UNC unified coarse threads

UNC Thread Dimensions

The UNC thread (Unified National Coarse), defined by the ANSI/ASME B1.1 standard, covers 29 standardized sizes with a thread pitch ranging from 64 to 4 threads per inch.

UNC Size	Threads per inch (TPI)	Pitch (mm / in)	Major diameter (mm / in)
#1 – 64	64	0,397 mm / 0,0156 in	1,854 mm / 0,073 in
#2 – 56	56	0,453 mm / 0,0179 in	2,184 mm / 0,086 in
#3 – 48	48	0,529 mm / 0,0208 in	2,515 mm / 0,099 in
#4 – 40	40	0,635 mm / 0,0250 in	2,845 mm / 0,112 in
#5 – 40	40	0,635 mm / 0,0250 in	3,175 mm / 0,125 in
#6 – 32	32	0,794 mm / 0,0313 in	3,505 mm / 0,138 in
#8 – 32	32	0,794 mm / 0,0313 in	4,166 mm / 0,164 in
#10 – 24	24	1,058 mm / 0,0417 in	4,826 mm / 0,190 in
#12 – 24	24	1,058 mm / 0,0417 in	5,486 mm / 0,216 in
1/4“ – 20	20	1,270 mm / 0,0500 in	6,350 mm / 0,250 in
5/16“ – 18	18	1,411 mm / 0,0556 in	7,938 mm / 0,313 in
3/8“ – 16	16	1,587 mm / 0,0625 in	9,525 mm / 0,375 in
7/16“ – 14	14	1,814 mm / 0,0714 in	11,112 mm / 0,438 in
1/2“ – 13	13	1,954 mm / 0,0769 in	12,700 mm / 0,500 in
9/16“ – 12	12	2,117 mm / 0,0833 in	14,288 mm / 0,563 in
5/8“ – 11	11	2,309 mm / 0,0909 in	15,875 mm / 0,625 in
3/4“ – 10	10	2,540 mm / 0,1000 in	19,050 mm / 0,750 in
7/8“ – 9	9	2,822 mm / 0,1111 in	22,225 mm / 0,875 in
1“ – 8	8	3,175 mm / 0,1250 in	25,400 mm / 1,000 in
1 1/8“ – 7	7	3,628 mm / 0,1429 in	28,575 mm / 1,125 in
1 1/4“ – 7	7	3,628 mm / 0,1429 in	31,750 mm / 1,250 in
1 3/8“ – 6	6	4,233 mm / 0,1667 in	34,925 mm / 1,375 in
1 1/2“ – 6	6	4,233 mm / 0,1667 in	38,100 mm / 1,500 in
1 3/4“ – 5	5	5,080 mm / 0,2000 in	44,450 mm / 1,750 in
2“ – 4 1/2	4 1/2	5,644 mm / 0,2222 in	50,800 mm / 2,000 in
2 1/4“ – 4 1/2	4 1/2	5,644 mm / 0,2222 in	57,150 mm / 2,250 in
2 1/2“ – 4	4	6,350 mm / 0,2500 in	63,500 mm / 2,500 in
2 3/4“ – 4	4	6,350 mm / 0,2500 in	69,850 mm / 2,750 in
3“ – 4	4	6,350 mm / 0,2500 in	76,200 mm / 3,000 in
3 1/4“ – 4	4	6,350 mm / 0,2500 in	82,550 mm / 3,250 in
3 1/2“ – 4	4	6,350 mm / 0,2500 in	88,900 mm / 3,500 in
3 3/4“ – 4	4	6,350 mm / 0,2500 in	95,250 mm / 3,750 in
4“ – 4	4	6,350 mm / 0,2500 in	101,600 mm / 4,000 in

Tap Drill Sizes

The recommended tap drill for a 1/4“–20 UNC is 5.35 mm / 0.211 in (No. 3), while for a 3/8“–16 a 8.25 mm / 0.325 in (drill “O”) is used.

