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 is a geometric starting estimate for a cutting tap, not a specified thread percentage; confirm the tap manufacturer’s hole range.

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.

UNEF (Unified Extra Fine) Thread Sizes

Every standard UNEF 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.

Page last changed 2026-10-19 · based on ASME B1.1-2019

Quick Answer

UNEF covers 18 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

UNEF 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]
1/4-32 #	32	0.2500	0.0312	0.2297	0.2117	0.2162	7/32	0.2188	0.03786
5/16-32 #	32	0.3125	0.0312	0.2922	0.2742	0.2787	9/32	0.2812	0.06248
3/8-32 #	32	0.3750	0.0312	0.3547	0.3367	0.3412	11/32	0.3438	0.09324
7/16-28 #	28	0.4375	0.0357	0.4143	0.3937	0.3988	Y	0.4040	0.12737
1/2-28 #	28	0.5000	0.0357	0.4768	0.4562	0.4613	15/32	0.4688	0.16997
9/16-24 #	24	0.5625	0.0417	0.5354	0.5114	0.5174	33/64	0.5156	0.21393
5/8-24 #	24	0.6250	0.0417	0.5979	0.5739	0.5799	37/64	0.5781	0.26824
11/16-24 #	24	0.6875	0.0417	0.6604	0.6364	0.6424	41/64	0.6406	0.32868
3/4-20 #	20	0.7500	0.0500	0.7175	0.6887	0.6959	45/64	0.7031	0.38626
13/16-20 #	20	0.8125	0.0500	0.7800	0.7512	0.7584	49/64	0.7656	0.45818
7/8-20 #	20	0.8750	0.0500	0.8425	0.8137	0.8209	53/64	0.8281	0.53623
1-20 #	20	1.0000	0.0500	0.9675	0.9387	0.9459	61/64	0.9531	0.71074
1 1/16-18 #	18	1.0625	0.0556	1.0264	0.9943	1.0024	1	1.0000	0.79860
1 1/8-18 #	18	1.1250	0.0556	1.0889	1.0568	1.0649	1 1/16	1.0625	0.90067
1 3/16-18 #	18	1.1875	0.0556	1.1514	1.1193	1.1274	1 1/8	1.1250	1.00887
1 1/4-18 #	18	1.2500	0.0556	1.2139	1.1818	1.1899	1 3/16	1.1875	1.12321
1 3/8-18 #	18	1.3750	0.0556	1.3389	1.3068	1.3149	1 5/16	1.3125	1.37029
1 1/2-18 #	18	1.5000	0.0556	1.4639	1.4318	1.4399	1 7/16	1.4375	1.64191

All dimensions are basic — the theoretical profile with no class-of-fit allowance or tolerance applied. Tap drills are cutting-tap starting sizes; confirm the selected tool’s hole range.

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 is a geometric starting estimate for a cutting tap, not a specified thread percentage; confirm the tap manufacturer’s hole range.
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

References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.

#	Source	Type	Revision / method
[1]	ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form)	reference	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)	reference	n/a — industry standard drill series — source
[5]	MIL-S-7742 — Screw Threads, Standard, Optimum Selected Series	reference	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

Verification scope

No page-specific independent source review is recorded here. The listed references and formulas explain the basis of the compilation; automated consistency checks do not verify the original sources or establish product conformity.

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. Use the source notes and recorded verification scope above to assess each value. Calculated values follow the stated derivations; compilation values may lack a confirmed original document or product condition. 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

Read the source notes, calculation methods and verification scope above before reusing a value. To cite this page:

Plain text

Engineering Reference. "UNEF (Unified Extra Fine) Thread Sizes." https://refmanual.com/charts/unef-thread-sizes/ (accessed 2026-10-19). Data from ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form), ASME B1.1-2019.

BibTeX

@misc{unef-thread-sizes,
  title        = {UNEF (Unified Extra Fine) Thread Sizes},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/unef-thread-sizes/}},
  note         = {Accessed: 2026-10-19},
  source       = {ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form), ASME B1.1-2019}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - UNEF (Unified Extra Fine) Thread Sizes
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/unef-thread-sizes/
PY  - 2026
DA  - 2026-10-19
N1  - Source: ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form), ASME B1.1-2019
ER  -

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