What is the difference between UNC and UNF?

Both are Unified inch thread forms; the letter sets the pitch. UNC is coarse — fewer threads per inch, deeper thread form. UNF is fine — more threads per inch, shallower form and a larger minor diameter for the same nominal size, which gives higher tensile strength and better vibration resistance.

How do I find the tap drill size for a thread?

Subtract the pitch from the major diameter (both in inches) and pick the nearest standard drill. For a 1/4-20 that is 0.2500 − 0.0500 = 0.2000 in, which lands on a #7 drill at 0.2010 in. This gives roughly 75% thread engagement, the normal target.

What does the number in a thread size mean?

For sizes 1/4 in and larger, the fraction is the nominal major diameter in inches. For sizes below 1/4 in, sizes are numbered #0 through #12 and the major diameter follows d = 0.060 + 0.013 × N, so a #10 screw has a major diameter of 0.1900 in.

UNF (Unified Fine) Thread Sizes

Every standard UNF size with pitch, basic pitch diameter, external and internal minor diameters, tap drill and tensile stress area — computed from the ASME B1.1 basic profile.

Data verified 2026-09-29 · based on ASME B1.1-2019

Quick Answer

UNF covers 24 standard sizes in this table. The series letter sets the pitch for each nominal diameter: UNC is coarse, UNF fine and UNEF extra fine. Dimensions below are basic — they carry no class-of-fit allowance.

Find a size

UNF Thread Size Chart

Size^[1]	TPI^[1]	Major dia in^[2]	Pitch in^[2]	Pitch dia in^[2]	Ext. minor in^[2]	Int. minor in^[2]	Tap drill^[4]	Drill dia in^[4]	Tensile area in²^[2]
#0-80 #	80	0.0600	0.0125	0.0519	0.0447	0.0465	3/64	0.0469	0.00180
#1-72 #	72	0.0730	0.0139	0.0640	0.0560	0.0580	1/16	0.0625	0.00278
#2-64 #	64	0.0860	0.0156	0.0759	0.0668	0.0691	5/64	0.0781	0.00393
#3-56 #	56	0.0990	0.0179	0.0874	0.0771	0.0797	5/64	0.0781	0.00523
#4-48 #	48	0.1120	0.0208	0.0985	0.0864	0.0894	3/32	0.0938	0.00660
#5-44 #	44	0.1250	0.0227	0.1102	0.0971	0.1004	#38	0.1015	0.00831
#6-40 #	40	0.1380	0.0250	0.1218	0.1073	0.1109	#33	0.1130	0.01014
#8-36 #	36	0.1640	0.0278	0.1460	0.1299	0.1339	#29	0.1360	0.01473
#10-32 #	32	0.1900	0.0312	0.1697	0.1517	0.1562	#21	0.1590	0.01999
#12-28 #	28	0.2160	0.0357	0.1928	0.1722	0.1773	#15	0.1800	0.02579
1/4-28 #	28	0.2500	0.0357	0.2268	0.2062	0.2113	#3	0.2130	0.03637
5/16-24 #	24	0.3125	0.0417	0.2854	0.2614	0.2674	I	0.2720	0.05807
3/8-24 #	24	0.3750	0.0417	0.3479	0.3239	0.3299	Q	0.3320	0.08783
7/16-20 #	20	0.4375	0.0500	0.4050	0.3762	0.3834	W	0.3860	0.11872
1/2-20 #	20	0.5000	0.0500	0.4675	0.4387	0.4459	29/64	0.4531	0.15995
9/16-18 #	18	0.5625	0.0556	0.5264	0.4943	0.5024	1/2	0.5000	0.20298
5/8-18 #	18	0.6250	0.0556	0.5889	0.5568	0.5649	9/16	0.5625	0.25596
3/4-16 #	16	0.7500	0.0625	0.7094	0.6733	0.6823	11/16	0.6875	0.37296
7/8-14 #	14	0.8750	0.0714	0.8286	0.7874	0.7977	51/64	0.7969	0.50947
1-12 #	12	1.0000	0.0833	0.9459	0.8978	0.9098	59/64	0.9219	0.66304
1 1/8-12 #	12	1.1250	0.0833	1.0709	1.0228	1.0348	1	1.0000	0.85572
1 1/4-12 #	12	1.2500	0.0833	1.1959	1.1478	1.1598	1	1.0000	1.07294
1 3/8-12 #	12	1.3750	0.0833	1.3209	1.2728	1.2848	1	1.0000	1.31471
1 1/2-12 #	12	1.5000	0.0833	1.4459	1.3978	1.4098	1	1.0000	1.58102

All dimensions are basic — the theoretical profile with no class-of-fit allowance or tolerance applied. Tap drill is sized for approximately 75% thread engagement.

