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Reference · screw threads

Screw Thread Reference — UNC, UNF, UNEF, Metric and NPT Sizes

https://shopmath.org/threads · data verified August 1, 2026

184 standard thread sizes across 7 series, each with basic dimensions, tap drill sizes from 50% to 85% thread engagement, clearance holes and manufacturing class limits — inch and millimetre on the same row. Every number names the standard it came from and the published table it was checked against.

Sizes
184
Series
7
Verified
August 1, 2026

Every Screw Thread Series in This Reference

Pick a series to see all of its sizes, or go straight to a size page for tap drills, clearance holes and class limits.

Screw thread series covered by this reference
SeriesSystemSizesSmallestLargestWhat it is
Unified Coarse (UNC)Unified inch33#1-64 UNC4"-4 UNCThe Unified coarse inch thread series — the default inch thread for general assembly.
Unified Extra Fine (UNEF)Unified inch25#12-32 UNEF1 11/16"-18 UNEFThe Unified extra-fine inch thread series, used for thin-wall and adjustment threads.
Unified Fine (UNF)Unified inch24#0-80 UNF1 1/2"-12 UNFThe Unified fine inch thread series — more threads per inch, better hold in thin material.
ISO Metric CoarseISO metric31M1.6×0.35M64×6ISO 261 coarse metric threads — the default pitch for each nominal diameter.
ISO Metric FineISO metric37M1.6×0.2M64×4ISO 261 fine metric pitches, for thin walls and vibration resistance.
NPT Taper Pipe ThreadTaper pipe241/16-27 NPT24-8 NPTAmerican National Standard taper pipe thread, ASME B1.20.1 — sealing needs a compound or tape.
NPTF Dryseal Pipe ThreadTaper pipe101/16-27 NPTF2-11.5 NPTFDryseal taper pipe thread, ASME B1.20.3 — crest and root interfere to seal without tape.

A series appears here only when it carries at least three verified sizes.

Sources and Method for the Screw Thread Data

ISO metric screw threads — M1.6 to M64, coarse and finecomputed+compiled

ISO 68-1 basic profile computed; ISO 261/262 diameter-pitch pairs and 6H class limits compiled

Verified against threads-metric: 215 checks passed across 3 harness(es)

Part computed from the standard's formulas, part compiled from agreeing public sources.

NPT and NPTF taper pipe threadscomputed+compiled

ASME B1.20.1 taper formulas computed; pipe OD, tpi, L1 and L4 compiled

Verified against threads-pipe: 98 checks passed across 1 harness(es)

Part computed from the standard's formulas, part compiled from agreeing public sources.

Unified inch screw threads — UNC, UNF, UNEFcomputed

NBS Handbook H28 (1969) Part I §§6-7 formula system (public domain, 17 U.S.C. 105)

Verified against threads-unified: 1064 checks passed across 3 harness(es)

Computed from the standard's formulas, then checked against published tables.

Assumptions
  • ISO metric screw threads — M1.6 to M64, coarse and fine — EXCLUDED: ISO 965-1 tolerance-grade formulas (not located — data-sourcing.md open item 2), 6g external limits (single-sourced), form tap drills, M68-M100 (verified demand cliff at M64).
  • NPT and NPTF taper pipe threads — L1 is tabulated, not derivable (data-sourcing.md §3.3). EXCLUDED: NPTF crest/root truncation limits (single-sourced), NPSM/NPSC/NPSF straight pipe threads, BSPT/BSPP.
  • Unified inch screw threads — UNC, UNF, UNEF — Le = 1D for UNC/UNF, 9P for UNEF — determined empirically against H28 Table 2.21, resolving data-sourcing.md open item 5. EXCLUDED: form (roll) tap drills, class 1A/1B limits, class 1AR allowances, constant-pitch UN series.
Data last verified
August 1, 2026
How this data is built and checked →

How to Cite This Screw Thread Reference

Citation

ShopMath. "Screw Thread Reference — UNC, UNF, UNEF, Metric and NPT Sizes." ShopMath, verified August 1, 2026, https://shopmath.org/threads

BibTeX

@misc{shopmath-threads,
  title        = {Screw Thread Reference — UNC, UNF, UNEF, Metric and NPT Sizes},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/threads}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/threads