Thread Pitch Converter (TPI ↔ mm)
Convert between TPI and mm pitch — Metric, BSP, NPT, UNC, UNF
TPI aur mm pitch ke beech conversion — Metric, BSP, NPT threads के लिए
Thread pitch describes the distance between adjacent thread crests — in millimetres for metric threads or as TPI (threads per inch) for imperial threads. The conversion is simple: pitch in mm = 25.4 ÷ TPI. So a 20 TPI thread has a pitch of 1.27 mm. Metric coarse threads follow ISO standards — M6 × 1.0, M8 × 1.25, M10 × 1.5, M12 × 1.75. Metric fine threads use finer pitches on the same diameters — M8 × 1.0, M10 × 1.25. BSP (British Standard Pipe, also called Whitworth) is the most common pipe thread in India — G1/4 is 19 TPI (1.337 mm pitch), G1/2 is 14 TPI (1.814 mm). NPT (National Pipe Taper, American) has a taper and is used for gas lines and US imports. UNC and UNF are Unified National Coarse / Fine for US bolts. This converter handles all five standards.
Calculator
ƒFormula
Pitch (mm) = 25.4 / TPI | TPI = 25.4 / Pitch (mm)25.4 mm = 1 inch (exact conversion). The formula assumes measuring at 90° to the thread axis. For threads already specified in one system (e.g. Metric M10 × 1.5), the pitch is stated; for imperial threads (e.g. 1/2-13 UNC), TPI is the original spec.
Units: Pitch: millimetre (mm) · TPI: threads per inch
Thread pitch kya hai? TPI aur mm mein kya difference?
Thread pitch do lagataar thread crests ke beech ki distance hoti hai. Metric threads mein yeh mm mein batayi jaati hai (jaise M10 × 1.5 — 10 mm diameter, 1.5 mm pitch). Imperial (British/American) threads mein TPI (threads per inch) use hoti hai — yaani ek inch mein kitne thread hain. Formula simple hai: pitch (mm) = 25.4 / TPI. BSP (British Standard Pipe) aur NPT (American pipe) threads sab imperial hain, lekin BSP parallel hoti hai aur NPT tapered. Indian pneumatic aur water fittings mostly BSP use karte hain; American imports aksar NPT hote hain.
Worked Examples
Matching a SPAC pneumatic fitting thread
Problem: A SPAC cylinder has a G1/4 port. You need a push-in fitting. What thread is that?
Solution: G1/4 BSP = 19 TPI = 1.337 mm pitch, major diameter 13.157 mm. Choose a push-in fitting marked 'G1/4' — NPT or Metric threads will not seal correctly on this port.
Converting an American bolt spec
Problem: An imported machine drawing calls for a '1/2-13 UNC bolt'. What is the pitch?
Solution: 1/2-13 UNC means 1/2-inch diameter, 13 TPI. Pitch = 25.4 / 13 = 1.954 mm. Closest metric equivalent is M12 × 1.75, but threads do not interchange — use an original UNC bolt.
Coarse vs fine metric
Problem: A drawing shows 'M10 × 1.25'. Is this coarse or fine?
Solution: M10 coarse is 1.5 mm pitch. M10 × 1.25 is the fine thread (closer threads). Fine threads are used where you need tighter clamping resolution or in thin materials where full coarse threads would strip.
Common Thread Standards — TPI, Pitch, and Typical Use
| Thread | Standard | TPI | Pitch (mm) | Common Application |
|---|---|---|---|---|
| M6 × 1.0 | Metric coarse | 25.4 | 1.000 | General bolts |
| M8 × 1.25 | Metric coarse | 20.3 | 1.250 | Machine mounts |
| M10 × 1.5 | Metric coarse | 16.9 | 1.500 | General bolts |
| M10 × 1.25 | Metric fine | 20.3 | 1.250 | Automotive, thin parts |
| M12 × 1.75 | Metric coarse | 14.5 | 1.750 | Structural bolts |
| M12 × 1.5 | Metric fine | 16.9 | 1.500 | Fine adjustment |
| G1/8 | BSP parallel | 28 | 0.907 | Small pneumatic fittings |
| G1/4 | BSP parallel | 19 | 1.337 | SPAC cylinder ports, fittings |
| G3/8 | BSP parallel | 19 | 1.337 | Medium pneumatic fittings |
| G1/2 | BSP parallel | 14 | 1.814 | Water, compressed air mains |
| G3/4 | BSP parallel | 14 | 1.814 | Plumbing, larger air lines |
| G1 | BSP parallel | 11 | 2.309 | Industrial pipes |
| 1/4-18 NPT | NPT taper | 18 | 1.411 | US-standard gas/air |
| 1/2-14 NPT | NPT taper | 14 | 1.814 | US imports |
| 1/4-20 UNC | UNC coarse | 20 | 1.270 | US bolts, furniture |
| 3/8-16 UNC | UNC coarse | 16 | 1.588 | US machinery |
| 1/2-13 UNC | UNC coarse | 13 | 1.954 | US heavy bolts |
BSP (Whitworth) is the dominant pipe thread standard in India — pneumatic, plumbing, hydraulic. NPT and UNC only appear on American-origin equipment.
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