Choosing Between UNC, UNF and UNEF

The series letter selects the pitch for a given nominal diameter. Coarser threads have fewer threads per inch, a deeper thread form and greater resistance to cross-threading and galling; finer threads have more threads per inch, a shallower form and a larger minor diameter, giving higher tensile strength for the same nominal size and finer adjustment.

UNC — the default for general fastening in steel, cast iron and soft materials. More tolerant of plating thickness and minor damage.
UNF — used where wall thickness is limited, where vibration is present, and in thinner or higher-strength materials.
UNEF — extra fine, used on thin-wall tubing and where very fine adjustment is needed.

A practical constraint: fine threads have a thinner wall at the crest of an internal thread, so in weak or thin materials coarse threads are usually the safer choice.

Frequently Asked Questions

What is the difference between UNC and UNF?: Both are Unified inch thread forms; the letter sets the pitch. UNC is coarse — fewer threads per inch, deeper thread form. UNF is fine — more threads per inch, shallower form and a larger minor diameter for the same nominal size, which gives higher tensile strength and better vibration resistance.
How do I find the tap drill size for a thread?: Subtract the pitch from the major diameter (both in inches) and pick the nearest standard drill. For a 1/4-20 that is 0.2500 − 0.0500 = 0.2000 in, which lands on a #7 drill at 0.2010 in. This gives roughly 75% thread engagement, the normal target.
What does the number in a thread size mean?: For sizes 1/4 in and larger, the fraction is the nominal major diameter in inches. For sizes below 1/4 in, sizes are numbered #0 through #12 and the major diameter follows d = 0.060 + 0.013 × N, so a #10 screw has a major diameter of 0.1900 in.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form)	standard	ASME B1.1-2019 — source
[2]	Basic dimensions computed from the ASME B1.1 basic profile	derived	Pitch P = 1/n. Fundamental triangle height H = √3/2 × P. Pitch diameter d2 = d − 0.75H. External minor diameter d1 = d − 2(0.61343P). Internal minor diameter D1 = d − 2(0.54127P). Tensile stress area As = π/4 × (d − 0.9743/n)².
[3]	Millimetre conversion	derived	Inch value × 25.4.
[4]	Standard twist drill size series (number, letter and fractional)	standard	n/a — industry standard drill series — source
[5]	MIL-S-7742 — Screw Threads, Standard, Optimum Selected Series	standard	MIL-S-7742D (free public-domain mirror at everyspec.com) — source

Data Sources

Standard	Revision	What it covers on this page
ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form)	ASME B1.1-2019	thread form geometry, the designation system and the standard pitch series
MIL-S-7742 — Screw Threads, Standard, Optimum Selected Series	MIL-S-7742D (free public-domain mirror)	selection of nominal diameters and pitches

Cross-checked against:

Recomputed from the ASME B1.1 basic profile formulas and compared against published fastener dimension tables
Build-time unit and formula consistency checks (major diameter, pitch diameter and tensile area are verified against each other on every build)

Derived values — the following values on this page are calculated, not taken directly from the standard:

Value	How it is derived
Pitch, pitch diameter, minor diameters, tensile stress area	Computed from the ASME B1.1 basic profile formulas — see the Value Sources table above.

Dimensions are basic and carry no class-of-fit allowance. For a threaded assembly specify the class and use the limit dimensions from ASME B1.1.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "UNF (Unified Fine) Thread Sizes." https://refmanual.com/charts/unf-thread-sizes/ (accessed 2026-09-29). Data from ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form), ASME B1.1-2019.

BibTeX

@misc{unf-thread-sizes,
  title        = {UNF (Unified Fine) Thread Sizes},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/unf-thread-sizes/}},
  note         = {Accessed: 2026-09-29},
  source       = {ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form), ASME B1.1-2019}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - UNF (Unified Fine) Thread Sizes
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/unf-thread-sizes/
PY  - 2026
DA  - 2026-09-29
N1  - Source: ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form), ASME B1.1-2019
ER  -

Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.