Table of Contents
What Is Thread Pitch?
Thread pitch is the distance between adjacent thread crests, measured parallel to the thread axis. It is the most fundamental measurement of a screw thread and determines how far a fastener advances per revolution. In the imperial system, threads are typically specified by TPI (threads per inch), while the metric system uses pitch in millimeters directly.
Understanding thread pitch is essential for selecting the correct fastener, matching mating components, identifying unknown threads, and programming CNC lathes for thread cutting operations. An incorrect pitch identification will result in cross-threading, stripped threads, or complete fastener failure.
Thread pitch can be measured using a thread pitch gauge (a set of metal blades with different thread profiles), a caliper, or a thread micrometer. For unknown threads, counting the number of crests over a known distance and dividing is the most reliable field method. This calculator supports all three input methods to accommodate different measurement scenarios.
Thread Pitch Formulas
Common Thread Pitches
| Thread | TPI | Pitch (mm) | Type |
|---|---|---|---|
| 1/4"-20 | 20 | 1.270 | UNC |
| 1/4"-28 | 28 | 0.907 | UNF |
| 3/8"-16 | 16 | 1.588 | UNC |
| 1/2"-13 | 13 | 1.954 | UNC |
| M6 x 1.0 | 25.4 | 1.000 | Metric |
| M8 x 1.25 | 20.3 | 1.250 | Metric |
| M10 x 1.5 | 16.9 | 1.500 | Metric |
| M12 x 1.75 | 14.5 | 1.750 | Metric |
Multi-Start Threads
Multi-start threads have two or more independent helical ridges wrapped around the cylinder. They are used when rapid linear motion is needed from minimal rotation. The lead (distance advanced per revolution) equals the pitch multiplied by the number of starts. For example, a double-start thread with a 2mm pitch has a 4mm lead, meaning it advances 4mm per revolution instead of 2mm.
- Single-start: Standard threading. Lead equals pitch. Used in most fasteners.
- Double-start: Two interleaved threads. Lead = 2 x Pitch. Common in bottle caps, quick-release mechanisms.
- Triple-start: Three threads. Used in lead screws requiring fast travel with moderate load.
- Quad-start: Four threads. Found in high-speed actuators and dispensing caps.
Frequently Asked Questions
How do I measure thread pitch without a gauge?
Place a ruler along the thread axis and count the number of complete thread crests over one inch (for TPI) or over 10mm (for metric pitch). Alternatively, wrap a piece of paper around the bolt, mark two adjacent thread impressions, and measure the distance between marks with a caliper. This gives you the pitch directly.
What is the difference between pitch and lead?
Pitch is the distance between adjacent threads. Lead is the distance a screw advances in one complete revolution. For single-start threads, pitch and lead are identical. For multi-start threads, lead = pitch x number of starts. This distinction is critical when programming CNC lathes or calculating mechanical advantage of lead screws.
Can I mix coarse and fine pitch fasteners?
Never mix coarse and fine pitch fasteners. Even though bolt diameters may match, different pitches will cause cross-threading and joint failure. Always verify both the diameter and pitch when selecting replacement fasteners. Use a thread pitch gauge to confirm before assembly.