What is the difference between UNC and UNF threads?

Both are 60° Unified inch threads defined by ASME B1.1. UNC (Unified National Coarse) has fewer threads per inch and is the general-purpose choice; UNF (Unified National Fine) has more threads per inch, giving higher strength in the same diameter and finer adjustment. UNF is preferred for thin-walled parts and vibration resistance.

How are UNC/UNF sizes designated?

By the size and threads per inch, for example 1/4-20 UNC or 1/4-28 UNF. For diameters below 1/4 inch, numbered sizes are used (#1 to #12), such as #10-24 UNC. Note that the #0 size exists only in UNF (#0-80); the UNC series begins at #1.

TPI is threads per inch — the number of thread crests within one inch of length. It is the inch-system equivalent of pitch; a higher TPI means a finer thread. UNF sizes have a higher TPI than the UNC size of the same diameter.

Thread Tables

ASME B1.1

American Unified Thread: UNC and UNF

Unified system — TPI (threads per inch), 60° angle

The American unified system (UNS) includes coarse pitch (UNC — Unified National Coarse) and fine pitch (UNF — Unified National Fine). Pitch is specified in TPI (threads per inch). It shares the 60° angle with metric thread but uses imperial units and slightly different profiles.

Dimension table

Designation	System	TPI	Nom. d (in)	d₂ pitch (in)	D₁ int (in)
#1-64	UNC	64	0.0730	0.0629	0.0561
#2-56	UNC	56	0.0860	0.0744	0.0667
#3-48	UNC	48	0.0990	0.0855	0.0764
#4-40	UNC	40	0.1120	0.0958	0.0849
#5-40	UNC	40	0.1250	0.1088	0.0979
#6-32	UNC	32	0.1380	0.1177	0.1042
#8-32	UNC	32	0.1640	0.1437	0.1302
#10-24	UNC	24	0.1900	0.1629	0.1449
#12-24	UNC	24	0.2160	0.1889	0.1709
1/4-20	UNC	20	0.2500	0.2175	0.1959
5/16-18	UNC	18	0.3125	0.2764	0.2524
3/8-16	UNC	16	0.3750	0.3344	0.3073
7/16-14	UNC	14	0.4375	0.3911	0.3602
1/2-13	UNC	13	0.5000	0.4500	0.4167
9/16-12	UNC	12	0.5625	0.5084	0.4723
5/8-11	UNC	11	0.6250	0.5660	0.5266
3/4-10	UNC	10	0.7500	0.6850	0.6417
7/8-9	UNC	9	0.8750	0.8028	0.7547
1-8	UNC	8	1.0000	0.9188	0.8647
1-1/4-7	UNC	7	1.2500	1.1572	1.0954
1-1/2-6	UNC	6	1.5000	1.3918	1.3196
#0-80	UNF	80	0.0600	0.0519	0.0465
#1-72	UNF	72	0.0730	0.0640	0.0580
#2-64	UNF	64	0.0860	0.0759	0.0691
#3-56	UNF	56	0.0990	0.0874	0.0797
#4-48	UNF	48	0.1120	0.0985	0.0895
#5-44	UNF	44	0.1250	0.1102	0.1004
#6-40	UNF	40	0.1380	0.1218	0.1109
#8-36	UNF	36	0.1640	0.1460	0.1339
#10-32	UNF	32	0.1900	0.1697	0.1562
#12-28	UNF	28	0.2160	0.1928	0.1773
1/4-28	UNF	28	0.2500	0.2268	0.2113
5/16-24	UNF	24	0.3125	0.2854	0.2674
3/8-24	UNF	24	0.3750	0.3479	0.3299
7/16-20	UNF	20	0.4375	0.4050	0.3834
1/2-20	UNF	20	0.5000	0.4675	0.4459
9/16-18	UNF	18	0.5625	0.5264	0.5024
5/8-18	UNF	18	0.6250	0.5889	0.5649
3/4-16	UNF	16	0.7500	0.7094	0.6823
7/8-14	UNF	14	0.8750	0.8286	0.7977
1-12	UNF	12	1.0000	0.9459	0.9098

Formulas ASME B1.1: d₂ = d − 0.6495/n · D₁ = d − 1.0825/n (basic diameters for class 2A/2B). Values rounded to 4 decimal places.

Key technical characteristics

Flank angle: 60°
Pitch: TPI — threads per inch
Standard: ASME B1.1 (UNS)
Designation: Fraction or No. × TPI-UNC/UNF
Common range: #0 (1.52 mm) – 4 in
Tolerances: Classes 1A/1B (loose) to 3A/3B (precision)

Typical applications

Industrial equipment designed under North American standards (USA, Canada, partial Mexico).
Tooling and accessories for Haas, Hurco, and other American machine tool manufacturers.
Structural aluminum assemblies, hex nuts in heavy machinery.
Replacement components in imported equipment with SAE/UNS fasteners.

Compatibility and interoperability

✗NOT interchangeable with metric thread even if external diameter appears similar — TPI and tolerance limits are incompatible.
✓UNC and UNF share the same 60° profile — only TPI differs. The same 60° thread mills can be used for both systems.

Selection guide

01UNC for general fastening in ductile materials: higher strength due to larger root area.
02UNF for vibration applications and finer adjustment without changing diameter.
03For new designs in Mexico: prefer ISO metric — tooling and spare parts availability is higher.
04Always specify the full designation: diameter × TPI - class (e.g. ½-13 UNC-2A) to avoid purchasing errors.

Back to Thread Tables