How To Measure Thread Size Diameter. Spread the ruler across the diameter of the pipe and measure. Both the tpi (threads per inch) and od (outside diameter) of the thread are required for positive identification of thread size because several sizes have the same tpi.
Find figure nearest this dimension in column 1 or 2 of chart. Note the reading of the caliper. Metric thread sizes are specified by diameter and pitch, which is the space between threads.
If you can slip a 5/8″ dowel into the hose end snug then you’ll know its size.
Generally speaking, the larger the diameter the more expensive the filter will be. Cylinder screws, the size is specified as an iso metric, where the number begins with “m”. For example, a bolt and nut may be described as being ½” diameter. Thread has 14 threads per inch.