Thread
1/4"-20
1/4"-20 (UNC) — 20 TPI, 0.25" major Ø, tap drill #7 (0.201").
Last verified: 2026-07-17
| SystemMeasurement system — Inch (Unified) or Metric (ISO). | Inch |
|---|---|
| SeriesCoarse or fine thread series (UNC/UNF for inch). | Coarse (UNC) |
| Pitch / TPIInch threads list threads-per-inch; metric threads list the pitch in mm. | 20 TPI |
| Major ØNominal major (outside) diameter of the thread. | 0.25" |
| Tap drill (~75%)Drill before tapping for ~75% thread engagement — the common shop default. Inch drills are given as number/letter/fraction with the decimal size; metric as the drill diameter in mm. | #7 (0.201") |
| Clearance holeBolt clearance hole where a reliable standard value exists — metric is the ISO 273 normal fit; inch fractional is the close fit (nominal + 1/64"). For other fits and inch machine-screw sizes, use the Clearance Hole Calculator. | 17/64" (0.266") |
| Typical use | General structural fastening — very common |
Other thread, drill & tap reference
- #4-40
- #6-32
- #8-32
- #10-24
- #12-24
- 5/16"-18
- 3/8"-16
- 7/16"-14
- 1/2"-13
- 9/16"-12
- 5/8"-11
- 3/4"-10
- #4-48
- #6-40
- #8-36
- #10-32
- 1/4"-28
- 5/16"-24
- 3/8"-24
- 7/16"-20
- 1/2"-20
- 9/16"-18
- 5/8"-18
- 3/4"-16
- M2 × 0.4
- M2.5 × 0.45
- M3 × 0.5
- M4 × 0.7
- M5 × 0.8
- M6 × 1.0
- M8 × 1.25
- M10 × 1.5
- M12 × 1.75
- M16 × 2.0
- M20 × 2.5
- M8 × 1.0
- M10 × 1.25
- M10 × 1.0
- M12 × 1.5
- M12 × 1.25
- M16 × 1.5
- M20 × 1.5
Sources
- ASME B1.1 — Unified Inch Screw Threads (UNC/UNF): TPI and major diameter
- ISO 261 / ISO 262 — ISO general-purpose metric screw threads (coarse and fine pitch)
- ISO 273 — clearance holes for bolts and screws (normal fit, metric)
- Standard machinist tap-drill sizing for ~75% thread engagement