a ₹5 lakh assembly. This reference pulls the numbers you need into one place, with the logic behind them so you are not just copying a chart.
What thread pitch actually means
For metric threads, pitch is the distance from one thread crest to the next, in millimetres. An M10×1.5 has 1.5 mm between threads. For unified (inch) threads we instead quote threads per inch (TPI) — a 3/8-16 UNC has 16 threads in one inch. They describe the same thing from opposite directions: pitch in mm versus a count per inch.
One trap worth clearing up early: pitch is not lead. On a single-start thread (almost everything you meet) pitch and lead are equal. On a two-start thread the lead is twice the pitch, so the nut advances two pitches per turn. Power screws and some quick-action clamps use multi-start threads; standard fasteners do not.
Metric thread pitch chart (ISO 261 / ISO 965)
Standard metric fasteners follow ISO 261 for the coarse series and ISO 965 for tolerances. Coarse pitch is the default — if a drawing says M12 with no pitch, it means M12×1.75. Fine pitches are called out explicitly, e.g. M12×1.25.
| Size | Coarse pitch (mm) | Common fine pitch (mm) | Tap drill, coarse (mm) |
|---|---|---|---|
| M3 | 0.50 | 0.35 | 2.50 |
| M4 | 0.70 | 0.50 | 3.30 |
| M5 | 0.80 | 0.50 | 4.20 |
| M6 | 1.00 | 0.75 | 5.00 |
| M8 | 1.25 | 1.00 | 6.80 |
| M10 | 1.50 | 1.25 | 8.50 |
| M12 | 1.75 | 1.25 | 10.20 |
| M16 | 2.00 | 1.50 | 14.00 |
| M20 | 2.50 | 1.50 | 17.50 |
| M24 | 3.00 | 2.00 | 21.00 |
The tap drill column above is for a standard ~70–75% thread engagement, which is the usual target for steel. Need an exact value for a different size, material, or thread percentage? The thread pitch and tap drill calculator returns the drill diameter for any metric or unified thread without you reading across a chart.
Unified thread chart: UNC and UNF (TPI)
Inch threads come mainly in two series: UNC (Unified National Coarse) and UNF (Unified National Fine), defined in ASME B1.1. Coarse is the general-purpose default; fine gives more threads per inch, higher tensile stress area, and better resistance to loosening under vibration — at the cost of being easier to cross-thread.
| Size | UNC (TPI) | UNF (TPI) | Major dia (in) |
|---|---|---|---|
| #6 | 32 | 40 | 0.138 |
| #8 | 32 | 36 | 0.164 |
| #10 | 24 | 32 | 0.190 |
| 1/4" | 20 | 28 | 0.250 |
| 5/16" | 18 | 24 | 0.3125 |
| 3/8" | 16 | 24 | 0.375 |
| 1/2" | 13 | 20 | 0.500 |
| 5/8" | 11 | 18 | 0.625 |
| 3/4" | 10 | 16 | 0.750 |
| 1" | 8 | 12 | 1.000 |
To convert TPI to pitch in millimetres: pitch (mm) = 25.4 ÷ TPI. So a 1/2-13 UNC has a pitch of 25.4 ÷ 13 = 1.95 mm. That conversion is how you sanity-check whether a thread gauge reading is metric or inch when the numbers look close.
How to identify an unknown thread
You have a fastener and no markings. Work through this in order:
- Measure the major diameter with a caliper across the crests. ~6 mm points to M6; ~0.25 in (6.35 mm) points to 1/4 inch. This first reading tells you which family you are in.
- Check the pitch with a thread pitch gauge. Try leaves until one seats with no rocking and no light gap. Metric leaves read in mm (1.0, 1.25, 1.5); inch leaves read in TPI (20, 18, 16).
- Confirm against the chart. M10 measures ~10 mm major; if the gauge reads 1.5 mm it is M10×1.5 coarse, if 1.25 mm it is M10×1.25 fine.
- If you have no gauge, count threads over a known length. Lay the bolt on a rule, count crests across 10 mm, and pitch = 10 ÷ (number of gaps). For inch, count over one inch to get TPI directly.
Tap drill size: the quick rule
For metric threads a fast approximation is: tap drill ≈ major diameter − pitch. M8×1.25 gives 8 − 1.25 = 6.75 mm, and the standard drill is 6.8 mm. That rule targets roughly 75% thread engagement, which balances strength against tapping torque — chasing 100% engagement only adds a few percent of strength while sharply raising the chance of a broken tap.
For unified threads the arithmetic is messier, so a chart or calculator is faster. In every case, harder materials and deeper holes are reasons to drop to a lower thread percentage (a slightly larger drill) to keep the tap from binding.
Common mistakes
- Assuming coarse on a fine-thread part. A drawing that calls M10×1.25 will not accept a standard M10×1.5 nut. Read the callout, do not assume.
- Confusing pitch with TPI. A larger pitch number is a coarser thread; a larger TPI number is a finer thread. They run in opposite directions.
- Ignoring lead on multi-start screws. On a 2-start leadscrew the nut moves two pitches per turn — size feeds and stops accordingly.
- Tapping with the wrong drill. Undersized drill means high torque and broken taps; oversized means weak, stripped threads.
Once a threaded part is made, the inspection drawing still has to be ballooned and checked — thread callouts included. CadNexa's auto-ballooning tool reads a PDF drawing with Smart Detect and Box+Balloon OCR, numbers every dimension and thread spec, and exports an inspection sheet. For the design side, pair this reference with the bolt torque guide for clamp load, the ISO 286 fits guide for the mating holes, and the ready-made engineering templates.
Frequently asked questions
How do I find the pitch of a thread without a gauge?
Lay the fastener against a steel rule, count the gaps between crests over a 10 mm span, and divide: pitch = 10 ÷ number of gaps. For an inch thread, count the crests over one inch to read threads per inch directly. A thread pitch gauge is faster and more reliable, but the rule method works in a pinch.
What is the difference between coarse and fine thread?
Coarse threads (UNC, metric coarse) have larger pitch and fewer threads per inch — faster to assemble and more damage-tolerant. Fine threads (UNF, metric fine) have smaller pitch, more threads per inch, a larger stress area, and better vibration resistance, but they cross-thread more easily and need cleaner handling.
How do I convert TPI to metric pitch?
Pitch in millimetres equals 25.4 divided by threads per inch. A 1/4-20 UNC therefore has a pitch of 25.4 ÷ 20 = 1.27 mm. Use this to compare an inch thread against the metric chart when a gauge reading is ambiguous.
What tap drill do I use for M8?
For M8×1.25 coarse, the standard tap drill is 6.8 mm, which gives about 75% thread engagement. The quick rule is major diameter minus pitch: 8 − 1.25 = 6.75 mm, rounded to the nearest standard drill at 6.8 mm. For M8×1.0 fine, use a 7.0 mm drill.