What is the circumference of 12.45cm diameter?

Remember the circle equation.

C = pi d

In Words, 'Circumference equals the constant 'pi' multiplied to a diameter'.

For circles of ANY size, pi = 3.141592.... ~ 3.14

NB Pi is an irrational number and the decimal recur to infinity. However, it is approximated to 3.14 for learning/school purposes.

Hence for a diameter of 12.45 cm

C = 3.14 X 12.45cm

C = 39.093 cm