How do I convert threads per inch to pitch?

Pitch in millimeters = 25.4 ÷ TPI. A 20 TPI thread has a 1.27 mm pitch.

What is the difference between UNC and UNF?

UNF has more threads per inch at the same diameter: finer adjustment and more strength in thin walls, but slower to assemble and easier to cross-thread.

Thread size chart

Basic major, pitch and minor diameters for Unified (UNC, UNF) and ISO metric threads, with threads per inch converted to pitch.

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UNC Basic thread sizes (in)
Thread	TPI	Pitch (mm)	Major Ø	Pitch Ø	Minor Ø
#1-64	64	0.3969	0.0730	0.0629	0.0538
#2-56	56	0.4536	0.0860	0.0744	0.0641
#3-48	48	0.5292	0.0990	0.0855	0.0734
#4-40	40	0.635	0.1120	0.0958	0.0813
#5-40	40	0.635	0.1250	0.1088	0.0943
#6-32	32	0.7938	0.1380	0.1177	0.0997
#8-32	32	0.7938	0.1640	0.1437	0.1257
#10-24	24	1.0583	0.1900	0.1629	0.1389
1/4-20	20	1.27	0.2500	0.2175	0.1887
5/16-18	18	1.4111	0.3125	0.2764	0.2443
3/8-16	16	1.5875	0.3750	0.3344	0.2983
7/16-14	14	1.8143	0.4375	0.3911	0.3499
1/2-13	13	1.9538	0.5000	0.4500	0.4056
5/8-11	11	2.3091	0.6250	0.5660	0.5135
3/4-10	10	2.54	0.7500	0.6850	0.6273
7/8-9	9	2.8222	0.8750	0.8028	0.7387
1"-8	8	3.175	1.0000	0.9188	0.8466

UNF Basic thread sizes (in)
Thread	TPI	Pitch (mm)	Major Ø	Pitch Ø	Minor Ø
#0-80	80	0.3175	0.0600	0.0519	0.0447
#1-72	72	0.3528	0.0730	0.0640	0.0560
#2-64	64	0.3969	0.0860	0.0759	0.0668
#4-48	48	0.5292	0.1120	0.0985	0.0864
#6-40	40	0.635	0.1380	0.1218	0.1073
#8-36	36	0.7056	0.1640	0.1460	0.1299
#10-32	32	0.7938	0.1900	0.1697	0.1517
1/4-28	28	0.9071	0.2500	0.2268	0.2062
5/16-24	24	1.0583	0.3125	0.2854	0.2614
3/8-24	24	1.0583	0.3750	0.3479	0.3239
7/16-20	20	1.27	0.4375	0.4050	0.3762
1/2-20	20	1.27	0.5000	0.4675	0.4387
9/16-18	18	1.4111	0.5625	0.5264	0.4943
3/4-16	16	1.5875	0.7500	0.7094	0.6733

Metric Basic thread sizes (mm)
Thread	Pitch (mm)	Major Ø	Pitch Ø	Minor Ø
M3×0.5	0.5	3.000	2.675	2.459
M4×0.7	0.7	4.000	3.545	3.242
M5×0.8	0.8	5.000	4.480	4.134
M6×1.0	1	6.000	5.350	4.917
M8×1.25	1.25	8.000	7.188	6.647
M10×1.5	1.5	10.000	9.026	8.376
M12×1.75	1.75	12.000	10.863	10.106
M14×2.0	2	14.000	12.701	11.835
M16×2.0	2	16.000	14.701	13.835
M20×2.5	2.5	20.000	18.376	17.294
M24×3.0	3	24.000	22.051	20.752

Common questions

How do I convert threads per inch to pitch?: Pitch in millimeters = 25.4 ÷ TPI. A 20 TPI thread has a 1.27 mm pitch.
What is the difference between UNC and UNF?: UNF has more threads per inch at the same diameter: finer adjustment and more strength in thin walls, but slower to assemble and easier to cross-thread.

Calculators that use this data

Thread Pitch CalculatorTPI ↔ pitch conversion and standard thread specs.Lathe ThreadingSingle-point threading pitch, multi-start lead and synchronized feed.Tap Drill & Thread % CalculatorPercent of thread for any drill (cut and form taps), 75 / 65 / 50% tap drills, UNC / UNF / metric chart with thread limits, every thread to 4 in and M64, pipe taps and STI / Helicoil.

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