Threads / Size reference
1/4-28 UNF tap drill sizes & dimensions
Guhring’s chart lists 5.45 mm for a cutting tap at 75% theoretical thread. Compare the chart options below; the selected tap and application determine the hole requirement.
1/4-28 UNF tap drill size chart
These are cutting-tap selections at the listed theoretical thread percentages, not forming-tap hole sizes. Confirm the hole range with the selected tap manufacturer for your material and application.
| Target theoretical thread | Listed drill | Diameter · in / mm |
|---|---|---|
| 55% | 5.70 mm | 0.224409 / 5.7 |
| 65% | 7/32 in | 0.21875 / 5.55625 |
| 75% | 5.45 mm | 0.214567 / 5.45 |
Source: Guhring tap drill chart · PDF page 2 ↗. Check form-tap options →
Earlier drill references
An earlier version of this page listed #3 (0.213 in); this source’s 75% cut-tap column lists 5.45 mm. These are different selections, not unit equivalents.
6.35 mm
0.907143 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
1/4-28 vs 1/4-32
1/4-32 UNEF shares the nominal diameter but has a 0.793750 mm pitch. The pitch difference is 0.113393 mm; these are different nominal threads.
Swipe to compare both sizes →
| Property | 1/4-28 UNF | 1/4-32 UNEF |
|---|---|---|
| Nominal diameter · in | 0.25 | 0.25 |
| Nominal diameter · mm | 6.35 | 6.35 |
| Threads per inchDifferent | 28 | 32 |
| Pitch · mmDifferent | 0.907143 | 0.79375 |
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 → |
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 →