Thread Pitch & Size Chart: UNC, UNF & Metric (PDF)

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.

Major Diameter (d):	-
Pitch Diameter (d2):	-
Minor Diameter (d3):	-
Pitch / TPI:	-

Thread Size ▲	Thread Type ▲	Pitch (TPI or mm) ▲	Major Dia (in) ▲	Minor Dia (in) ▲	Pitch Dia (in) ▲
#2-56	UNC	56 TPI	0.0860	0.0641	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	0.0594	0.0685
M2.5 x 0.45	Metric	0.45 mm	0.0984	0.0767	0.0869
M3 x 0.5	Metric	0.5 mm	0.1181	0.0940	0.1053
M4 x 0.7	Metric	0.7 mm	0.1575	0.1237	0.1396
M5 x 0.8	Metric	0.8 mm	0.1969	0.1582	0.1764
M6 x 1.0	Metric	1.0 mm	0.2362	0.1879	0.2106
M8 x 1.25	Metric	1.25 mm	0.3150	0.2546	0.2830
M8 x 1.0 (fine)	Metric	1.0 mm	0.3150	0.2667	0.2894
M10 x 1.5	Metric	1.5 mm	0.3937	0.3212	0.3553
M10 x 1.25 (fine)	Metric	1.25 mm	0.3937	0.3333	0.3617
M12 x 1.75	Metric	1.75 mm	0.4724	0.3879	0.4277
M14 x 2.0	Metric	2.0 mm	0.5512	0.4546	0.5000
M16 x 2.0	Metric	2.0 mm	0.6299	0.5333	0.5788
M20 x 2.5	Metric	2.5 mm	0.7874	0.6666	0.7235
M24 x 3.0	Metric	3.0 mm	0.9449	0.8000	0.8682

No threads match your search. Try a different term or filter.

Quick Answer: What Is Thread Pitch?

Thread pitch is the distance from one thread crest to the next. Inch threads state pitch as threads per inch (TPI), so a higher number means finer threads. Metric threads state pitch in millimeters, so a smaller number means finer threads. Machinists use pitch to pick the right tap, die or fastener and to set up CNC threading.

This chart lists 43 common sizes per ASME B1.1 and ASME B1.13M and shows: pitch (TPI or mm), major diameter, minor diameter and pitch diameter. Filter by UNC, UNF or metric, or search a size like M10 or 1/4-20. Diameters are basic (nominal) external-thread values. The Class 2A and 2B tables below give the max and min each class must hold.

Notes on This Chart

All values follow ASME B1.1 (inch) and ASME B1.13M (metric) threading standards.
Minor and pitch diameters in the main table are basic (nominal) external-thread values. Class 2A and 2B limits are in the tables below.
Class 2A (external) and 2B (internal) are the default fit for general-purpose fasteners. Class 3A/3B is tighter and used where fit and strength are critical.
Minor diameter here is the external thread root diameter, D − 1.226869 × pitch, for inch and metric alike. The internal (tapped hole) minor diameter is larger; see the Class 2B table or a tap drill chart.
Metric pitch is the distance in mm between thread crests. TPI is threads per inch.
Always verify thread specs with a thread gauge before machining.

UNC and UNF Thread Size Chart: Class 2A Limits (External Threads)

The tables below list UNC thread dimensions and their UNF counterparts for 28 sizes from #2-56 to 1-8, the values a drawing, thread gage or inspection report refers to. Class 2A is the standard tolerance class for bolts, screws and studs. Each size carries a small allowance below the basic diameter so plated or slightly damaged parts still assemble, plus a tolerance below that. A part is in spec when its major and pitch diameters both fall between the max and min shown. Values are computed from the ASME B1.1 formulas (length of engagement equal to the nominal diameter) and rounded to 0.0001″; a few cells in the printed standard differ by 0.0001″ because of its own rounding steps.

