UNF Thread Chart
Basic dimensions for Unified National Fine threads (ASME B1.1), from #0-80 to 1-1/2"-12 — threads per inch, major, pitch and minor diameter, and recommended tap drill size.
UNF (Unified National Fine) is the fine-pitch series of the Unified Thread Standard, using the same 60° symmetric profile as UNC but more threads per inch at a given nominal diameter. Because the finer pitch cuts a shallower thread, UNF keeps a larger pitch and minor diameter than UNC at the same major diameter — giving it a bigger stress area and higher tensile strength for the same bolt size.
Engineers reach for UNF when they need that extra strength, finer linear adjustment, or a thread shallow enough for a thin-walled part or short engagement length — common in aerospace, automotive and precision-instrument fasteners.
The trade-off is the same as with metric fine threads: the shallower profile is more prone to cross-threading and stripping, especially in soft metals, plastics or castings, and less forgiving of repeated assembly into tapped holes — so UNC remains the default there.
| Size | TPI | Major Ø (in) | Major Ø (mm) | Pitch Ø (in) | Minor Ø — bolt (in) | Minor Ø — nut (in) | Tap drill | Tap drill (mm) |
|---|---|---|---|---|---|---|---|---|
| #0-80 | 80 | 0.0600 | 1.524 | 0.0519 | 0.0447 | 0.0465 | 3/64 | 1.19 |
| #2-64 | 64 | 0.0860 | 2.184 | 0.0759 | 0.0668 | 0.0691 | #50 | 1.78 |
| #4-48 | 48 | 0.1120 | 2.845 | 0.0985 | 0.0864 | 0.0895 | #42 | 2.37 |
| #6-40 | 40 | 0.1380 | 3.505 | 0.1218 | 0.1073 | 0.1109 | #33 | 2.87 |
| #8-36 | 36 | 0.1640 | 4.166 | 0.1460 | 0.1299 | 0.1339 | #29 | 3.45 |
| #10-32 | 32 | 0.1900 | 4.826 | 0.1697 | 0.1517 | 0.1562 | #21 | 4.04 |
| #12-28 | 28 | 0.2160 | 5.486 | 0.1928 | 0.1722 | 0.1773 | #14 | 4.62 |
| 1/4-28 | 28 | 0.2500 | 6.350 | 0.2268 | 0.2062 | 0.2113 | #3 | 5.41 |
| 5/16-24 | 24 | 0.3125 | 7.938 | 0.2854 | 0.2614 | 0.2674 | I | 6.91 |
| 3/8-24 | 24 | 0.3750 | 9.525 | 0.3479 | 0.3239 | 0.3299 | Q | 8.43 |
| 7/16-20 | 20 | 0.4375 | 11.113 | 0.4050 | 0.3762 | 0.3834 | 25/64 | 9.92 |
| 1/2-20 | 20 | 0.5000 | 12.700 | 0.4675 | 0.4387 | 0.4459 | 29/64 | 11.51 |
| 9/16-18 | 18 | 0.5625 | 14.288 | 0.5264 | 0.4943 | 0.5024 | 33/64 | 13.10 |
| 5/8-18 | 18 | 0.6250 | 15.875 | 0.5889 | 0.5568 | 0.5649 | 37/64 | 14.68 |
| 3/4-16 | 16 | 0.7500 | 19.050 | 0.7094 | 0.6733 | 0.6823 | 11/16 | 17.46 |
| 7/8-14 | 14 | 0.8750 | 22.225 | 0.8286 | 0.7874 | 0.7977 | 13/16 | 20.64 |
| 1-12 | 12 | 1.0000 | 25.400 | 0.9459 | 0.8978 | 0.9098 | 59/64 | 23.42 |
| 1-1/8-12 | 12 | 1.1250 | 28.575 | 1.0709 | 1.0228 | 1.0348 | 1-3/64 | 26.59 |
| 1-1/4-12 | 12 | 1.2500 | 31.750 | 1.1959 | 1.1478 | 1.1598 | 1-11/64 | 29.77 |
| 1-3/8-12 | 12 | 1.3750 | 34.925 | 1.3209 | 1.2728 | 1.2848 | 1-19/64 | 32.94 |
| 1-1/2-12 | 12 | 1.5000 | 38.100 | 1.4459 | 1.3978 | 1.4098 | 1-27/64 | 36.12 |
No thread size matches your search.
Values are ASME B1.1 basic (theoretical) dimensions. Tap drill sizes target roughly 75% thread engagement — the standard used in general mechanical design. This table lists the most common UNF sizes; the full ASME B1.1 series also includes #1-72, #3-56 and #5-44. Data provided for reference — always verify against the current standard for critical or safety-related applications.
Understanding the columns
- TPI (Threads Per Inch)
- The number of thread crests per axial inch of the fastener. Pitch, in the metric sense, equals 1/TPI.
- Minor Ø — bolt vs. nut
- The bolt (external) minor diameter is the root diameter of the male thread — the value to use when checking a screw's own strength. The nut (internal) minor diameter is the corresponding root diameter of the mating female thread, which is always larger to provide clearance.
- UNC vs UNF vs UNEF
- Coarse, Fine and Extra-Fine series of the same 60° thread form, differing only in TPI for a given diameter. UNC is the general-purpose default; UNF and UNEF trade easier assembly for higher stress area and finer adjustment.
- Stress area
- The effective tensile cross-section of a thread, between the pitch and minor diameter — the value that actually resists axial (tensile) load, and the reason a finer pitch gives more strength at the same major diameter.
- Class of fit
- Tolerance grade for mating tightness: 1A/2A/3A for external (bolt) threads, 1B/2B/3B for internal (nut) threads, loosest to tightest. 2A/2B is the standard commercial fit.
Frequently asked questions
Why does UNF have a higher tensile stress area than UNC at the same nominal diameter?
The finer pitch cuts a shallower thread, so the pitch and minor diameters stay closer to the major diameter — increasing the effective cross-section that resists tensile load.
Are UNC and UNF interchangeable?
No. A UNC nut or tap will not thread onto a UNF bolt of the same nominal size, or vice versa — TPI, tap drills and mating parts must all match the same series.
What tolerance classes apply to UNF?
The same as UNC: 1A/2A/3A for external threads, 1B/2B/3B for internal threads, per ASME B1.1 — 2A/2B is the standard commercial fit.
How does UNEF relate to UNF?
UNEF (Unified National Extra Fine) uses even more threads per inch than UNF at a given diameter, reserved for thin-wall tubing, instrument bodies and very short thread-engagement applications. UNF is the general-purpose "fine" choice; UNEF is a further step for special cases.
References & further reading
- ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form) — the standard this chart's basic dimensions are built from.
- Unified Thread Standard — Wikipedia — background on the UTS system and its coarse/fine/extra-fine series.
- List of drill and tap sizes — Wikipedia — a cross-reference used to verify this chart's tap drill designations.
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