Use the formula C = πd to find the circumference if you know the diameter. In this equation, "C" represents the circumference of the circle, and "d" represents its diameter. That is to say, you can find the circumference of a circle just by multiplying the diameter by pi.
https://www.wikihow.com/Calculate-the-Circumference-of-a-Circle
How to find the circumference of a circle: The circumference of a circle can be found by multiplying pi ( π = 3.14 ) by the diameter of the circle. If a circle has a diameter of 4, its circumference is 3.14*4=12.56. If you know the radius, the diameter is twice as large.
https://en.m.wikipedia.org/wiki/Pi
How do you find the radius when you have the circumference? Divide the circumference by π, or 3.14 for an estimation. The result is the circle’s diameter. Divide the diameter by 2. There you go, you found the circle’s radius.
https://www.mvorganizing.org/how-do-you-find-the-radius-when-you-have-the-circumference/
To find the radius of a circle with a circumference of 10 centimeters, you have to do the following: Divide the circumference by π, or 3.14 for an estimation. The result is the circle's diameter, 3.18 centimeters. Divide the diameter by 2. And there you go, the radius of a circle with a circumference of 10 centimeters is 1.59 centimeters.
https://www.reference.com/world-view/formula-circumference-circle-7e508134800bdd34
pdf for "find diameter of circle from circumference".(Page 1 of about 18 results)