TPI means threads per inch—the number of thread crests in one inch of length. It's used for imperial (US) thread designation (e.g. 1/4-20 = 1/4 inch diameter, 20 TPI).

What Is TPI?

Q: How do you convert TPI to pitch?

Pitch in inches = 1 ÷ TPI. So 20 TPI = 0.05 inch pitch. For metric, use 25.4 ÷ TPI for approximate mm pitch, or use our thread pitch to TPI converter.

Q: Is higher TPI finer or coarser?

Higher TPI means finer threads (smaller pitch). Lower TPI means coarser threads (larger pitch).

Threads per inch (TPI) explained for imperial bolts and screws—and how to convert to metric pitch.

TPI definition

TPI means threads per inch: the number of complete thread crests in one inch of length. It's used for US (imperial) thread specs—for example, 1/4-20 is 1/4 inch diameter with 20 TPI. Higher TPI = finer thread; lower TPI = coarser. Use our thread pitch to TPI converter to switch between TPI and mm pitch.

TPI to pitch conversion

TPI	Pitch (inches)	Approx. pitch (mm)
13	0.0769	1.95
18	0.0556	1.41
20	0.050	1.27
24	0.0417	1.06
28	0.0357	0.91

Formula: pitch (in) = 1 ÷ TPI. For mm: pitch (mm) ≈ 25.4 ÷ TPI.

Common TPI values

UNC coarse threads often use 20, 18, 16, 13 TPI for 1/4", 5/16", 3/8", 1/2". UNF fine threads use higher TPI (e.g. 28, 24). Match your measurement to standards with the thread identifier and confirm with our screw size chart or tap drill chart.

FAQ

What does TPI mean?

TPI = threads per inch—the number of thread crests in one inch. Used for imperial thread designation (e.g. 1/4-20).

How do you convert TPI to pitch?

Pitch in inches = 1 ÷ TPI. So 20 TPI = 0.05 in. Use our converter for TPI ↔ mm pitch.

Is higher TPI finer or coarser?

Higher TPI = finer threads (smaller pitch). Lower TPI = coarser threads.