Determine flange and bolt dimensions by measuring several key areas of your flange and then correlating these measurements to a flange and bolt dimension chart. Key dimensions to measure for the flange itself are nominal pipe size and flange diameter. For the bolts, measure the bolt diameter, bolt hole diameter and total diameter of the entire bolt circle.
To determine the flange's nominal pipe size, measure the outer diameter of the flange's pipe as well as the pipe's wall thickness. Using this information, consult a nominal pipe size chart and correlate your findings with the chart to find the appropriate nominal pipe size. Determine the flange diameter by measuring the distance from one of the flange's outer walls to the outer wall on the opposite side.
Bolt diameter is the distance from one of the bolt's unthreaded outer walls to the other. The bolt hole diameter is the distance between inner walls of each bolt hole on the flange. Bolt hole diameter is usually very slightly larger than bolt diameter. To determine the bolt circle diameter, place one end of your measuring tool directly in the center of one of the flange's bolt holes, and measure the distance to the center of the bolt hole on the opposite side of the flange.