Thread Pitch to TPI Converter: Threads per Inch ↔ Metric Pitch, With the Nearest Standard Thread
Convert threads per inch (TPI) to metric pitch in mm and back, find the nearest ISO metric, UNC or UNF thread, and get pitch and minor diameters for a bolt size.
How it works
Pitch (mm) = 25.4 ÷ TPI · TPI = 25.4 ÷ pitch (mm) · Lead = pitch × number of starts · Thread height H = 0.866025 × P · Basic pitch diameter = D − 0.649519 × P · Basic minor diameter (nut) = D − 1.082532 × P · Threads along a length = length ÷ pitch
Inch threads are described by how many threads fit in one inch (TPI); metric threads by the pitch, the distance from one crest to the next in millimetres. Because an inch is exactly 25.4 mm, the two are reciprocals: pitch = 25.4 ÷ TPI and TPI = 25.4 ÷ pitch. A 1/4"-20 UNC thread has a 1.27 mm pitch; an M8 × 1.25 thread has 20.32 threads per inch. Enter a major diameter to see which standard thread you have. The calculator compares your pitch and diameter with every preferred ISO metric size from M1 to M64 (coarse and fine) and every UNC and UNF size from #0 to 2 in, then lists the closest ones with their pitch difference. With a diameter it also gives the basic pitch diameter and nut minor diameter from the 60° thread profile shared by ISO and Unified threads, the thread height H and a rough tap drill (D − P) for about 75% thread. The tap drill size calculator on this site picks the drill for a chosen thread and percentage of thread, and the thread engagement calculator checks strip strength. This page answers the step before: what pitch a TPI number is, and which standard thread it belongs to. Pipe threads (NPT, BSP) and Acme or trapezoidal threads use other profiles and are not in the match list.
Worked example
With the defaults, 20 TPI and a 1/4 in diameter: pitch = 25.4 ÷ 20 = 1.2700 mm (0.05000 in), or 7.874 threads per cm. The closest standard thread is an exact match, 1/4"-20 UNC. Its basic pitch diameter is 6.350 − 0.649519 × 1.27 = 5.525 mm (0.2175 in), the nut minor diameter is 6.350 − 1.082532 × 1.27 = 4.975 mm (0.1959 in), the thread height H is 0.866025 × 1.27 = 1.100 mm, and a rough tap drill is 6.350 − 1.27 = 5.080 mm (0.2000 in, a #7 drill in practice).
Sources: Omni Calculator: Thread pitch (P = 1/TPI, 2 in / 48 threads and 3/4-10 UNC examples) · Wikipedia: Unified Thread Standard (UNC/UNF sizes and TPI, H = 0.866P) · Wikipedia: ISO metric screw thread (ISO 262 coarse and fine pitches, D1 = D − 1.0825P)
Frequently asked questions
How do I convert TPI to thread pitch in mm?
Divide 25.4 by the threads per inch. For example, 13 TPI is 25.4 ÷ 13 = 1.954 mm and 32 TPI is 0.794 mm. To go the other way, divide 25.4 by the pitch: a 1.5 mm pitch is 16.93 TPI.
Is M6 × 1 the same as 1/4-20?
No. M6 is 6.00 mm across with a 1 mm pitch (25.4 TPI), while 1/4"-20 is 6.35 mm across with a 1.27 mm pitch. They look similar but will not thread together.
What is the TPI of an M8 bolt?
A standard M8 coarse bolt has a 1.25 mm pitch, which is 20.32 threads per inch. The fine M8 × 1 thread is 25.4 TPI.
Can I use a metric nut on an inch bolt with almost the same pitch?
Only for a few turns. Even a 1–2% pitch difference adds up over the length of engagement, so the threads bind and the joint is weak. Always match both the diameter and the pitch to a standard.
How do I measure thread pitch without a gauge?
Lay a ruler along the threads, count the crests over 1 in (or 25 mm) and divide. For a metric screw, measure across 10 crests and divide by 10 to get the pitch in mm. Enter the TPI or pitch and the measured diameter here to find the standard thread.
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This calculator is for educational purposes. Double-check important results before acting on them.