UNC vs UNF Threads

ASME B1.1 · Unified inch screw threads · coarse and fine

FIELD IDENTIFICATION

What it is

UNC is the Unified coarse thread series and UNF the fine series. Same diameter, different threads per

inch: the nuts are not interchangeable.

1/4-20 UNC (coarse)

1/4-28 UNF (fine)

Same diameter, different threads per inch

Threads per inch

Size

UNC

UNF

1/4

20

28

5/16

18

24

3/8

16

24

7/16

14

20

1/2

13

20

9/16

12

18

5/8

11

18

3/4

10

16

7/8

9

14

1

8

12

1-1/4

7

12

1-1/2

6

12

Key points

Coarse assembles faster and tolerates damage and soft materials; fine gives finer adjustment and a slightly larger stress area.

Write the full callout: 1/2-13 UNC-2A, not just 1/2.

Governing standards: ASME B1.1-2023. Editions shown are the latest confirmed as of September 2026. The drawing and the governing standard always take precedence over this

sheet.

torquing any fastener. Verify the current governing standard, project documents, OEM requirements, size, thread, coating and lot certification. To the fullest extent permitted by law, The Fastener App disclaims liability

arising from reliance on this reference.