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Screw Gauge Lookup Calculator

Enter a US screw gauge number (0–24) to find its major diameter.

How it is calculated

A US screw gauge number maps to the major (outside thread) diameter by a straight line: diameter_in = 0.060 + 0.013 × gauge in inches. The metric value is diameter_mm = diameter_in × 25.4. Every gauge step therefore adds 0.013 in (about 0.33 mm), starting from 0.060 in for gauge 0.

The tool accepts gauges from 0 to 24 and returns nothing for negative values or numbers above 24. Inches are shown to three decimals and millimetres to two, and the millimetre figure is computed from the unrounded inch value. A decimal gauge such as 10.5 is calculated as given, but the echoed label rounds it to a whole number (#11), so use whole gauge numbers. The result is the nominal outside diameter of the thread, not the size of the hole or the head.

Worked example

You want the diameter of a #8 screw. Multiply the gauge by the step: 0.013 × 8 = 0.104. Add the base: 0.060 + 0.104 = 0.164 in. In millimetres, 0.164 × 25.4 = 4.1656, which the tool shows as 4.17 mm. For comparison, a #10 screw comes out at 0.190 in, or 4.83 mm, so each gauge number is about a third of a millimetre larger than the one below.

Try this example →

Common mistakes

  • Taking the gauge number as a diameter in millimetres or sixteenths of an inch. A #8 screw is 0.164 in (4.17 mm) across, not 8 mm.
  • Treating the result as the drill size. It is the major thread diameter; a pilot or clearance hole needs a separate chart and is smaller or slightly larger.
  • Confusing screw gauge with wire or sheet-metal gauge. Those systems run the opposite way, where a bigger number means thinner material. For screws a bigger number is a thicker screw.
  • Entering a metric size like M4 as gauge 4. M4 means a 4 mm diameter, while #4 is 0.112 in (2.84 mm).
  • Entering a fractional gauge. Only whole numbers 0 to 24 are real gauge sizes; 10.5 is calculated but labelled #11.

About this calculator

US screw sizes below 1/4 inch are labeled with a gauge number (like #4, #8, #10) rather than a fraction of an inch, a legacy numbering system that isn't self-evident from the number alone. The major (outer thread) diameter follows a simple linear formula: diameter = 0.060 + 0.013 × gauge inches.

This lookup is a practical reference for machinists, woodworkers, and DIYers who need to match a drill bit or nut to a screw's actual size, or convert an American gauge number into millimeters when working alongside metric hardware — a common point of confusion since the gauge number itself gives no direct sense of physical size.

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Comments (1)

  • Editorial team

    The result is the nominal thread diameter, so use it to choose the hole, not as the hole itself. A #8 at 4.17 mm will not pass through a 4 mm hole, so a clearance hole has to be larger. For a metric substitute, #8 sits near M4, but the thread pitches differ, so they are not interchangeable. Which metric equivalents would you like to see beside each gauge?

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