#12-28 UNF Thread Dimensions and Tap Drill
#12-28 UNF is the Unified fine thread for the number 12 size (major diameter 0.216 in, 5.49 mm): 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 as a secondary size. Against the coarse #12-24 UNC it has 28 threads per inch instead of 24, a stress area 7% larger and a lead angle of 3.37° instead of 4.02°.
- Tap drill#1473% thread, 4.62 mm
- Pitch28 TPI0.907 mm
- Major diameter0.2160 in5.486 mm
- Pitch diameter0.1928 in4.897 mm
- Minor diameter (nut)0.1773 in4.504 mm
- Tensile stress area0.0258 in²16.6 mm²
Thread profile
#12-28 UNF basic dimensions
| Dimension | in | mm |
|---|---|---|
| Major diameter d = D | 0.2160 | 5.486 |
| Pitch P | 0.03571 | 0.907 |
| Pitch diameter d2 = D2 | 0.1928 | 4.897 |
| Minor diameter, internal thread D1 | 0.1773 | 4.504 |
| Minor diameter, external UNR thread d3 | 0.1735 | 4.406 |
| 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 | 3.37° |
| Tensile stress area As | 0.0258 in² (16.6 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.
#12-28 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 | 0.2150 | 0.2085 | 0.0065 |
| Pitch diameter | 0.1918 | 0.1886 | 0.0032 |
| Internal thread 2B | Max (in) | Min (in) | Tolerance (in) |
|---|---|---|---|
| Minor diameter | 0.186 | 0.177 | 0.0090 |
| Pitch diameter | 0.1970 | 0.1928 | 0.0042 |
Limits of size from ASME B1.1 Table 2.
#12-28 UNF tap drill size
Tap drill for #12-28 UNF: #14 (0.1820 in, 4.62 mm)
This leaves about 73% thread in the tapped hole.
#14 is the drill listed for #12-28 UNF in standard tap drill charts. It is 0.0340 in under the 0.2160 in major diameter.
The 2B minor diameter of the finished nut thread must lie between 0.177 and 0.186 in. The recommended drill falls inside that band.
Other drills that work for #12-28 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 |
|---|---|---|---|---|---|
| #16 | 4.50 | 0.1770 | 84% | yes | |
| #15 | 4.57 | 0.1800 | 78% | yes | |
| 4.6 mm | 4.60 | 0.1811 | 75% | yes | |
| #14recommended | 4.62 | 0.1820 | 73% | yes | |
| #13 | 4.70 | 0.1850 | 67% | yes | |
| 3/16" | 4.76 | 0.1875 | 61% | no | |
| #12 | 4.80 | 0.1890 | 58% | no | |
| #11 | 4.85 | 0.1910 | 54% | no |
% thread = (major diameter − drill diameter) ÷ (1.299 × pitch). See the tap drill chart for how to choose a percentage.
Clearance hole for #12
ASME B18.2.8 does not list the #12 size, so there is no standard clearance hole for the #12 size. Size the hole from the fit the joint needs.
#12-28 UNF strength
SAE J429 starts at 1/4 in, so the SAE grades give no proof load for #12-28 UNF. For an estimate, multiply the tensile stress area of 0.0258 in² by the strength of the material: for example, 60,000 psi gives 1,550 lbf.
#12 pitches compared
All standard pitches on the #12 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 |
|---|---|---|---|---|---|
| #12-24 UNC | 24 | 0.1889 | 0.1709 | 0.0242 | #16 |
| #12-28 UNF | 28 | 0.1928 | 0.1773 | 0.0258 | #14 |
| #12-32 UNEF | 32 | 0.1957 | 0.1822 | 0.0270 | #13 |
How the #12-28 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 = 0.2160 − 0.0232 = 0.1928 in
- D1 = d − 1.082532 × P = 0.2160 − 0.0387 = 0.1773 in
- d3 = d − 1.190785 × P = 0.2160 − 0.0425 = 0.1735 in (external root, 11H/16 deep)
- As = 0.7854 × (d − 0.9743 × P)² = 0.7854 × 0.1812² = 0.02579 in²
- Lead angle = arctan(P ÷ (π × d2)) = 3.37°
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.