Threads

Unified inch threads UNC and UNF: sizes and threads per inch

UNC (Unified coarse) and UNF (Unified fine) are 60° inch threads designated by diameter and threads per inch, such as 1/2-13 UNC or 1/2-20 UNF. They are governed by ASME B1.1 in the USA, by BS 1580 in the UK and internationally by ISO 263 and ISO 725, and they are the standard inch fastener thread in North America.

Names by standard

Standards in each region

Region	System	Designation	Norma	Match
USA	Unified Thread Standard	1/2-13 UNC-2AGoverning standard. Classes 1A/2A/3A are external, 1B/2B/3B internal; 2A/2B is the general-purpose fit.	ASME B1.1	Same material
UK	BS unified	1/2-13 UNC-2APart 1 covers 1/4 in and larger, Part 3 sizes below 1/4 in. BS 1768 covers unified precision hexagon bolts, screws and nuts.	BS 1580-1 and BS 1580-3	Same material
EU	ISO inch	1/2-13 UNCNo separate EN thread standard; ISO 263 gives the general plan and ISO 725 the basic dimensions. Used mainly for imported and export equipment.	ISO 263, ISO 725	Same material
Índia	ASME / ISO inch (used directly)	1/2-13 UNCNo separate Indian Standard for unified threads was identified. IS 4218 metric threads are standard for new Indian fasteners; UNC/UNF appear on export work and imported equipment.	ASME B1.1 / ISO 725	Same material
Japan	JIS unified	1/2-13 UNCBoth 1973 and based on ISO 263. Limits of size are in JIS B 0210 (coarse) and JIS B 0212 (fine).	JIS B 0206 (coarse), JIS B 0208 (fine)	Same material

Thread form

Geometria

Thread angle: 60°, symmetric V (same form as ISO metric)
Pitch expressed as: Threads per inch (TPI); pitch = 25.4 mm ÷ TPI
Number sizes: #0–#12 below 1/4 in; major diameter = 0.060 in + 0.013 in × number
Series: UNC coarse, UNF fine, UNEF extra fine, plus constant-pitch UN series (e.g. 8-UN)
Designation: Size-TPI series-class, e.g. 3/8-16 UNC-2A (external) or 3/8-24 UNF-2B (internal)
Hand: Right-hand unless LH is added

Size chart

Common sizes

Tamanho	Major diameter (in)	Major diameter (mm)	UNC TPI	UNF TPI	Tap drill, UNC (75%)
#4	0.112	2.845	40	48	#43 (0.0890 in)
#5	0.125	3.175	40	44	#38 (0.1015 in)
#6	0.138	3.505	32	40	#36 (0.1065 in)
#8	0.164	4.166	32	36	#29 (0.1360 in)
#10	0.190	4.826	24	32	#25 (0.1495 in)
#12	0.216	5.486	24	28	#16 (0.1770 in)
1/4 in	0.250	6.350	20	28	#7 (0.2010 in)
5/16 in	0.3125	7.938	18	24	F (0.2570 in)
3/8 in	0.375	9.525	16	24	5/16 in (0.3125 in)
7/16 in	0.4375	11.113	14	20	U (0.3680 in)
1/2 in	0.500	12.700	13	20	27/64 in (0.4219 in)
9/16 in	0.5625	14.288	12	18	31/64 in (0.4844 in)
5/8 in	0.625	15.875	11	18	17/32 in (0.5312 in)
3/4 in	0.750	19.050	10	16	21/32 in (0.6562 in)
7/8 in	0.875	22.225	9	14	49/64 in (0.7656 in)
1 in	1.000	25.400	8	12	7/8 in (0.8750 in)

Tap drills are the usual workshop choice for about 75% thread engagement; # sizes are number drills and letters are letter drills. Harder materials often use a slightly larger drill. ASME B1.1 also lists 1 in-14 UNS, which some catalogues show as 1 in UNF.

In practice

Uses and compatibility

Where it is used

Inch bolts, screws, studs and nuts in North America
Threaded rod and anchor bolts specified to US standards such as ASTM A193, A307 and F1554
Aerospace, automotive and machinery built to US drawings
UNF where fine adjustment, thin walls or higher stress area are needed

What interchanges

UNC and UNF of the same diameter do not mate: 1/2-13 UNC will not run into a 1/2-20 UNF nut.
Unified threads do not interchange with metric threads, even where the diameter is close (1/4-20 UNC is 6.35 mm, M6 is 6 mm with a 1 mm pitch).
1/4 in UNC and 1/4 in BSW both have 20 TPI but different angles (60° vs 55°); they may turn together loosely but are not an approved substitute. Most sizes from 1/4 to 1 in share TPI the same way; 1/2 in does not (13 UNC vs 12 BSW).
UNF and BSF are not interchangeable: 1/4 in UNF has 28 TPI, 1/4 in BSF has 26.
Older American National (NC/NF) threads mate with UNC/UNF of the same size and TPI in most sizes.

Perguntas respondidas

Common questions

What does 1/4-20 UNC mean?

It is a 1/4 in diameter Unified coarse thread with 20 threads per inch. A class suffix such as 2A (bolt) or 2B (nut) is often added.

Can I use a UNC bolt in a metric nut?

No. UNC pitches are based on threads per inch and metric pitches on millimetres, so even close sizes such as 5/16-18 UNC and M8 bind after a few turns or strip under load.

What is the difference between UNC and UNF?

UNF has more threads per inch for the same diameter, for example 3/8-24 UNF against 3/8-16 UNC. Fine threads allow finer adjustment and carry slightly more load; coarse threads assemble faster and resist cross-threading and damage.

What drill size do I use for a 1/2-13 tap?

A 27/64 in (0.4219 in, about 10.7 mm) drill gives about 75% thread in common materials.

From the Corona catalog

Products listed with UNC / UNF

Barras e hastes

Barras roscadas

Diâmetro: 1/4-2 1/2 in / M6-M60 · Comprimento: 3-20 ft / 1-6 m

Especificações

Barras e hastes

Tirantes (rosca nas duas extremidades)

Diâmetro: 1/2-2 1/2 in / M12-M60 · Comprimento: 3-20 ft / 1-6 m

Especificações

Parafusos e ferragens para armação

Parafuso máquina / parafuso de cabeça quadrada

Diâmetro: 3/8-1 1/4 in (comuns 1/2, 5/8, 3/4 in) / M10-M32 · Comprimento: 2-40 in / 50-1000 mm

Especificações

Âncoras e porcas

Porca quadrada

Tamanho: 3/16-1 1/4 in / M6-M32 · Rosca: UNC; porca com rosca de folga para parafusos galvanizados

Especificações

Parafusos e acessórios

Parafuso francês (cabeça abaulada)

Tamanho: 1/4-1/2 in (5/16 e 3/8 in para acessórios de cerca) / M6-M12 · Comprimento: 1-1/4-12 in / 25-300 mm

Especificações

Componentes de suporte para tubulações

Barras roscadas

Diâmetro: 1/4" - 2-1/2" / M6 - M60 · Comprimento: 2 ft - 19 ft (3, 6, 10, 12 ft comuns) / 1 m - 5,8 m (1, 2, 3 m comuns)

Especificações

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References

Sources

Values summarize published limits and typical data. The governing standard and the current edition control; check them before specifying.