Screw Sizes & Thread Types


There are literally thousands of different sizes of screws, but just a few thread types. The shank of a screw may either be tapered or straight, depending on what it is designed to be used for. Screws give a far more stable joint between two materials than nails do, and the type or name of screw usually depicts the material on which it is to be used.

Wood Screws

  • You can find wood screws at all hardware retail outlets. The full range of sizes starts at 0, which corresponds to a shaft diameter of 0.06 inch, and goes up to 24, or 0.372 inch. A wood screw has a tapered shaft with a short untapered section immediately below the head. It usually has a deep thread to enable it to get a good grip onto the wood.

Sheet Metal or Self -Tapping Screws

  • Sheet metal or self-tapping screws come in the same sizes as wood screws, namely 0 through 24. They have tapered shafts with threads extending all the way to the heads. These screws have a sharp thread that enables them to cut their own way into the substrate. This is usually metal but can be plastic or other similar materials.

Machine Screws

  • The distinguishing feature of machine screws is that they have a straight shaft and a much finer thread than wood screws or self-tapping screws. They are designed to be screwed into a bolt or into a pre-tapped hole, usually in metal. Machine screws can be found in the standard sizes of 0 to 24, but the most common are sizes 6 (0.138 inch), 8 (0.164 inch) and 10 (0.190 inch) diameter.

Concrete or Masonry Screws

  • Concrete screws, sometimes known as masonry screws, are self-tapping screws that are designed to be screwed into pre-drilled pilot holes in concrete. They have a shaft diameter of at least 0.325 inch and each has a hexagonal head so it can be tightened with a wrench. They are used for attaching machinery or similar items to a concrete base. Some concrete screws come with an attached anchor that expands as the screw is tightened, thus giving a stronger bond.

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  • Photo Credit screw image by Henryk Olszewski from wood screws image by Darko Draskovic from screws image by Eldin Muratovic from slotted machine screws image by Tom Oliveira from bolt image by Grebenshchikov from
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