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Thread pitch chart for UNC, UNF and metric bolts

A 1/2 in bolt is 13 threads per inch coarse (UNC) or 20 fine (UNF). An M12 is 1.75 mm coarse or 1.25 mm fine. The fine thread has the larger stress area, about 13% more at 1/2 in, so it carries more clamp load at the same stress.

By the Torque Reference editors 3 sources cited Last reviewed Method
Contents7 sections

Key takeaways

  • Inch threads are counted in threads per inch (TPI). Metric threads are measured as pitch, the distance between crests in mm.
  • Fine threads have 5% to 15% more stress area than coarse threads in the same size.
  • Some inch and metric pairs are close enough to confuse: 5/16-18 and M8 x 1.25, 3/8-16 and M10 x 1.5.
  • Stress areas follow ASME B1.1 (inch) and ISO 898-1 (metric) ⁠1 ⁠2.

Unified inch threads (UNC / UNF)

SizeUNC, TPIUNF, TPIStress area (UNC), in²
1/4 in20280.0318
5/16 in18240.0525
3/8 in16240.0775
7/16 in14200.1060
1/2 in13200.1420
9/16 in12180.1820
5/8 in11180.2260
3/4 in10160.3350
7/8 in9140.4620
1 in8120.6060

ASME B1.1 values as tabulated by Fastenal⁠1.

ISO metric threads

SizeCoarse pitch, mmCommon fine pitch, mmStress area (coarse), mm²
M30.50.355.0
M40.70.58.8
M50.80.514.2
M610.7520.1
M81.25136.6
M101.51.2558.0
M121.751.2584.3
M1421.5115.0
M1621.5157.0
M202.51.5245.0
M2432353.0
M303.52561.0

Coarse stress areas from ISO 898-1 Table 4⁠2; fine pitches as tabulated by Fastenal⁠1.

How to measure TPI or pitch

  1. Measure the diameter

    Put a caliper across the thread crests. A 1/2 in bolt measures close to 12.7 mm, and an M12 close to 12 mm. That alone usually tells inch from metric.

  2. Try a thread gauge

    Press gauge leaves into the thread until one sits with no light showing between the teeth. Inch gauges are stamped in TPI and metric gauges in mm pitch.

  3. Count if there is no gauge

    For inch bolts, count crests over 1 in of length. For metric, measure across 10 crests and divide by 10.

Watch the near pairs. 5/16-18 has a 1.41 mm pitch against 1.25 mm for M8, and the diameters are 7.94 mm and 8 mm. 3/8-16 has a 1.59 mm pitch against 1.5 mm for M10, at 9.53 mm and 10 mm. Over a few threads, a gauge can seem to fit both. Check along at least 1/2 in of thread.

UNC vs UNF

UNC (coarse) and UNF (fine) are the two main Unified inch series. The size pages calculate torque for both. A finer pitch leaves more metal at the thread root, so the tensile stress area is larger. On the same proof-load basis, that means more clamp load and more torque:

  • Inch: UNF has 9.4% more area at 1 in (1-12 vs 1-8) and 14.5% more at 1/4 in (1/4-28 vs 1/4-20).
  • Metric: the common fine pitch has 5.5% more at M10 x 1.25 and 13.4% more at M5 x 0.5.

The 1 in size also comes in 1-14 UNS, a pitch ASME B1.1 lists outside UNF ⁠1.

ISO vs other metric references

The coarse pitches here match ISO 898-1 Tables 4 and 5 ⁠2. Some fine pitches have a second common choice. M12 comes as 1.25 or 1.5, and M10 as 1.25 or 1. These come from the ISO thread tables as reproduced on Wikipedia ⁠3. Check the pitch before choosing a fine-thread value.

Sources

  1. 1
  2. 2
  3. 3

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