Thread	TPI	Major Dia Max (in)	Major Dia Min (in)	Pitch Dia Max (in)	Pitch Dia Min (in)	Allowance (in)
#2-56 UNC	56	0.0854	0.0813	0.0738	0.0716	0.0006
#4-40 UNC	40	0.1112	0.1061	0.0950	0.0925	0.0008
#6-32 UNC	32	0.1372	0.1312	0.1169	0.1140	0.0008
#8-32 UNC	32	0.1631	0.1572	0.1428	0.1399	0.0009
#10-24 UNC	24	0.1890	0.1818	0.1619	0.1586	0.0010
#12-24 UNC	24	0.2150	0.2078	0.1879	0.1845	0.0010
1/4-20 UNC	20	0.2489	0.2407	0.2164	0.2127	0.0011
5/16-18 UNC	18	0.3113	0.3026	0.2752	0.2712	0.0012
3/8-16 UNC	16	0.3737	0.3642	0.3331	0.3287	0.0013
7/16-14 UNC	14	0.4361	0.4258	0.3897	0.3850	0.0014
1/2-13 UNC	13	0.4985	0.4877	0.4485	0.4436	0.0015
9/16-12 UNC	12	0.5609	0.5495	0.5068	0.5016	0.0016
5/8-11 UNC	11	0.6233	0.6112	0.5643	0.5588	0.0017
3/4-10 UNC	10	0.7482	0.7353	0.6833	0.6774	0.0018
7/8-9 UNC	9	0.8731	0.8592	0.8009	0.7946	0.0019
1-8 UNC	8	0.9980	0.9830	0.9168	0.9100	0.0020
#4-48 UNF	48	0.1113	0.1067	0.0978	0.0954	0.0007
#6-40 UNF	40	0.1372	0.1321	0.1210	0.1184	0.0008
#8-36 UNF	36	0.1632	0.1577	0.1451	0.1423	0.0008
#10-32 UNF	32	0.1891	0.1831	0.1688	0.1658	0.0009
1/4-28 UNF	28	0.2490	0.2425	0.2258	0.2225	0.0010
5/16-24 UNF	24	0.3114	0.3042	0.2843	0.2807	0.0011
3/8-24 UNF	24	0.3739	0.3666	0.3468	0.3430	0.0011
7/16-20 UNF	20	0.4362	0.4281	0.4038	0.3996	0.0013
1/2-20 UNF	20	0.4987	0.4906	0.4662	0.4620	0.0013
9/16-18 UNF	18	0.5611	0.5524	0.5251	0.5205	0.0014
5/8-18 UNF	18	0.6236	0.6149	0.5875	0.5829	0.0014
3/4-16 UNF	16	0.7485	0.7390	0.7079	0.7029	0.0015

The minor diameter of an external thread is not toleranced directly; it follows from the pitch diameter and the root form. Class 3A uses the same tolerances with zero allowance.

Class 2B Limits (Internal Threads: Nuts and Tapped Holes)

Class 2B is the matching class for nuts and tapped holes. There is no allowance: the pitch diameter minimum is the basic size and the tolerance runs upward from it. The minor diameter range is what the tap drill produces, so a drill that lands between the min and max gives roughly 65 to 75% thread. A GO thread plug gage checks the pitch diameter minimum, a NOGO gage checks the maximum and a plain plug gage checks the minor diameter.

