How Is Bolt Size Measured?
Measure the length of a bolt from the underside of the bolt head to the end of the shank, and to measure diameter, measure the widest part of the shank of the bolt. Length and diameter can be measured in millimeters or inches, and they are the two principal dimensions of a bolt.
Length of a bolt can also be measured from the top of the head of a bolt to the end of the shank for certain types of bolts. These include oval head screws, flat head screws, elevator bolts and plow bolts. Any screw that sinks in to form a level surface when properly installed should be measured from the top of the head to the end of the fastener. All other bolts, including hex head cap screws, round head screws, carriage bolts and pan head screws should be measured from the underside of the bolt head. Diameter can be measured in three different ways. The most common way is to measure the widest part of the bolt shank. Bolt diameter can also be measured using thread diameter, shank diameter and root diameter. Thread diameter is measured using the outside of the bolt threads. Shank diameter is measured just under the bolt head before the threading begins. Root diameter is measured using the inside, or narrowest point, of the bolt shank.