Threads / Size reference
1/4-20 UNC
1/4-20 has a nominal outside diameter of 6.35 mm and a 1.27 mm pitch.
6.35 mm
1.27 mm pitch
Nominal dimensions; no class allowances or acceptance limits applied. Diameter is in inches; the number after the dash is TPI.
same diameter comparison
1/4-20 vs 1/4-28
1/4-28 UNF shares the nominal diameter but has a 0.907143 mm pitch. The pitch difference is 0.362857 mm; these are different nominal threads.
Swipe to compare both sizes →
| Property | 1/4-20 UNC | 1/4-28 UNF |
|---|---|---|
| Nominal diameter · in | 0.25 | 0.25 |
| Nominal diameter · mm | 6.35 | 6.35 |
| Threads per inchDifferent | 20 | 28 |
| Pitch · mmDifferent | 1.27 | 0.907143 |
All 3 options at this diameter
Swipe to see all columns →
| Designation | Major Ø · in / mm | TPI | Pitch · mm | Drill evidence | Next step |
|---|---|---|---|---|---|
| 1/4-20 UNC | 0.25 / 6.35 | 20 | 1.27 | Chart row | Tap-drill lookup → |
| 1/4-28 UNF | 0.25 / 6.35 | 28 | 0.907143 | Chart row | Tap-drill lookup → |
| 1/4-32 UNEF | 0.25 / 6.35 | 32 | 0.79375 | Maker data needed | Tap-drill lookup → |
Cutting-tap drill references
| Target theoretical thread | Listed drill | Diameter · in / mm |
|---|---|---|
| 55% | #3 | 0.213 / 5.4102 |
| 65% | #5 | 0.2055 / 5.2197 |
| 75% | #7 | 0.201 / 5.1054 |
Need acceptance limits?
The complete class and applicable drawing requirements are needed. These nominal values are not maximum/minimum inspection limits.
Part has no marking?
Measure the correct diameter and pitch, then compare candidates across metric and inch systems.
Basis for this size
Nominal designation and family transcribed from the live directory on 2026-10-19; unit conversions are calculated using exactly 25.4 mm/in. The drill row was read from Guhring PDF page 2; numbered and letter drill diameters use page 1. Paid standard limit tables have not been reviewed.
ASME B1.1 · standard scope ↗ · Guhring · Tap Drill Sizes ↗ Full source scope →