UNC vs UNF Threads: What’s the Difference?
UNC (coarse) and UNF (fine) are two thread series for the same screw diameter. Both are defined in ASME B1.1, and both use the same 60° thread form; they differ only in how many threads fit into an inch. A 1/4 in screw is 1/4-20 in the coarse series and 1/4-28 in the fine series. Coarse threads are fewer and deeper; fine threads are more numerous and shallower. They are not interchangeable, and the tap drill changes with the series. If you have the screw in your hand, count the threads per inch, then look up the size in the machine screw size chart.
Key takeaways
- UNC and UNF share a diameter and differ in threads per inch (TPI): 1/4-20 against 1/4-28, #8-32 against #8-36, #10-24 against #10-32.
- A UNC screw will not run into a UNF nut, or the reverse. Counting TPI is the only reliable test.
- The tap drill follows the thread: the finer series usually calls for a slightly larger drill.
- Whether fine or coarse suits a job is an engineering decision. Follow the drawing or specification rather than a rule of thumb.
What UNC and UNF actually mean
The letters stand for Unified National Coarse and Unified National Fine, the names of the two main series in the Unified inch system. Each diameter gets a standard TPI in each series, so “1/4-20” and “1/4-28” are complete names: the diameter, then the TPI. Many callouts add the series and a fit class, as in 1/4-20 UNC-2A for an external thread. The 2A (external) or 2B (internal) part is the fit class, which sets how tightly the parts mate. A few of the very smallest sizes exist in only one series; #0, for example, is made as 0-80 only.
The numbers side by side
| Size | UNC | UNF | Tap drill, UNC | Tap drill, UNF |
|---|---|---|---|---|
| #8 | 32 TPI | 36 TPI | #29 | #29 |
| #10 | 24 TPI | 32 TPI | #25 | #21 |
| 1/4 | 20 TPI | 28 TPI | #7 | #3 |
Two things stand out. First, the gap in TPI is small for the small sizes: #8-32 and #8-36 differ by only four threads per inch, which you cannot see by eye. A pitch gauge or a careful count is the only way to tell them apart. Second, the tap drills are not the same, except at #8.
Why the tap drill changes with the series
A tap drill leaves enough material for the tap to cut a thread of the right depth, and a finer thread is shallower. So the hole for the finer series sits closer to the screw’s full diameter. A standard shortcut gives about 75% thread depth: tap drill ≈ major diameter − 0.975 ÷ TPI. For 1/4-20 that is 0.2500 − 0.975 ÷ 20 = 0.2013 in, which points to a #7 (0.201 in). For 1/4-28 it is 0.2500 − 0.975 ÷ 28 = 0.2152 in, close to a #3 (0.213 in). At #8 the two series happen to land on the same #29 (0.136 in), so one drill serves both.
When a fine thread is chosen
Designers pick the fine series for reasons that come down to geometry. Each turn advances the screw by one pitch, which is 0.0500 in at 20 TPI and 0.0357 in at 28 TPI, so a fine thread gives finer adjustment. In a short or thin section, a fine thread fits more turns into the same depth: a 1/4 in deep engagement holds 5 turns of a 20 TPI thread and 7 turns of a 28 TPI thread. And often the reason is simply that the drawing says UNF.
You will also read that fine threads are stronger or resist vibration better, and that coarse threads are tougher. Whether any of that matters depends on the material, the joint and the specification, so this guide does not rank them. For anything load-bearing or safety-related, follow the specification or ask a qualified engineer.
When coarse is the default
Coarse is the general-purpose series. It is the one most hardware is made in, it starts faster by hand, and it tolerates a nick or a little dirt in the thread better than a shallow fine thread does. If a drawing does not say otherwise and a part came with coarse screws, coarse is the likely series for replacements, once a count confirms it.
How to tell which one you have
- Measure the major diameter with calipers and find the size (a numbered size, or a fraction).
- Count the threads per inch, or match a pitch gauge leaf with no light between the teeth.
- Compare the count with the table above or the full chart.
Test a nut by hand. If it spins on for a turn or two and then binds, stop; it is the wrong series, and forcing it will damage the threads on both parts. Metric threads follow the same idea: the coarse pitch is the default (M8 x 1.25) and fine pitches are marked in the callout (M8 x 1).
For the full inch list, see the machine screw size chart, or start from the screw size chart hub if you are not sure which family you have.
The series are defined in ASME B1.1. Tap drill sizes follow the standard 75% thread chart. Sources and method explains how this site checks its figures.
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