---
type: "hub"
name: "Screw thread reference"
series_count: 7
size_count: 184
systems: ["Unified inch", "ISO metric", "Taper pipe"]
verified: "August 1, 2026"
url: https://shopmath.org/threads
---

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

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.

## Every Screw Thread Series in This Reference

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

| Series | System | Sizes | Smallest | Largest | What it is |
| --- | --- | --- | --- | --- | --- |
| [Unified Coarse (UNC)](https://shopmath.org/threads/unc.md) | Unified inch | 33 | #1-64 UNC | 4"-4 UNC | The Unified coarse inch thread series — the default inch thread for general assembly. |
| [Unified Extra Fine (UNEF)](https://shopmath.org/threads/unef.md) | Unified inch | 25 | #12-32 UNEF | 1 11/16"-18 UNEF | The Unified extra-fine inch thread series, used for thin-wall and adjustment threads. |
| [Unified Fine (UNF)](https://shopmath.org/threads/unf.md) | Unified inch | 24 | #0-80 UNF | 1 1/2"-12 UNF | The Unified fine inch thread series — more threads per inch, better hold in thin material. |
| [ISO Metric Coarse](https://shopmath.org/threads/metric-coarse.md) | ISO metric | 31 | M1.6×0.35 | M64×6 | ISO 261 coarse metric threads — the default pitch for each nominal diameter. |
| [ISO Metric Fine](https://shopmath.org/threads/metric-fine.md) | ISO metric | 37 | M1.6×0.2 | M64×4 | ISO 261 fine metric pitches, for thin walls and vibration resistance. |
| [NPT Taper Pipe Thread](https://shopmath.org/threads/npt.md) | Taper pipe | 24 | 1/16-27 NPT | 24-8 NPT | American National Standard taper pipe thread, ASME B1.20.1 — sealing needs a compound or tape. |
| [NPTF Dryseal Pipe Thread](https://shopmath.org/threads/nptf.md) | Taper pipe | 10 | 1/16-27 NPTF | 2-11.5 NPTF | Dryseal 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 fine

- **Source**: ISO 68-1 basic profile computed; ISO 261/262 diameter-pitch pairs and 6H class limits compiled
- **Method**: computed+compiled
- **Verified against**: threads-metric: 215 checks passed across 3 harness(es)

### NPT and NPTF taper pipe threads

- **Source**: ASME B1.20.1 taper formulas computed; pipe OD, tpi, L1 and L4 compiled
- **Method**: computed+compiled
- **Verified against**: threads-pipe: 98 checks passed across 1 harness(es)

### Unified inch screw threads — UNC, UNF, UNEF

- **Source**: NBS Handbook H28 (1969) Part I §§6-7 formula system (public domain, 17 U.S.C. 105)
- **Method**: computed
- **Verified against**: threads-unified: 1064 checks passed across 3 harness(es)

### 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

## Related pages

- Cross-series chart: [Tap drill chart](https://shopmath.org/charts/tap-drill.md)
- Drill index: [Drill size chart](https://shopmath.org/charts/drill-sizes.md)
- Drill sizes: [Drill bit size reference](https://shopmath.org/drills.md)

---

*Source: [ShopMath](https://shopmath.org/threads) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*