Thread pitch and TPI, explained

Thread pitch is just the spacing of the threads — but two measuring systems describe the same thing, and mixing them is where it goes wrong. Metric counts millimetres between threads; inch counts threads per inch. Once you can move between them, the rest of threading falls into place.

Two ways to say the same spacing

metric: pitch in mm (M8 × 1.25 → 1.25 mm) · inch: TPI, pitch = 1/TPI · pitch(mm) = 25.4 / TPI

A metric thread names the pitch outright: the 1.25 in M8 × 1.25 is 1.25 mm crest-to-crest. An inch thread names the count instead: 1/2-13 has 13 threads per inch, a pitch of 25.4 / 13 = 1.954 mm. The same M8 × 1.25 works out to about 20.3 TPI — useful when you only have an inch pitch gauge. The full lookup is the thread pitch chart.

Coarse vs fine

Each diameter has a coarse and a fine standard. Coarse (UNC, metric coarse) has the larger pitch — fewer, deeper threads that run fast, tolerate plating and nicks, and hold well in soft or brittle stock. Fine (UNF, metric fine) has the smaller pitch — more thread engagement per turn, finer preload adjustment, and better vibration resistance, at the cost of easier cross-threading. 1/2-13 is coarse; 1/2-20 is the fine version of the same diameter.

Why pitch drives everything downstream

Pitch sets the tap drill, the thread depth and the feed when single-point threading. The tap drill, for instance, comes straight from it:

tap drill = major − (%thread × pitch) / 76.98

An M8 × 1.25 at 75% thread needs 8 − (75 × 1.25)/76.98 ≈ 6.8 mm — the standard M8 tap drill. Run any size and engagement on the tap drill size calculator; the pitch-diameter side (the dimension that actually gauges a thread) is the thread pitch diameter calculator, and the tolerance grades live in the thread classes guide.

Common mistakes

• Reading pitch as TPI. M8 × 1.25 is 1.25 mm pitch, not 1.25 TPI — metric states the gap, inch states the count, and they are reciprocals through 25.4.
• Assuming one pitch per diameter. Most diameters have both a coarse and a fine standard; "1/2 inch thread" is ambiguous until you say 13 or 20 TPI.
• Measuring one thread. Count across 10 threads and divide — a single gap is too small to read accurately with a rule.
• Mixing metric and inch hardware. M12 × 1.75 and 1/2-13 are close in size but different pitches and angles; they will cross-thread and strip.

Frequently asked questions

What is thread pitch?

The distance from one thread crest to the next, measured along the screw axis. Metric threads state it directly in millimetres — an M8 × 1.25 has 1.25 mm between threads. Inch threads state it as threads per inch (TPI) instead, so pitch = 1 ÷ TPI.

How do you convert TPI to pitch in mm?

Pitch (mm) = 25.4 ÷ TPI. A 1/2-13 UNC thread has 13 TPI, so its pitch is 25.4 ÷ 13 = 1.954 mm. Going the other way, an M8 × 1.25 metric thread is equivalent to 25.4 ÷ 1.25 ≈ 20.3 TPI.

What is the difference between coarse and fine thread pitch?

Coarse threads (UNC, metric coarse) have larger pitch and fewer threads per inch — faster to run, more tolerant of damage and plating, better in soft or brittle material. Fine threads (UNF, metric fine) have smaller pitch — more holding power and finer adjustment, less likely to vibrate loose, but easier to cross-thread. 1/2-13 is coarse; 1/2-20 is fine.

How do you measure thread pitch?

Quickest is a thread pitch gauge — match the leaf that seats cleanly in the threads. Without one, lay a rule along the threads and count: for metric, measure across 10 threads and divide by 10 for the pitch in mm; for inch, count the threads in one inch for the TPI.