How to Measure Thread Pitch (and Calculate Pitch Diameter)

Thread pitch is the distance from one thread crest to the next, measured along the fastener’s axis. Metric fasteners state it directly in millimeters (a 1.5 mm pitch); inch fasteners state it as TPI — threads per inch — instead. The two are just inverses of each other: TPI = 25.4 ÷ pitch (mm), and pitch (mm) = 25.4 ÷ TPI. Once you have pitch, you can also calculate two numbers that don’t show up on a caliper directly — pitch diameter and minor diameter — using standard thread-geometry formulas.

This guide assumes you already know the fastener’s major diameter (if not, see How to Measure Thread Size first — pitch is only half of a complete thread-size reading).

Method 1: Thread Pitch Gauge (Fastest)

A pitch gauge is a folding set of thin blades, each stamped with a pitch or TPI value. Press each blade against the thread until you find the one whose teeth seat fully into the thread profile — no visible gap, no rocking. The number on that blade is your pitch or TPI.

The one mistake worth avoiding: try both a metric and an imperial gauge, even if you think you already know which system you’re dealing with. Because metric and imperial thread profiles are geometrically similar, a metric blade can sometimes look like a reasonable fit on an imperial thread — close enough to seem right if you don’t have anything to compare it against. The blade that seats with zero rocking, checked against both gauge sets, is the one to trust.

Method 2: Measure and Average (No Gauge)

If you don’t have a pitch gauge, a caliper or a fine ruler works, with one adjustment for accuracy: don’t measure a single thread space. Measure across several consecutive threads — 10 is a common choice — and divide the total length by the number of spaces.

• Metric: measure the distance across 10 thread spaces in mm, divide by 10, and that’s your pitch.
• Inch: position the calipers to span exactly one inch along the thread and count the complete thread peaks inside that span — that count is your TPI.

Measuring across more threads spreads out any single-reading error, which matters more on fine-pitch threads where one misjudged crest can throw off the whole reading.

Converting Between TPI and Metric Pitch

Because pitch and TPI describe the same physical spacing in different units, you can convert between them directly:

(Calculated from pitch = 25.4 ÷ TPI, rounded to two decimals — not a measured or quoted table, just the conversion formula applied to common TPI values.)

Calculating Pitch Diameter, Minor Diameter, and Thread Depth

Major diameter and pitch are what you measure directly. From those two numbers, standard thread-geometry formulas (ISO 68-1 for metric threads, ASME B1.1 for Unified/inch threads) let you calculate three more figures that machinists and thread-gauge manufacturers rely on:

Metric (external thread), where d = major diameter and P = pitch, both in mm:

• Pitch diameter: d2 = d − 0.6495 × P
• Minor diameter: d1 = d − 1.2269 × P
• Thread depth: h = 0.6134 × P

Unified/inch (external thread), where D = major diameter (in) and n = TPI:

• Pitch diameter: E = D − 0.6495 ÷ n
• Minor diameter: D − 1.2990 ÷ n
• Thread depth: h = 0.6134 ÷ n

These constants (0.6495, 1.2269, 1.2990, 0.6134) all come from the geometry of the standard 60° thread angle used by both the ISO metric and Unified systems — that’s why the metric and inch formulas share the same 0.6495 constant once pitch and 1/TPI are treated as equivalent.

Worked example (metric): For an M10 × 1.5 bolt (d = 10 mm, P = 1.5 mm):

• Pitch diameter = 10 − (0.6495 × 1.5) = 9.03 mm
• Minor diameter = 10 − (1.2269 × 1.5) = 8.16 mm
• Thread depth = 0.6134 × 1.5 = 0.92 mm

Pitch diameter is the figure that actually determines whether two threaded parts fit together correctly — it’s the diameter measured at the point where the thread groove and thread ridge are the same width, and it’s what thread gauges and go/no-go tolerance checks are built around. Major diameter alone tells you the nominal size; pitch diameter is what you’d check if a thread is fitting too tight or too loose despite reading the “right” nominal size on a caliper.

Common Metric Thread Pitches (Coarse Series)

If your measured pitch doesn’t match the coarse value for that diameter, don’t assume a measurement error — check the fine-pitch option before re-measuring. Many common metric sizes are available in both.

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

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.

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