Thread pitch is the distance between one thread crest and the next. Reading it correctly tells you whether a bolt, nut, or fitting will match its partner, and getting it wrong by even a fraction of a millimeter can mean a stripped connection or a dangerous leak. The method depends on whether you’re working with metric or imperial threads, and both are straightforward once you know what to look for.
What Thread Pitch Actually Measures
Picture the spiral ridges running along a bolt. Thread pitch is the gap between the peaks of two neighboring ridges. In the metric system, that distance is expressed in millimeters. A bolt labeled M10 × 1.5, for example, has a 10 mm major diameter and a pitch of 1.5 mm, meaning each thread crest sits 1.5 mm from the next.
Imperial threads flip the concept. Instead of measuring the distance between crests, the imperial system counts how many thread crests fit inside one inch of length. This is called Threads Per Inch, or TPI. A 1/4″-20 bolt has a quarter-inch diameter and 20 threads packed into every inch. The two systems are inversely related: smaller pitch values in millimeters correspond to higher TPI numbers, because the threads are closer together.
Converting between the two is simple. Divide 25.4 by the metric pitch in millimeters and you get TPI. Divide 1 by TPI and you get pitch in inches. A 1.5 mm pitch, for instance, works out to about 17 TPI (25.4 ÷ 1.5).
Reading Metric Thread Pitch
Metric fasteners are labeled in a consistent format: the letter M, followed by the major diameter, then the pitch. M8 × 1.25 means 8 mm diameter, 1.25 mm pitch. If you already have the fastener in hand but no label, you need to measure that crest-to-crest distance yourself.
The quickest way is with a ruler or caliper. Count a set number of thread crests, measure the distance from the first crest to the last, and divide by the number of gaps between them. Counting across 10 gaps reduces rounding error. If 10 gaps span 12.5 mm, each pitch is 1.25 mm.
Reading Imperial TPI
For imperial bolts, lay a ruler along the threads and count how many crests fall within exactly one inch. That count is your TPI. If you can’t fit a full inch along the threaded section, count the crests in half an inch and double it.
Imperial fasteners come in two common series. Unified National Coarse (UNC) has fewer, wider-spaced threads. Unified National Fine (UNF) packs more threads per inch for the same diameter. A 1/2″ UNC bolt has 13 TPI, while a 1/2″ UNF bolt has 20 TPI. Here are TPI values for some of the most common sizes:
- 1/4″: 20 TPI (UNC), 28 TPI (UNF)
- 5/16″: 18 TPI (UNC), 24 TPI (UNF)
- 3/8″: 16 TPI (UNC), 24 TPI (UNF)
- 1/2″: 13 TPI (UNC), 20 TPI (UNF)
- 5/8″: 11 TPI (UNC), 18 TPI (UNF)
- 3/4″: 10 TPI (UNC), 16 TPI (UNF)
- 1″: 8 TPI (UNC), 12 TPI (UNF)
As bolt diameter increases, TPI drops. A 2″ UNC bolt has only 4.5 threads per inch, while a tiny #1 machine screw has 64.
Using a Thread Pitch Gauge
A thread pitch gauge is a fan-shaped set of thin metal blades, each cut with a different thread profile. It’s the most reliable hand tool for identifying an unknown thread, and it costs just a few dollars.
Start by cleaning any dirt or grease off the threads. Pick a blade that looks like a visual match for the spacing. Press it flat against the threads so the blade’s teeth nestle into the grooves. If the blade sits flush with no light showing between the teeth and the thread crests, you have the right pitch. If you see gaps or the blade rocks, try the next size up or down. Once you find a perfect match, the pitch value is stamped right on the blade.
Metric and imperial gauges are sold separately. If you aren’t sure which system you’re dealing with, try both sets. A metric blade will never sit perfectly on imperial threads, and vice versa, so the gauge itself will tell you which system the fastener uses.
Coarse vs. Fine Threads at a Glance
You can often tell coarse from fine threads without any tools. Coarse threads have taller, wider-spaced peaks that are easy to see and feel individually. Fine threads look tighter and more closely packed, almost smooth at a distance. Run your fingernail across the threads: coarse threads produce a distinct “click” with each crest, while fine threads feel more like a textured surface.
This matters because coarse and fine threads are not interchangeable. A coarse nut forced onto a fine bolt will cross-thread almost immediately. If you’re replacing a fastener and can’t read a label, identifying coarse versus fine is the first decision to get right.
Lead vs. Pitch on Multi-Start Threads
On most standard bolts, a single spiral winds from end to end. These are single-start threads, and the pitch and lead (the distance a nut travels in one full turn) are the same number. But some screws, especially ball screws and lead screws used in machinery, have two, three, or four independent spirals wound in parallel. These are multi-start threads, and they create a common source of confusion.
Pitch is still the distance between two adjacent crests on the same spiral. Lead, however, is the total distance the nut advances in one revolution. On a double-start screw, the lead is twice the pitch. On a four-start screw, the lead is four times the pitch. If you measure from one crest to the very next crest (which belongs to a different spiral), you’ll get the lead, not the pitch. To measure pitch on a multi-start thread, you need to follow one individual spiral and measure from one of its crests to the next crest on that same spiral.
Avoiding Common Measurement Mistakes
The most frequent error is measuring from the wrong reference points. Pitch is always crest to crest, not root to root or crest to root. Using the wrong pair of reference points will give you a number that doesn’t match any standard size.
Worn or damaged threads are another trap. If the crests are rounded or partially stripped, a pitch gauge may seem to fit a size that’s actually wrong. When threads look beat up, try to find an undamaged section further along the shaft before taking your reading.
Mixing up metric and imperial is surprisingly easy in the middle sizes. A 1.25 mm metric pitch is very close to 20 TPI (which works out to 1.27 mm). They look almost identical, but they will not mate properly. When two pitch gauge blades from different systems both seem close, pay attention to which one sits truly flush. Only one will have zero gaps.
Finally, be careful not to confuse pitch with lead on any thread that has more than one start. If the nut advances farther than expected per turn, you’re likely looking at a multi-start design, and the pitch is a fraction of the lead.