Machining and Fastener Charts
UNC and UNF thread chart.
Pick an inch screw size and coarse (UNC) or fine (UNF). You get threads per inch, pitch, pitch diameter, minor diameter and tensile stress area. The full chart is below. Runs in your browser; nothing is sent or stored.
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Result
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Answer first. 1/4-20 means a 1/4 inch screw with 20 threads per inch (UNC, coarse). 1/4-28 is the fine (UNF) version. Pitch is 1 divided by threads per inch.
Worked example
1/4-20 UNC: pitch = 1/20 = 0.050 in; pitch diameter = 0.250 - 0.649519 x 0.050 = 0.2175 in; minor diameter of the internal thread = 0.250 - 1.082532 x 0.050 = 0.1959 in; tensile stress area = pi/4 x (0.250 - 0.9743/20)² = 0.0318 in².
How it works
Numbered sizes. The major diameter of a numbered screw is 0.060 in plus 0.013 in for each size number, so a #10 is 0.060 + 10 x 0.013 = 0.190 in. Fractional sizes are the fraction of an inch.
UNC and UNF. For the same diameter, a UNF thread has more threads per inch than a UNC thread. For the same diameter a fine thread has a slightly larger tensile stress area. The chart lists the standard UNC and UNF counts; some sizes (#0) have only a fine thread.
Stress area. The tensile stress area uses the factor 0.9743 for unified threads. Multiply it by a grade's proof or tensile strength for a load; the bolt torque calculator does this.
Note. These are basic dimensions. A 1 inch UNF is listed here with 12 threads per inch, although 14 per inch is also widely used in practice. Check the thread standard for limits and classes.
UNC and UNF thread chart (#0 to 1 in)
| Size | Major dia. (in) | UNC TPI | UNF TPI | Minor dia., UNC (in) | Stress area, UNC (in²) |
|---|---|---|---|---|---|
| #0 | 0.0600 | – | 80 | – | – |
| #1 | 0.0730 | 64 | 72 | 0.0561 | 0.0026 |
| #2 | 0.0860 | 56 | 64 | 0.0667 | 0.0037 |
| #3 | 0.0990 | 48 | 56 | 0.0764 | 0.0049 |
| #4 | 0.1120 | 40 | 48 | 0.0849 | 0.0060 |
| #5 | 0.1250 | 40 | 44 | 0.0979 | 0.0080 |
| #6 | 0.1380 | 32 | 40 | 0.1042 | 0.0091 |
| #8 | 0.1640 | 32 | 36 | 0.1302 | 0.0140 |
| #10 | 0.1900 | 24 | 32 | 0.1449 | 0.0175 |
| #12 | 0.2160 | 24 | 28 | 0.1709 | 0.0242 |
| 1/4 | 0.2500 | 20 | 28 | 0.1959 | 0.0318 |
| 5/16 | 0.3125 | 18 | 24 | 0.2524 | 0.0524 |
| 3/8 | 0.3750 | 16 | 24 | 0.3073 | 0.0775 |
| 7/16 | 0.4375 | 14 | 20 | 0.3602 | 0.1063 |
| 1/2 | 0.5000 | 13 | 20 | 0.4167 | 0.1419 |
| 9/16 | 0.5625 | 12 | 18 | 0.4723 | 0.1819 |
| 5/8 | 0.6250 | 11 | 18 | 0.5266 | 0.2260 |
| 3/4 | 0.7500 | 10 | 16 | 0.6417 | 0.3345 |
| 7/8 | 0.8750 | 9 | 14 | 0.7547 | 0.4617 |
| 1 | 1.0000 | 8 | 12 | 0.8647 | 0.6057 |
Sources
- Unified Thread Standard - Wikipedia (threads per inch, numbered sizes, profile formulas) (opened 2026-10-16)
- Threaded Bolts - Stress Area - Engineering ToolBox (unified stress area formula) (opened 2026-10-16)
Threads per inch and the profile formulas are from the first page; the stress area factor 0.9743 is from the second.
FAQ
How many threads per inch is 1/4-20?
1/4-20 has 20 threads per inch, so the pitch is 0.050 in. It is the UNC (coarse) thread for a 1/4 inch screw.
What is the difference between UNC and UNF?
Both are unified inch threads. For a given diameter, UNF has more threads per inch (finer pitch) and UNC has fewer (coarser pitch). 1/4-20 is UNC and 1/4-28 is UNF.
What is the major diameter of a #10 screw?
A #10 screw has a major diameter of 0.190 in: 0.060 in plus 10 times 0.013 in.
What is the difference between 10-24 and 10-32?
Both are #10 screws with a 0.190 in major diameter. 10-24 is UNC (24 threads per inch); 10-32 is UNF (32 threads per inch).
Which tap drill goes with these threads?
Use the tap drill calculator: it subtracts the pitch from the major diameter, or applies the 85 or 90 percent rule.