Originally Posted by Wilfred Laurier

the easiest and most accurate way to figure out your wheel circumference
is a roll out test

put the bike with the valve on the front wheel at the bottom of the rotation
and put a mark on the ground beside it
then roll the bike forward until the valve is at the bottom again and mark that point
then measure between the two marks

this is most accurate if you are sitting on the bike during this test
but i dont think it makes a noticeable difference