UNC Size	Drill (No. / fraction)	Drill diameter (mm / in)
#1 – 64	#53	1,50 mm / 0,059 in
#2 – 56	#50	1,80 mm / 0,070 in
#3 – 48	#47	2,10 mm / 0,079 in
#4 – 40	#43	2,35 mm / 0,089 in
#5 – 40	#38	2,65 mm / 0,102 in
#6 – 32	#36	2,85 mm / 0,107 in
#8 – 32	#29	3,50 mm / 0,136 in
#10 – 24	#25	4,00 mm / 0,150 in
#12 – 24	#16	4,65 mm / 0,177 in
1/4“ – 20	No. 7	5,35 mm / 0,201 in
5/16“ – 18	F	6,80 mm / 0,257 in
3/8“ – 16	5/16“	8,25 mm / 0,332 in
7/16“ – 14	U	9,65 mm / 0,368 in
1/2“ – 13	27/64“	11,15 mm / 0,439 in
9/16“ – 12	31/64“	12,60 mm / 0,484 in
5/8“ – 11	17/32“	14,05 mm / 0,553 in
3/4“ – 10	21/32“	17,00 mm / 0,669 in
7/8“ – 9	49/64“	20,00 mm / 0,787 in
1“ – 8	7/8“	22,85 mm / 0,900 in
1 1/8“ – 7	63/64“	25,65 mm / 1,010 in
1 1/4“ – 7	1 7/64“	28,85 mm / 1,136 in
1 3/8“ – 6	1 7/32“	31,55 mm / 1,242 in
1 1/2“ – 6	1 11/32“	34,70 mm / 1,366 in
1 3/4“ – 5	1 19/32“	40,40 mm / 1,591 in
2“ – 4 1/2	1 13/16“	46,30 mm / 1,823 in
2 1/4“ – 4 1/2	2 1/16“	52,65 mm / 2,073 in
2 1/2“ – 4	2 5/16“	58,50 mm / 2,303 in

Recommended Tightening Torques

For a 1/2“–13 UNC Grade 5 bolt in dry assembly, 102 Nm / 75 lb‑ft is recommended, a value that rises to 148 Nm / 109 lb‑ft for Grade 8.

UNC Size	TPI	Torque Grade 5 (Nm / lb‑ft)	Torque Grade 8 (Nm / lb‑ft)
1/4“ – 20	20	11 Nm / 8 lb‑ft	16 Nm / 12 lb‑ft
5/16“ – 18	18	23 Nm / 17 lb‑ft	33 Nm / 24 lb‑ft
3/8“ – 16	16	41 Nm / 30 lb‑ft	58 Nm / 43 lb‑ft
7/16“ – 14	14	65 Nm / 48 lb‑ft	92 Nm / 68 lb‑ft
1/2“ – 13	13	102 Nm / 75 lb‑ft	148 Nm / 109 lb‑ft
9/16“ – 12	12	149 Nm / 110 lb‑ft	210 Nm / 155 lb‑ft
5/8“ – 11	11	203 Nm / 150 lb‑ft	285 Nm / 210 lb‑ft
3/4“ – 10	10	353 Nm / 260 lb‑ft	495 Nm / 365 lb‑ft
7/8“ – 9	9	515 Nm / 380 lb‑ft	725 Nm / 535 lb‑ft
1“ – 8	8	746 Nm / 550 lb‑ft	1050 Nm / 775 lb‑ft

Reference values for dry zinc-plated threaded joint; reduce ~20% if lubricant is used.

Allowable Loads

A 1/2“–13 UNC Grade 5 bolt supports a proof load of 53.7 kN / 12,062 lbf, based on a stress area of 0.1419 in².

UNC Size	TPI	Stress area (in²)	Proof load Grade 5 (kN / lbf)	Proof load Grade 8 (kN / lbf)
1/4“ – 20	20	0,0318	12,0 kN / 2 705 lbf	23,0 kN / 5 175 lbf
5/16“ – 18	18	0,0524	19,8 kN / 4 454 lbf	37,8 kN / 8 505 lbf
3/8“ – 16	16	0,0775	29,3 kN / 6 587 lbf	56,0 kN / 12 600 lbf
7/16“ – 14	14	0,1063	40,2 kN / 9 036 lbf	76,8 kN / 17 280 lbf
1/2“ – 13	13	0,1419	53,7 kN / 12 062 lbf	102,5 kN / 23 062 lbf
9/16“ – 12	12	0,1820	68,8 kN / 15 470 lbf	131,5 kN / 29 585 lbf
5/8“ – 11	11	0,2260	85,5 kN / 19 210 lbf	163,3 kN / 36 735 lbf
3/4“ – 10	10	0,3340	126,3 kN / 28 390 lbf	241,3 kN / 54 270 lbf
7/8“ – 9	9	0,4620	174,7 kN / 39 270 lbf	333,7 kN / 75 060 lbf
1“ – 8	8	0,6060	229,2 kN / 51 510 lbf	437,8 kN / 98 440 lbf

Proof load = proof stress × stress area (Grade 5: 85 ksi; Grade 8: 120 ksi).

