Standard Screw Sizes, Choosing the Right Size Made Easier
By Joan Whetzel
The dimensions, or sizes, of screws include the type of screwdriver to be used with it (e.g. Phillips, hex or flat blade), as well as the length of the screw, the diameter of the shank, and the thread pitch. Worldwide, the use of screws and other fasteners (nuts, bolts, nails) has grown significantly, which has increased the need to standardize screw sizes and fastener sizing in general. Standardized screw sizes avoids the use of wrong sized screws because the right size screw was not available. Standardization prevents the use of screws that are incompatible with the materials being used, or screws that don't match the available bolt sizes..
Standardized US and Metric Numbering System
Screws come in two standardized numbering systems - US and metric. The diameter measurements of metric screws are listed in millimeters (the first number) and thread pitch (the second number). So a 4mm x 0.7 screw has a shaft diameter of 4 millimeters and a thread pitch of 0.7 threads per millimeter. US screw diameters are listed by an arbitrary numbering system for smaller screws and fractions of an inch for larger screws.. The US diameter numbers are 0, 1, 2 ,3, 4, 5, 6, 7, 8, 9, 10, 12, 14 and 16 (smallest screws) and fractional sizes 1/4 inch to 2 inches (in increments of 1/16 inch). The second number of the screw size lists the thread pitch in 6 to 80 threads per inch, and the third number lists the length of the shaft. Example 1/4 - 30 x 1/2, which indicates a screw with a shaft that is 1/4 inch in diameter, has 30 threads per inch, and is 1/2 inch long.
The screw diameter is the first number listed on both standard screw sizing charts. The screw's diameter is measured just underneath the screw head. The screw's diameter must mach the inner diameter of the nuts and washers that it is paired with. For the US screws, washers and nuts, the standardized sizing is determined and set by the American Society of Mechanical Engineers (ASME) using hardware sizing systems. Increases in the size numbers (0 through 16) correspond to increases in the diameters of the screw shafts and screw heads. Shaft lengths and thread pitches vary depending on the job they will be used for.
A screw's thread pitch is indicated by the second number on both the US and metric sizing charts. Thread pitch on US screws are measured as threads per inch, and metric screws are listed as threads per millimeter. Thread pitch is separated from the shaft diameter size by a dash (1/4-35 x 1/2)
The shaft length of round head screws are measured starting at the flat bottom of the head down to the screw tip. Flat head screws, on the other hand, are measured form the top of the screw head down to the tip. The length of the screw shaft is the third number listed in screw measurement charts. The length measurement is separated from the other measurements by an "x" ((1/4-35 x 1/2 or #10-40 x 1/8).
Check out this machine screw size conversion chart;
CSG Network. Size Tables and Conversions.
These charts list screw sizes and the sizes of tools to be used with them:
Wiha Quality Tools. Typical Dimensional and Torque Specifications.
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W I Fuller. Wood Screw Recommendation Chart. http://www.wlfuller.com/html/wood_screw_chart.html
Bolt Depot. Standard US Machine Screw Sizes. Downloaded 2/19/2012.
Bolt Depot. Standard Metric Machine Screw Sizes. Downloaded 2/19/2012.
Thomas Net. Explaining Screw and Nail Sizes. Downloaded 2/19/2012.
In Stock Fasteners. Guide to Fastener Size. Downloaded 2/19/2012.
Hinge Dummy. Screw Sizing. Downloaded 2/19/2012.
Sizes: Fastener Screw Thread Systems. Downloaded 2/19/2012.
Votaw Tool. Standard and Metric Thread Sizes. Downloaded 2/19/2012.