How to Measure Thread Size: Diameter, Pitch & TPI

You have a loose bolt in your hand and no idea what it is. Sizing it takes two numbers: how wide the thread is, and how tightly the threads are spaced. Get both, match them to a chart, and you have your answer. This guide walks through each measurement with the tools most people already own, plus what to do when you do not have a thread gauge.

The two numbers that define any thread

Every thread size, in any system, boils down to two values:

• Diameter — how wide the thread is, measured across the crests of a male thread. This is the "M10" or "3/8" part of the callout.
• Pitch or TPI — how closely the threads sit together. Metric threads state the pitch (the gap between crests, in millimetres). Imperial threads state threads per inch (TPI), the count of crests in one inch.

Nail those two and everything else (tap drill, tolerance class, torque) follows from a reference table. Miss either one and you are guessing.

Step 1: Measure the diameter

Set a caliper across the outside of a male thread, touching the tips of the crests on both sides. That reading is the major diameter, which is the nominal size for a bolt. An M10 bolt reads right around 10 mm; a 3/8" bolt reads about 9.53 mm.

Two things trip people up here. First, worn or dirty crests read slightly small, so clean the thread and take the largest stable reading. Second, a measured value a hair under nominal is normal. Threads are cut below the exact nominal so they fit their mating part, so 9.8–9.9 mm on a caliper still means M10.

Step 2: Measure the pitch (or TPI)

This is the measurement that actually pins down the thread, and the one most people get wrong. The fastest, most reliable tool is a thread pitch gauge: a fan of thin blades, each cut to one specific pitch or TPI. Press each blade into the thread until one seats perfectly, with the teeth dropping fully into the grooves and no light showing between them. Read the number stamped on that blade.

Metric gauge blades are marked in millimetres (0.8, 1.0, 1.25, 1.5, 1.75). Imperial blades are marked in TPI (16, 20, 24, 28). Owning a gauge that covers both is the single best few dollars you can spend if you deal with mixed hardware.

No gauge? Measure across several threads

You can still get pitch with just a ruler or caliper, and the trick is to measure across several threads rather than one. Line the fastener up against the scale, pick two crests as far apart as you can clearly count, and measure the distance between them. Then divide by the number of gaps between those crests.

The reason this works better than measuring a single thread is simple: any small error in your endpoints gets divided by the number of gaps. Spread across five threads, a half-millimetre misread shrinks to a tenth. For imperial threads, count how many crests fall inside one inch to read TPI directly.

Step 3: Match your numbers to a standard

With a diameter and a pitch in hand, the pattern usually reveals the system before you even open a chart:

For example, a 10 mm diameter with a 1.5 mm pitch is a textbook M10. A 0.5"-ish diameter reading 13 TPI points to 1/2"-13 UNC. Plug your two numbers into our cross-reference tool to confirm the exact match and catch metric/imperial look-alikes.

Watch for tapered pipe threads

One case breaks the "measure the diameter" rule: pipe threads like NPT and BSP are often tapered, so the diameter changes along the thread. If your bolt or fitting is visibly cone-shaped and destined for plumbing or hydraulics, measure the diameter about four or five threads in from the end, and expect the size name to reflect a nominal bore rather than the measured diameter. Our NPT vs BSP guide covers how to tell those two apart.

The bottom line

Thread sizing is never a guess once you have the right two numbers. Caliper the crests for diameter, seat a pitch gauge (or count crests across a measured span) for pitch, then match both to a chart. Two measurements and thirty seconds beat threading a mismatched fastener and stripping a hole every time.