Equivalences Between Standards

There is no direct interchangeability between UNC threads and ISO/DIN metric threads; however, 1/4“–20 UNC is dimensionally close to M6 x 1.0 (diameters 6.35 mm vs 6.00 mm).

UNC Size	Nominal diameter (mm)	Suggested metric equivalent (DIN/ISO)	Equivalent pitch	Note
#1 – 64	1,854	M2	0,40 mm	Approximation by diameter only
#2 – 56	2,184	M2,2 (non-standard)	0,45 mm	Very rare in metric
#3 – 48	2,515	M2,5	0,45 mm	Noticeable pitch difference
#4 – 40	2,845	M3	0,50 mm	Similar diameter and pitch
#5 – 40	3,175	M3,5 (non-standard)	0,60 mm	–
#6 – 32	3,505	M3,5 × 0,6	0,60 mm	Limited metric use
#8 – 32	4,166	M4 × 0,7	0,70 mm	Not interchangeable
#10 – 24	4,826	M5 × 0,8	0,80 mm	Substantial pitch difference
1/4“ – 20	6,350	M6 × 1,0	1,00 mm	Most common dimensional approximation
5/16“ – 18	7,938	M8 × 1,25	1,25 mm	–
3/8“ – 16	9,525	M10 × 1,5	1,50 mm	–
7/16“ – 14	11,112	M12 × 1,75	1,75 mm	M12 has larger diameter 12 mm
1/2“ – 13	12,700	M12 × 1,75	1,75 mm	Close diameter, different pitch
9/16“ – 12	14,288	M14 × 2,0	2,00 mm	–
5/8“ – 11	15,875	M16 × 2,0	2,00 mm	–
3/4“ – 10	19,050	M18 × 2,5 or M20 × 2,5	2,50 mm	M20 closer in diameter
7/8“ – 9	22,225	M22 × 2,5	2,50 mm	–
1“ – 8	25,400	M24 × 3,0	3,00 mm	M24 x 3,0 is the best approximation

Equivalences are for guidance only; never force interchange between thread systems.

Frequently Asked Questions (FAQ)

What distinguishes the pitch of a UNC thread from that of a UNF thread for the same diameter?

A UNC thread always has fewer threads per inch than UNF: for example, 1/4“–20 UNC has a pitch of 1.270 mm / 0.0500 in, while 1/4“–28 UNF has a pitch of 0.907 mm / 0.0357 in.

What is the flank angle of the UNC thread?

The basic profile of the UNC thread is symmetric V with an included angle of 60°, identical to that of ISO metric threads.

Which standard regulates the dimensions of UNC threads?

UNC threads are defined in the American standard ANSI/ASME B1.1 – Unified Inch Screw Threads, which unifies the standards of the USA and Canada.

How is an external UNC 1/4“–20 thread correctly designated?

It is written as 1/4–20 UNC, indicating first the nominal diameter in inches, then the number of threads per inch, and finally the series abbreviation.

What drill size should I use for a 3/8“–16 UNC tap?

For a 3/8“–16 UNC tap, a drill of 8.25 mm / 0.325 in is used, which corresponds to the fraction 5/16“ or the letter “O”.

What is the typical proof load of a 1/2“–13 UNC Grade 5 bolt?

A 1/2“–13 UNC Grade 5 bolt has a proof load of approximately 53.7 kN / 12,062 lbf, calculated over a stress area of 0.1419 in² and a proof stress of 85 ksi.

References

• engineeringtoolbox.com: https://www.engineeringtoolbox.com/unified-screw-threads-unc-unf-d_1809.html
• bossard.com: https://www.bossard.com/global-en/knowledge-hub/resources/technical-information/metric-iso-threads/
• mcmaster.com: https://www.mcmaster.com/products/thread-size-identifiers/thread-type~unc/