The pitch of a screw is the distance between two adjacent thread peaks, and you can find it by counting threads, using a gauge, or applying a simple formula. For metric screws, pitch is expressed in millimeters. For imperial screws, pitch is expressed as threads per inch (TPI). Once you know which system you’re working with, the actual measurement takes under a minute.
What Screw Pitch Actually Means
Pitch is the distance from the crest of one thread to the crest of the next, measured along the length of the screw. On a metric screw like an M8×1.25, the “1.25” means each thread is 1.25 mm apart. The imperial system flips this around: instead of measuring the gap, you count how many thread peaks fit in one inch. A screw with 20 TPI has threads spaced 1/20 of an inch apart.
These two systems describe the same physical property from opposite directions. If you know one, you can convert to the other. A screw with 20 TPI has a pitch of 0.05 inches (1 divided by 20). A metric screw with a 2 mm pitch has the equivalent of 12.7 TPI (25.4 divided by 2).
The Counting Method (No Special Tools)
The simplest way to find pitch is to count threads over a known distance. Lay a ruler along the screw so the edge sits parallel to the shaft. For metric screws, count the number of thread peaks that fall within 10 mm, then divide 10 by that count. For imperial screws, count the peaks within one inch. That count is your TPI.
Counting over a longer span gives you a more accurate result, especially on small fasteners where the threads are tightly packed. If you can see 20 threads across 10 mm, the pitch is 0.5 mm. If you only measured across 2 mm, a slight miscount throws the whole number off. Whenever possible, count across at least 10 threads and divide the total distance by the number of gaps between them (which is one fewer than the number of peaks you counted).
The Pitch Formula
The math behind this is straightforward: P = L / n, where P is the pitch, L is the length of threaded section you measured, and n is the number of complete thread gaps in that section.
For a metric example: if you measure 20 mm of threaded length and count 10 complete threads, the pitch is 20 / 10 = 2 mm. For an imperial example: if you count 40 threads across 1 inch, the pitch is 1 / 40 = 0.025 inches, and the TPI is simply 40.
Using a Thread Pitch Gauge
A thread pitch gauge is a fan-shaped set of thin metal blades, each cut with teeth matching a specific pitch. They’re inexpensive and available at most hardware stores, typically in separate metric and imperial sets.
To use one, clean the screw threads first so dirt doesn’t throw off the fit. Pick a blade that looks close to the thread spacing and press it against the threads, aligning the teeth with the grooves. If it fits snugly with no gaps or rocking, you’ve found the right pitch, and it’s printed right on the blade. If there are gaps between the blade teeth and the screw threads, try the next size up or down. The correct blade will nestle perfectly into every groove across its full length.
This is the fastest method when you need a precise answer, and it eliminates the guesswork of counting tiny threads on small screws.
Coarse vs. Fine Thread: Spotting the Difference
Most screws come in either coarse or fine thread versions. Coarse threads have taller peaks spaced farther apart. Fine threads are shorter and packed more closely together. You can usually tell the difference just by looking: hold two screws of the same diameter side by side, and the coarse one will have noticeably fewer, larger ridges.
This matters because coarse and fine versions of the same diameter screw have different pitches. An M8 coarse screw has a 1.25 mm pitch, while an M8 fine screw drops to 1.0 mm. In the imperial system, a 3/8-inch coarse (UNC) bolt has 16 TPI, while the fine (UNF) version has 24 TPI. Using the wrong pitch means the nut won’t thread on properly, which is exactly why measuring matters rather than guessing.
Measuring Pipe Threads
Pipe threads like NPT (National Pipe Thread) add a wrinkle because they’re tapered. The diameter changes along the length of the thread, getting slightly wider or narrower. You measure pitch the same way, by counting threads per inch, but you need to be aware that NPT and NPS (National Pipe Straight) threads can share the same TPI while behaving very differently. NPT threads seal by wedging together as the taper tightens. NPS threads are parallel and need a gasket or sealant.
If you’re measuring a pipe fitting, first check whether the threads are tapered by holding a straight edge along the threaded section. If there’s a visible angle, you’re dealing with tapered threads. Then count TPI normally to identify the size.
Getting More Accurate Results
For most home and shop projects, counting threads with a ruler or using a pitch gauge gives you everything you need. But if precision matters, a few other tools exist.
- Thread micrometers measure the pitch diameter (the effective middle diameter of the threads) directly and give very accurate readings, provided the thread angle is correct.
- The three-wire method uses three small precision wires placed in the thread grooves while you measure across them with a standard micrometer. This is one of the most accurate manual methods and is least affected by small errors in the thread angle.
- Calipers work well for measuring the major (outside) diameter of a screw, which you’ll need alongside the pitch to fully identify a thread. Measure at the widest point for male threads and the narrowest point for female threads.
For most people identifying a replacement screw or checking compatibility, a pitch gauge or the counting method will get the job done. The three-wire method and thread micrometers are mainly used in machining and quality control where tolerances are measured in thousandths of an inch.