Unified Inch Thread Table (UNC and UNF)

Inch threads from #4 to 1 inch: threads per inch, diameters, stress area and tap drill.

Unified threads are the inch screw threads used throughout North American hardware. They are written as the size, the number of threads per inch and the series: 1/4-20 UNC is a quarter-inch thread with 20 threads per inch in the coarse series; 1/4-28 UNF is the fine series of the same size. Sizes below 1/4 inch are numbered, from #0 up: #10 is 0.190 inch.

The table gives both series from #4 to 1 inch, with the pitch, the pitch and minor diameters, the tensile stress area and an approximate tap drill.

The table

28 rows
ThreadMajor diameter (in)Threads per inchPitch (in)Pitch diameter (in)Minor diameter, external (in)Tensile stress area (in2)Approx. tap drill, 75 percent thread: d - 1/n (in)
#4-40 UNC0.112400.0250.09580.08490.006030.087
#4-48 UNF0.112480.020830.09850.08940.00660.0912
#6-32 UNC0.138320.031250.11770.10420.009090.1068
#6-40 UNF0.138400.0250.12180.11090.010140.113
#8-32 UNC0.164320.031250.14370.13020.014010.1328
#8-36 UNF0.164360.027780.1460.13390.014730.1362
#10-24 UNC0.19240.041670.16290.14490.017530.1483
#10-32 UNF0.19320.031250.16970.15620.019990.1588
1/4-20 UNC0.25200.050.21750.19590.031820.2
1/4-28 UNF0.25280.035710.22680.21130.036370.2143
5/16-18 UNC0.3125180.055560.27640.25240.052430.2569
5/16-24 UNF0.3125240.041670.28540.26740.058070.2708
3/8-16 UNC0.375160.06250.33440.30730.077490.3125
3/8-24 UNF0.375240.041670.34790.32990.087830.3333
7/16-14 UNC0.4375140.071430.39110.36020.106310.3661
7/16-20 UNF0.4375200.050.4050.38340.118720.3875
1/2-13 UNC0.5130.076920.450.41670.14190.4231
1/2-20 UNF0.5200.050.46750.44590.159950.45
9/16-12 UNC0.5625120.083330.50840.47230.181940.4792
9/16-18 UNF0.5625180.055560.52640.50240.202980.5069
5/8-11 UNC0.625110.090910.5660.52660.2260.5341
5/8-18 UNF0.625180.055560.58890.56490.255960.5694
3/4-10 UNC0.75100.10.68510.64180.334460.65
3/4-16 UNF0.75160.06250.70940.68230.372960.6875
7/8-9 UNC0.87590.111110.80280.75470.461730.7639
7/8-14 UNF0.875140.071430.82860.79770.509470.8036
1-8 UNC180.1250.91880.86470.605750.875
1-12 UNF1120.083330.94590.90980.663040.9167

Source and notes: ASME B1.1 (Unified inch threads): d2 = d - 0.6495/n, d3 = d - 1.0825/n, At = 0.7854 (d - 0.9743/n)2. Tap drill: choose the nearest stock drill.

How to use this table

  1. Find the size and series in the first column.
  2. Pick the tap drill from the last column (the nearest stock drill).
  3. Use the tensile stress area (in²) with a stress in psi to get a load in pounds.

Worked example: 1/4-20 UNC against 1/4-28 UNF

  • Coarse 1/4-20: 20 threads per inch, pitch 0.0500 in, stress area 0.0318 in².
  • Fine 1/4-28: 28 threads per inch, pitch 0.0357 in, stress area 0.0364 in².
  • A Grade 5 bolt has a proof strength of 85 000 psi (SAE J429), so the coarse bolt holds 2 705 lbf and the fine bolt 3 091 lbf.

Notes and cautions

  • Thread classes (2A and 2B for general use, 3A and 3B for close fits) change the tolerance, not the nominal sizes in this table.
  • Pipe threads (NPT) are tapered and are a different family; do not use this table for them.

FAQ

What does 1/4-20 UNC mean?

A thread of 1/4 inch nominal diameter with 20 threads per inch, in the Unified National Coarse series.

What do the numbered sizes mean?

Sizes below 1/4 inch are numbered. The major diameter in inches is 0.060 + 0.013 × the number, so #10 is 0.190 inch.

Which series should I use?

Coarse (UNC) for general use and for soft materials such as aluminum and cast iron. Fine (UNF) where you need more strength per diameter, a fine adjustment, or better resistance to vibration.

Sources

  • ASME B1.1, Unified Inch Screw Threads (UN and UNR Thread Form).
  • SAE J429, Mechanical and Material Requirements for Externally Threaded Fasteners.
  • ASME B1.1 (Unified inch threads): d2 = d – 0.6495/n, d3 = d – 1.0825/n, At = 0.7854 (d – 0.9743/n)2. Tap drill: choose the nearest stock drill.