Thread Pitch Chart: TPI (UNC/UNF) + Metric mm

Q: What is the difference between UNC and UNF threads?

UNC stands for Unified National Coarse. UNF stands for Unified National Fine. UNC threads have fewer threads per inch and are easier to assemble. UNF threads have more threads per inch and hold better in thin materials or under vibration.

Q: How do I measure thread pitch?

Use a thread pitch gauge. Place the gauge teeth against the threads until they mesh perfectly. The gauge reads out the TPI or metric pitch. You can also count threads over one inch with a ruler for inch-based threads.

Q: Can I mix UNC and metric threads?

No. UNC and metric threads are not interchangeable. They use different pitch angles and profiles. Forcing a metric bolt into a UNC hole will strip the threads. Always match thread type, size, and pitch.

Thread Size ▲	Thread Type ▲	Pitch (TPI or mm) ▲	Major Dia (in) ▲	Minor Dia (in) ▲	Pitch Dia (in) ▲
#2-56	UNC	56 TPI	0.0860	0.0628	0.0744
#4-40	UNC	40 TPI	0.1120	0.0813	0.0958
#6-32	UNC	32 TPI	0.1380	0.0997	0.1177
#8-32	UNC	32 TPI	0.1640	0.1257	0.1437
#10-24	UNC	24 TPI	0.1900	0.1389	0.1629
#12-24	UNC	24 TPI	0.2160	0.1649	0.1889
1/4-20	UNC	20 TPI	0.2500	0.1887	0.2175
5/16-18	UNC	18 TPI	0.3125	0.2443	0.2764
3/8-16	UNC	16 TPI	0.3750	0.2983	0.3344
7/16-14	UNC	14 TPI	0.4375	0.3499	0.3911
1/2-13	UNC	13 TPI	0.5000	0.4056	0.4500
9/16-12	UNC	12 TPI	0.5625	0.4603	0.5084
5/8-11	UNC	11 TPI	0.6250	0.5135	0.5660
3/4-10	UNC	10 TPI	0.7500	0.6273	0.6850
7/8-9	UNC	9 TPI	0.8750	0.7387	0.8028
1-8	UNC	8 TPI	1.0000	0.8466	0.9188
#4-48	UNF	48 TPI	0.1120	0.0864	0.0985
#6-40	UNF	40 TPI	0.1380	0.1073	0.1218
#8-36	UNF	36 TPI	0.1640	0.1299	0.1460
#10-32	UNF	32 TPI	0.1900	0.1517	0.1697
1/4-28	UNF	28 TPI	0.2500	0.2062	0.2268
5/16-24	UNF	24 TPI	0.3125	0.2614	0.2854
3/8-24	UNF	24 TPI	0.3750	0.3239	0.3479
7/16-20	UNF	20 TPI	0.4375	0.3762	0.4050
1/2-20	UNF	20 TPI	0.5000	0.4387	0.4675
9/16-18	UNF	18 TPI	0.5625	0.4943	0.5264
5/8-18	UNF	18 TPI	0.6250	0.5568	0.5889
3/4-16	UNF	16 TPI	0.7500	0.6733	0.7094
M2 x 0.4	Metric	0.4 mm	0.0787	0.0637	0.0712
M2.5 x 0.45	Metric	0.45 mm	0.0984	0.0812	0.0898
M3 x 0.5	Metric	0.5 mm	0.1181	0.0975	0.1078
M4 x 0.7	Metric	0.7 mm	0.1575	0.1299	0.1437
M5 x 0.8	Metric	0.8 mm	0.1969	0.1654	0.1811
M6 x 1.0	Metric	1.0 mm	0.2362	0.1968	0.2165
M8 x 1.25	Metric	1.25 mm	0.3150	0.2657	0.2903
M8 x 1.0 (fine)	Metric	1.0 mm	0.3150	0.2756	0.2953
M10 x 1.5	Metric	1.5 mm	0.3937	0.3345	0.3641
M10 x 1.25 (fine)	Metric	1.25 mm	0.3937	0.3445	0.3691
M12 x 1.75	Metric	1.75 mm	0.4724	0.4033	0.4378
M14 x 2.0	Metric	2.0 mm	0.5512	0.4723	0.5118
M16 x 2.0	Metric	2.0 mm	0.6299	0.5510	0.5905
M20 x 2.5	Metric	2.5 mm	0.7874	0.6887	0.7381
M24 x 3.0	Metric	3.0 mm	0.9449	0.8265	0.8857

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Frequently Asked Questions

What is thread pitch?

Thread pitch is the distance between two thread crests. For inch threads, pitch is given in threads per inch (TPI). More TPI means finer threads. For metric threads, pitch is the crest-to-crest distance in millimeters. A smaller number means finer threads.

What is the difference between UNC and UNF threads?

UNC is Unified National Coarse. UNF is Unified National Fine. UNC has fewer threads per inch. It is easier to start and more forgiving with dirt or damage. UNF has more threads per inch. It holds better in thin walls and resists vibration loosening.

How do I measure thread pitch?

Use a thread pitch gauge. Press the gauge teeth against the thread until they fit perfectly. The gauge shows TPI or metric pitch. You can also count threads over one inch with a ruler for inch threads.

Can I mix UNC and metric threads?

No. UNC and metric threads use different profiles and angles. A metric bolt will not fit a UNC tapped hole. Forcing them together strips the threads. Always match the thread type, size, and pitch.

Thread Pitch Chart

Notes on This Chart

Frequently Asked Questions

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Thread Pitch Chart

Notes on This Chart

Frequently Asked Questions

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