how to describe a wood screw

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In the United States, a wood screw is sized by its

Stringing all these parameters together, one might buy, for example, a box of one hundred 3½ inch #18 flat head Phillips wood screws. 

Length

drawing showing how to measure lengths of wood screws

The length of a wood screw is measured from the tip of the point to the surface of the material into which the screw is driven, which is the head's widest part. So the length of a flat head wood screw is measured from the tip to the top of the head, but the length of a round head screw is measured from the tip to the bottom of the head. 

In the common wood screw the thread extends 2/3rds of the length of the screw. Some other types (such as sheet metal and dry wall screws) are threaded all the way to the head.

Gauge

For many years American wood screws were made to two different standards, one a series that originated with the American Screw Co., and the other from the Asa I. Cook Co. In both series, sizes were indicated by gauge numbers, which went up to #30. (see historical table) The overall dimensions in the two series were the same, but the number of threads differed slightly. When an American Standard for wood screws was adopted, most of the diameters were within a few thousandths of an inch of the old dimensions, but the sizes extended only to #24. For the current body diameters of the various gauges, see the pilot holes table.

Head style

Usually flat-head, round or oval.

Type of drive

Usually slotted or Phillips, but for a fuller discussion see screw drive systems.

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