What this means is that a screw considered to be 8 mm long may actually measure 7.6mm or 8.0mm. It may be measured from under the head or the overall length. The actual diameter of an M3 screw is usually about 2.9mm, an M2.5 screw is 2.4mm and an M2 is 1.9mm. So for hex-, pan-, button-, round-, and truss-head screws , length is measured from directly under the head to the tip. One exception: an oval-head screw , which can be partially countersunk, is measured from the widest point of the head to the tip. Dec 30, 2006 The length of most screws is measured from the bottom of the head, as shown. Note, however, that a flathead screw , designed to sit flush in a countersunk material, is measured to the top of the head. For unified threads, the length is given in inches.

6 is a #6-size screw . Screw gauges are a measure of the head size and shaft size, and are roughly linear but not quite a 1:1 relationship (a #8 screw is a little less than twice the diameter of a #4 screw ). Screw size is measured in diameter. For example a 1/4-20 screw has 20 threads per inch and a diameter of 1/4 inch. All you need is a steel rule to determine the size and thread count of any screw . How to Measure Fastener Length Fastener length is measured from where the material surface is assumed to be, to the end of the fastener. Round head screws are measured from under the head. Machine Screws Machine screw measurement depends on the style of head it has.

Diameter, thread pitch, and length. Examples, 1/4-20 x 2 : 1/4 in diameter, 20 threads per inch of length, 2 long not including head usually. In metric might be something like 8 x 1.25 x 40. 8 mm in diameter, threads are 1.25 mm apart, Bolt thread size is a the measure of the small threads that run in a spiral along the exterior of a bolt or other attachment device. Getting the correct measurements is critical for purchasing pieces for a construction project or to replace attachments in a preexisting mechanism. How to Measure Bolt Thread Size. Length of Screws / Bolts. The simplest way to remember how to measure length: Length is the distance from the flat part of the head, to the tip or blunt end of the screw or bolt. Almost all screw head types will either be flat on the top or on the bottom.

US Sheet Metal Screw Diameter Table; US Wood Screw Diameter Table; Specific Exceptions. Some fasteners such as shoulder bolts, and many anchors use a different mesurement for diameter. Throughout our catalog you will find images showing exactly where the diameter is measured on each fastener when you are choosing a diameter.