I've never been particularly strong at math, and it does seem to be a requirement in knitting. I'm constantly looking up mathematical formulas and equivalents when I'm designing.
circumference of a circle = pi (3.1416) x diameter (which is radius x 2)
area of a circle = 3.1416 (pi) x radius squared.
(If radius=2.5 formula would be 3.1416 x (2.5 x 2.5)= 3.1416 x 6.25 = 19.635