How to Measure Thread Size: Diameter, Pitch, and Reading the Callout

Thread size means two numbers: the major diameter (how wide the thread is) and the pitch or threads-per-inch (how closely the threads are spaced). Together they’re written as a callout like M8 × 1.25 (metric) or 1/4-20 UNC (inch). To find both, you measure the outside diameter with calipers, then measure the spacing between thread peaks with a pitch gauge or by counting threads over a known length.

What You Need

• Digital or vernier calipers — the tool every source agrees on for the diameter measurement
• A thread pitch gauge (a set of thin, notched blades, metric and imperial) — the fastest way to read pitch or TPI directly
• A ruler, as a fallback if you don’t have a pitch gauge

Step 1: Measure the Major Diameter

External threads (bolts, screws): Place the caliper jaws across the outside edges — the crests — of the thread, not the smooth shank below the head. A reading close to a round number in millimeters usually points to a metric size (about 6 mm → likely M6; about 10 mm → likely M10). A reading close to a fraction of an inch points to an inch size (about 0.375 in → likely 3/8 in).

Internal threads (nuts, tapped holes): Use the caliper’s upper jaws, designed for inside measurements, and measure across the thread grooves (root-to-root) rather than the crests.

Check for taper before you trust the number. Most bolts and screws are straight (parallel) threads, but pipe fittings often aren’t. Measure the diameter at the first thread, then again a few threads further in. If the number changes, the thread is tapered — most likely NPT or a similar pipe thread — and a single diameter reading won’t identify it correctly. Tapered threads are outside the scope of this guide; if you’re measuring a pipe fitting, you’ll need a pipe-thread-specific reference rather than the straight-thread method below.

Step 2: Measure the Pitch

Pitch is the second half of the callout, and it’s easy to get wrong by eye, so use one of these two methods:

1. Thread pitch gauge (fastest and most reliable). Press the gauge’s blades against the thread until you find the one that seats fully — no gaps, no rocking. The number stamped on that blade is your pitch (in mm) or TPI, depending on which gauge you’re using.

1. No gauge on hand. Measure the distance across several consecutive thread peaks with a caliper or ruler, then divide by the number of spaces. Measuring across more threads (say, 10) and dividing by 10 spreads out any reading error and gives a more reliable number than measuring a single space.

One detail most quick guides skip: try both a metric and an imperial pitch gauge if you’re not sure which system you’re dealing with. Some imperial threads can make a metric gauge blade look like it fits when it doesn’t fit as tightly as the correct imperial blade would. Whichever gauge seats with zero rocking is the right one.

This guide covers pitch only far enough to complete a full thread-size reading. If you need the deeper mechanics — TPI-to-millimeter conversion, or calculating pitch diameter and minor diameter once you know the pitch — see How to Measure Thread Pitch (and Calculate Pitch Diameter).

Step 3: Read (or Write) the Full Callout

Once you have both numbers, the format depends on which system you’re in:

• Metric: M<diameter> × <pitch> — e.g., M10 × 1.5 means a 10 mm major diameter with 1.5 mm between threads.
• Inch: <diameter>-<TPI> <series> — e.g., 1/2-13 UNC means a 1/2-inch major diameter, 13 threads per inch, in the Unified National Coarse series. The same 1/2-inch diameter at 20 TPI would be 1/2-20 UNF (Unified National Fine) — same diameter, different pitch, different part.

If you’re staring at a printed callout rather than a physical part, reading it works the same way in reverse: split the string into diameter, pitch/TPI, and series, and each segment tells you exactly what to check the part against.

Quick Reference: Common Sizes

Inch (UTS) — UNC and UNF threads per inch, by major diameter:

Source: Unified Thread Standard (UTS) reference tables.

Metric (ISO 724) — coarse-pitch series, common sizes:

Source: ISO 724 metric thread standard reference tables. Many of these sizes also come in a finer optional pitch (e.g., M10 × 1.25 fine vs. M10 × 1.5 coarse) — if you’re not sure which one you have, measure the pitch directly rather than assuming coarse.

Common Ways to Misread a Measurement

A caliper reading that’s slightly off a standard size is usually a rounding problem, not an unusual part. A bolt that measures 9.8 mm isn’t a mystery size — it’s most likely an M10 with normal manufacturing tolerance and a bit of wear on the crests. This is why pitch is worth confirming even when the diameter looks obvious: two adjacent standard sizes can read close enough on a diameter-only measurement that the pitch is what actually tells them apart.

Related reading: How to Measure Thread Pitch (and Calculate Pitch Diameter)

Danny Ni Engineering & Mechanical Systems Writer

Danny Ni is an engineering-focused technical writer at SYZ Machine, specializing in mechanical components, linkage systems, and real-world application engineering. His work covers aftermarket vehicle parts, industrial joints, and mechanical principles, translating complex engineering concepts into practical insights for engineers, fabricators, and industry buyers.

See Full Bio