1/4-20 UNC thread pitch and dimensions
The 1/4-20 UNC thread has 20 threads per inch, a pitch of 0.0500 in (1.27 mm), on a 0.250 in major diameter (ASME B1.1). Its basic pitch diameter is 0.2175 in and its basic minor diameter is 0.1959 in.
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| Dimension | Formula | inch | mm |
|---|---|---|---|
| Major diameter D | nominal | 0.2500 | 6.35 |
| Pitch P | 1 / TPI | 0.0500 | 1.27 |
| Pitch diameter E | D − 0.649519 / n | 0.2175 | 5.53 |
| Minor diameter (internal) | D − 1.082532 / n | 0.1959 | 4.98 |
| Thread height (basic) | 0.541266 / n | 0.0271 | 0.69 |
| Tensile stress area As | 0.7854 (D − 0.9743 / n)² | 0.0318 in² | 20.5 mm² |
| Thread | TPI | Pitch Ø E | Stress area As | Tap drill |
|---|---|---|---|---|
| 1/4-20 UNC (this page) | 20 | 0.2175 in | 0.0318 in² | #7 |
| 1/4-28 UNF | 28 | 0.2268 in | 0.0364 in² | #3 |
| Thread | TPI | Pitch (mm) | Minor Ø | Stress area As |
|---|---|---|---|---|
| #8-32 UNC | 32 | 0.79 | 0.1302 in | 0.0140 in² |
| #10-24 UNC | 24 | 1.06 | 0.1449 in | 0.0175 in² |
| 1/4-20 UNC (this page) | 20 | 1.27 | 0.1959 in | 0.0318 in² |
| 5/16-18 UNC | 18 | 1.41 | 0.2524 in | 0.0524 in² |
| 3/8-16 UNC | 16 | 1.59 | 0.3073 in | 0.0775 in² |
Practical notes
Unified threads are named diameter–TPI–series: 1/4-20 UNC is a 1/4 nominal diameter with 20 threads per inch in the coarse series.
These are basic (zero-tolerance) dimensions. Most screws are made to class 2A and nuts to 2B (ASME B1.1), so measured sizes sit slightly below the basic values on the screw.
20 TPI is a pitch of 1.27 mm, which does not match a metric ISO pitch exactly; metric and Unified fasteners of similar size are not interchangeable.
Tensile stress area As is the area used for bolt strength (proof load = As × proof stress for the SAE J429 or ASTM grade). For 1/4-20 UNC it is 0.0318 in².
FAQ
How many threads per inch is 1/4-20 UNC?
20 TPI, a pitch of 0.0500 in (1.27 mm), in the Unified coarse series (ASME B1.1).
What is the minor diameter of 1/4-20 UNC?
The basic minor diameter of the internal thread is D − 1.082532/n = 0.1959 in (4.98 mm).
What is the tensile stress area of 1/4-20 UNC?
As = 0.7854 × (D − 0.9743/n)² = 0.0318 in² (20.5 mm²).
What tap drill does 1/4-20 UNC need?
The standard chart drill is #7, 0.2010 in (5.11 mm), about 75% thread.
What is the difference between 1/4-20 UNC and 1/4-28 UNF?
Same 1/4 diameter, different pitch: 20 TPI versus 28 TPI. They are not interchangeable.
Sources: ASME B1.1 — Unified inch screw threads (UN and UNR thread form): basic dimensions, tensile stress area · ASME B18.2.1 — square, hex, heavy hex and askew head bolts and hex, heavy hex, hex flange screws (width across flats)
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