Thread Pitch Calculator
Enter the thread length and thread count, or type in a known TPI or pitch value, to find all key thread geometry dimensions instantly. Switch between metric (ISO) and imperial (UNC/UNF) systems, and get the recommended tap drill size alongside major, minor, and pitch diameters.
What is thread pitch?
Thread pitch is the axial distance from one thread crest to the next adjacent crest, measured in millimetres for metric threads. For imperial threads the same spacing is described as TPI (threads per inch), which is simply the reciprocal: TPI = 25.4 / P(mm). A finer pitch means more threads per unit length, giving better locking ability but reduced load-carrying capacity per engagement length.
Thread geometry: major, pitch, and minor diameters
Every screw thread has three key diameters. The major diameter is the nominal size stamped on the fastener (e.g. 10 mm for an M10 bolt). The pitch diameter, also called the effective diameter, is the imaginary cylinder where thread ridge width equals groove width; it governs the fit class (clearance, transition, or interference). The minor diameter is the smallest diameter at the root of the thread, and it determines the tensile stress area. For an ISO 60-degree metric thread the theoretical full depth H = (sqrt(3)/2) x P, the minor diameter = D - (5/4)H, and the pitch diameter = D - (3/4)H.
Tap drill size and thread engagement
When drilling a hole before tapping, the drill diameter determines how much thread material remains. The standard recommendation for approximately 75% thread engagement, which is optimal for most steel-to-steel joints, is: tap drill diameter = major diameter - pitch. Going smaller (below 65%) risks tap breakage; going larger (above 85%) wastes the threading operation and weakens the joint. For softer materials like aluminium or plastic, use a slightly larger tap drill (major diameter - 0.8 x pitch) to reduce torque and tap breakage.
Metric vs. imperial thread standards
Metric threads follow ISO 261 and ISO 965, using millimetre-based pitch values (e.g. M8 x 1.25 means 8 mm nominal diameter, 1.25 mm pitch). Imperial threads follow the Unified Thread Standard (UTS), where the same information is expressed as a fraction plus TPI (e.g. 1/4-20 means 0.25 inch diameter, 20 threads per inch). Both use a 60-degree flank angle, so the geometry formulas are identical once you convert pitch to a common unit. Never mix metric and imperial fasteners in the same threaded hole; even if they engage initially, the pitch mismatch causes rapid wear and potential joint failure.
Common metric thread sizes (ISO 261)
| Designation | Major dia. (mm) | Pitch (mm) | TPI | Minor dia. (mm) | Tap drill (mm) |
|---|---|---|---|---|---|
| M3 | 3 | 0.5 | 50.8 | 2.38 | 2.5 |
| M4 | 4 | 0.7 | 36.3 | 3.14 | 3.3 |
| M5 | 5 | 0.8 | 31.8 | 4.02 | 4.2 |
| M6 | 6 | 1 | 25.4 | 4.77 | 5 |
| M8 | 8 | 1.25 | 20.3 | 6.47 | 6.8 |
| M10 | 10 | 1.5 | 16.9 | 8.16 | 8.5 |
| M12 | 12 | 1.75 | 14.5 | 9.85 | 10.2 |
| M16 | 16 | 2 | 12.7 | 13.55 | 14 |
| M20 | 20 | 2.5 | 10.2 | 16.93 | 17.5 |
| M24 | 24 | 3 | 8.5 | 20.32 | 21 |
Standard coarse-pitch metric threads. Tap drill = Major diameter - Pitch.
Frequently asked questions
How do I measure thread pitch without a thread gauge?
Place a ruler alongside the threaded shaft and count the number of crests over a known length (say 25 mm). Divide that length by the thread count to get the pitch. For a more precise result, press the thread against a piece of soft lead or clay, then measure the imprint. A dedicated thread pitch gauge (comb gauge) is the most reliable method for identifying unknown fasteners.
What does M10 x 1.5 mean?
M10 x 1.5 is an ISO metric thread designation. The "M" means metric, "10" is the nominal major diameter in millimetres, and "1.5" is the pitch in millimetres (the distance between crests). It is the standard coarse-pitch version of the M10 thread. A fine-pitch variant would be written M10 x 1.25 or M10 x 1.0.
What is the difference between UNC and UNF threads?
Both are Unified inch thread series using a 60-degree flank angle. UNC (Unified National Coarse) has fewer threads per inch for a given diameter, making it faster to assemble and more resistant to cross-threading. UNF (Unified National Fine) has more threads per inch, providing better resistance to vibration loosening and finer adjustment. For example, a 1/4-inch bolt is 1/4-20 UNC or 1/4-28 UNF.
Why is the tap drill size smaller than the major diameter?
A tap cuts thread grooves into the drilled hole. If the hole were already at the major diameter, there would be nothing for the tap to cut and no thread material to engage the bolt. The tap drill creates the minor diameter of the internal thread, leaving the wall material that becomes the thread ridges. Drilling to the major diameter minus one pitch gives roughly 75% thread depth, which provides full load capacity for most joint designs.
How do I convert between TPI and thread pitch in mm?
The conversion is straightforward: P (mm) = 25.4 / TPI, and TPI = 25.4 / P (mm). For example, a 20-TPI thread has a pitch of 25.4 / 20 = 1.27 mm. Conversely, an M6 x 1.0 thread has a TPI of 25.4 / 1.0 = 25.4 threads per inch.
What is thread pitch diameter used for?
The pitch diameter is the most important dimension for determining how tightly or loosely two mating threads fit together. Tolerance specifications (such as 6g for a metric external thread or 6H for an internal thread) define acceptable upper and lower limits for the pitch diameter. When the pitch diameter of a bolt exceeds the maximum or falls below the minimum for its class, the thread will either bind or have excessive play in the mating nut.