Lay a piece of black construction paper on a table in front of you.
Dip a bubble wand in the bubble solution. When you raise the wand out of the solution, there should be a film of soapy water stretched across the hoop on the end.
Blow through the hoop gently while pointing the hoop at the construction paper. Blow slowly for larger bubbles. When the bubbles hit the construction paper, they will burst, leaving behind a damp bubble print.
Measure across the bubble print with a tape measure. This is the diameter. To find the radius, divide the diameter by 2. To find the circumference, multiply the diameter by pi.