Sure, you can use a calculator or even your head to find the circumference of a round duct by measuring its diameter, but with the Cooper Tool Tinner’s Circumference Rule all you need to measure is the diameter — the ruler will read the circumference.
The tempered medium-weight steel rule measures in inches by 1/16″ on the top edge and circumference inches by 1/8″ on the bottom edge. The black markings are easy to read, and on the reverse side Cooper has printed relevant formulas and tables for easy reference.
The rulers come in 3′ and 4′ lengths — both rulers measure 1-1/4″ wide and 1/16″ thick. At $60 on the bottom end these circumference rulers aren’t cheap, so practicing your arithmetic might be a little more cost effective.