1/4-28 UNF Thread Dimensions and Tap Drill
1/4-28 UNF is the Unified fine thread for the 1/4 in (6.35 mm) diameter: 28 threads per inch, a pitch of 0.03571 in (0.907 mm), on a 60° profile. ASME B1.1 lists it in the UNF series. Against the coarse 1/4-20 UNC it has 28 threads per inch instead of 20, a stress area 14% larger and a lead angle of 2.87° instead of 4.18°.
1/4-28 is used for grease fittings and small aircraft and automotive fasteners.
- Tap drill#380% thread, 5.41 mm
- Pitch28 TPI0.907 mm
- Major diameter0.2500 in6.350 mm
- Pitch diameter0.2268 in5.761 mm
- Minor diameter (nut)0.2113 in5.368 mm
- Tensile stress area0.0364 in²23.5 mm²
- Clearance hole9/32 innormal fit, ASME B18.2.8
Thread profile
1/4-28 UNF basic dimensions
| Dimension | in | mm |
|---|---|---|
| Major diameter d = D | 0.2500 | 6.350 |
| Pitch P | 0.03571 | 0.907 |
| Pitch diameter d2 = D2 | 0.2268 | 5.761 |
| Minor diameter, internal thread D1 | 0.2113 | 5.368 |
| Minor diameter, external UNR thread d3 | 0.2075 | 5.270 |
| Height of fundamental triangle H | 0.0309 | 0.786 |
| Thread height, internal (5H/8) | 0.0193 | 0.491 |
| Thread height, external (11H/16) | 0.0213 | 0.540 |
| Crest flat of external thread (P/8) | 0.0045 | 0.113 |
| Crest flat of internal thread (P/4) | 0.0089 | 0.227 |
| Property | Value |
|---|---|
| Threads per inch | 28 |
| Flank angle | 60° (30° each side) |
| Lead angle at the pitch diameter | 2.87° |
| Tensile stress area As | 0.0364 in² (23.5 mm²) |
Basic (nominal) sizes per ASME B1.1, before tolerances. A UN external thread with a flat root has the same basic minor diameter as the nut (D1); d3 applies to the rounded UNR root.
1/4-28 UNF limits of size (class 2A bolt, 2B nut)
Classes 2A and 2B are the general-purpose fits for commercial bolts, screws and nuts. The 2A external thread has an allowance of 0.0010 in, so its maximum size is that much below basic; the 2B nut starts at the basic size.
| External thread 2A | Max (in) | Min (in) | Tolerance (in) |
|---|---|---|---|
| Major diameter | 0.2490 | 0.2425 | 0.0065 |
| Pitch diameter | 0.2258 | 0.2225 | 0.0033 |
| Internal thread 2B | Max (in) | Min (in) | Tolerance (in) |
|---|---|---|---|
| Minor diameter | 0.220 | 0.211 | 0.0090 |
| Pitch diameter | 0.2311 | 0.2268 | 0.0043 |
Limits of size from ASME B1.1 Table 2.
1/4-28 UNF tap drill size
Tap drill for 1/4-28 UNF: #3 (0.2130 in, 5.41 mm)
This leaves about 80% thread in the tapped hole.
#3 is the drill listed for 1/4-28 UNF in standard tap drill charts. It is 0.0370 in under the 0.2500 in major diameter.
The 2B minor diameter of the finished nut thread must lie between 0.211 and 0.220 in. The recommended drill falls inside that band.
Other drills that work for 1/4-28 UNF
A larger drill cuts less thread and taps more easily; a smaller one gives a fuller thread but loads the tap harder.
| Drill | mm | in | % thread | Within 2B limits | Thread engagement |
|---|---|---|---|---|---|
| #4 | 5.31 | 0.2090 | 88% | no | |
| 5.4 mm | 5.40 | 0.2126 | 81% | yes | |
| #3recommended | 5.41 | 0.2130 | 80% | yes | |
| 7/32" | 5.56 | 0.2188 | 67% | yes | |
| #2 | 5.61 | 0.2210 | 63% | no |
% thread = (major diameter − drill diameter) ÷ (1.299 × pitch). See the tap drill chart for how to choose a percentage.
Clearance hole for 1/4 in
The clearance hole depends only on the 0.2500 in diameter, not on the thread series. The normal fit, a 9/32 in drill, is the usual choice for general assembly.
| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Clearance on Ø (in) |
|---|---|---|---|---|
| Close | 17/64 in | 0.2656 | 6.75 | 0.0156 |
| Normal | 9/32 in | 0.2812 | 7.14 | 0.0312 |
| Loose | 19/64 in | 0.2969 | 7.54 | 0.0469 |
Nominal drill sizes from ASME B18.2.8.
1/4-28 UNF proof and breaking loads
Loads for steel bolts by SAE grade, from the tensile stress area of 0.0364 in². The proof load is the highest load the bolt must carry without permanent set.
| SAE grade | Proof stress (psi) | Min tensile strength (psi) | Proof load (lbf) | Min breaking load (lbf) | Proof load (kN) |
|---|---|---|---|---|---|
| Grade 2 | 55,000 | 74,000 | 2,000 | 2,690 | 8.91 |
| Grade 5 | 85,000 | 120,000 | 3,090 | 4,370 | 13.8 |
| Grade 8 | 120,000 | 150,000 | 4,370 | 5,460 | 19.4 |
Strengths from SAE J429 for this diameter; stress area from ASME B1.1. Loads rounded to three significant figures.
1/4 in pitches compared
All standard pitches on the 1/4 in diameter. A finer pitch gives a shallower thread, a larger core and a smaller lead angle; a coarser pitch is faster to assemble and more tolerant of damage and plating.
| Thread | TPI | Pitch Ø (in) | Minor Ø D1 (in) | Stress area (in²) | Tap drill |
|---|---|---|---|---|---|
| 1/4-20 UNC | 20 | 0.2175 | 0.1959 | 0.0318 | #7 |
| 1/4-28 UNF | 28 | 0.2268 | 0.2113 | 0.0364 | #3 |
| 1/4-32 UNEF | 32 | 0.2297 | 0.2162 | 0.0379 | 7/32" |
Closest metric thread
No metric thread is interchangeable with 1/4-28 UNF. The nearest is M6×1, which is 0.35 mm smaller in diameter with a pitch of 1.000 mm against 0.907 mm. A nut may start on the wrong thread and then bind or strip, so identify by both diameter and pitch.
How the 1/4-28 UNF dimensions are calculated
Every basic dimension follows from the major diameter and the pitch:
- P = 0.03571 in = 1 ÷ 28 threads per inch
- H = 0.866025 × P = 0.0309 in
- d2 = d − 0.649519 × P = 0.2500 − 0.0232 = 0.2268 in
- D1 = d − 1.082532 × P = 0.2500 − 0.0387 = 0.2113 in
- d3 = d − 1.190785 × P = 0.2500 − 0.0425 = 0.2075 in (external root, 11H/16 deep)
- As = 0.7854 × (d − 0.9743 × P)² = 0.7854 × 0.2152² = 0.03637 in²
- Lead angle = arctan(P ÷ (π × d2)) = 2.87°
Related sizes and tools
Sources: ASME B1.1 (profile, series, limits of size and stress area), ASME B18.2.8 (clearance holes), SAE J429 (mechanical properties) and the tap drill tables of Machinery’s Handbook. How this site checks its data. Last reviewed 29 September 2026.