How To Measure Thread Size Of A Nut. Nuts are measured by the width of the hex shape and by their thread pitch. Both nuts and washers are sized by the fastener they fit.
Open the caliper jaws far enough to allow the nut to fit between them. If you don't know the pitch, choose coarse threads; Find figure nearest this dimension in column 1 or 2 of chart.
Identify threads per inch on external threads and find the center of your workpiece to line up thread cutting tools.
Size is stamped on nut. Measure the inverted size with the tube outer diameter portion. Metric thread sizes are specified by diameter and pitch, which is the space between threads. Finding the thread size with a tape measure to determine the size of pipe fitting thread you need, first use a tape measure or scale to measure the inside diameter (id) of the threaded hole were you would like to install the fitting or the outside diameter (od) of your existing fitting as shown below.