Thread	TPI	Minor Dia Min (in)	Minor Dia Max (in)	Pitch Dia Min (in)	Pitch Dia Max (in)	Major Dia Min (in)
#2-56 UNC	56	0.0667	0.0737	0.0744	0.0772	0.0860
#4-40 UNC	40	0.0849	0.0939	0.0958	0.0990	0.1120
#6-32 UNC	32	0.1042	0.1139	0.1177	0.1214	0.1380
#8-32 UNC	32	0.1302	0.1388	0.1437	0.1475	0.1640
#10-24 UNC	24	0.1449	0.1555	0.1629	0.1673	0.1900
#12-24 UNC	24	0.1709	0.1807	0.1889	0.1934	0.2160
1/4-20 UNC	20	0.1959	0.2074	0.2175	0.2224	0.2500
5/16-18 UNC	18	0.2524	0.2650	0.2764	0.2817	0.3125
3/8-16 UNC	16	0.3073	0.3214	0.3344	0.3401	0.3750
7/16-14 UNC	14	0.3602	0.3760	0.3911	0.3972	0.4375
1/2-13 UNC	13	0.4167	0.4336	0.4500	0.4565	0.5000
9/16-12 UNC	12	0.4723	0.4903	0.5084	0.5152	0.5625
5/8-11 UNC	11	0.5266	0.5460	0.5660	0.5731	0.6250
3/4-10 UNC	10	0.6417	0.6627	0.6850	0.6927	0.7500
7/8-9 UNC	9	0.7547	0.7776	0.8028	0.8110	0.8750
1-8 UNC	8	0.8647	0.8897	0.9188	0.9276	1.0000
#4-48 UNF	48	0.0894	0.0968	0.0985	0.1015	0.1120
#6-40 UNF	40	0.1109	0.1186	0.1218	0.1252	0.1380
#8-36 UNF	36	0.1339	0.1416	0.1460	0.1496	0.1640
#10-32 UNF	32	0.1562	0.1641	0.1697	0.1736	0.1900
1/4-28 UNF	28	0.2113	0.2198	0.2268	0.2311	0.2500
5/16-24 UNF	24	0.2674	0.2771	0.2854	0.2902	0.3125
3/8-24 UNF	24	0.3299	0.3396	0.3479	0.3529	0.3750
7/16-20 UNF	20	0.3834	0.3949	0.4050	0.4104	0.4375
1/2-20 UNF	20	0.4459	0.4574	0.4675	0.4731	0.5000
9/16-18 UNF	18	0.5024	0.5150	0.5264	0.5323	0.5625
5/8-18 UNF	18	0.5649	0.5775	0.5889	0.5950	0.6250
3/4-16 UNF	16	0.6823	0.6964	0.7094	0.7159	0.7500

The major diameter of an internal thread is controlled only at its minimum, the basic size. Metric tolerance classes (6g, 6H) follow ASME B1.13M and ISO 965 instead; see the threading standards guide.

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.

What is the difference between Class 2A and Class 2B threads?

Class 2A is the standard tolerance class for external threads (bolts, screws and studs) and Class 2B for internal threads (nuts and tapped holes). 2A includes a small allowance below the basic size so plated or slightly damaged parts still assemble; 2B starts at the basic size. Together they are the general-purpose fit. Class 3A/3B has no allowance and tighter tolerances for critical joints.

What are major, minor and pitch diameters?

The major diameter is the largest diameter of the thread (the crest of a bolt). The minor diameter is the smallest (the root of a bolt or the drilled hole of a nut). The pitch diameter sits between them, where the thread and the groove are the same width. Pitch diameter controls how tight or loose the fit is, which is why thread gages check it.

How do I pick the right tap drill size?

Start from the minor diameter of the internal thread: the tap drill should land between the Class 2B minor diameter minimum and maximum, which gives roughly 65 to 75% thread. For 1/4-20 UNC that is a #7 drill (0.2010 in). Use a tap drill chart for the listed size rather than the nearest fraction.

Thread Pitch & Size Chart

Unified & Metric 60° Thread Profile

Select a Thread

Quick Answer: What Is Thread Pitch?

Notes on This Chart

UNC and UNF Thread Size Chart: Class 2A Limits (External Threads)

Class 2B Limits (Internal Threads: Nuts and Tapped Holes)

Frequently Asked Questions

Thread & Fastener Reference Series

More Resources

Need High-Integrity, Precision Threaded Parts?

Thread Pitch & Size Chart

Unified & Metric 60° Thread Profile

Select a Thread

Quick Answer: What Is Thread Pitch?

Notes on This Chart

UNC and UNF Thread Size Chart: Class 2A Limits (External Threads)

Class 2B Limits (Internal Threads: Nuts and Tapped Holes)

Frequently Asked Questions

Thread & Fastener Reference Series

Related Resources

Metric vs Imperial Thread Comparison Chart | Closest Equivalents

Tap & Drill Size Chart | UNC, UNF & Metric Thread Sizes

Threading Standards Guide | UNC, UNF, Metric & NPT

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Need High-Integrity, Precision Threaded Parts?

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