To calculate the diameter of a circle, multiply the radius by 2. If you don't have the radius, divide the circumference of the circle by π to get the diameter. If you don't have the radius or the circumference, divide the area of the circle by π and then find that number's square root to get the radius.
2 x radius

Just to clarify Baltuilhe's post- Draw a line from the center of the circle to the midpoint of the chord and another from the center of the circle to an endpoint of the chord. It is simple geometry that the line to the midpoint of the chord is perpendicular to the chord so we have a right triangle with the radius of the circle, R, as hypotenuse, the distance from the center, D, as the length of one leg, and half the length of the chord, C/2, as the length of the other leg. Then the Pythagorean Theorem gives (D)^2+ (C/2)^2= R^2.

Good afternoon!!

If Radius of the circle R, Chord measure is C and distance from the center of the circle to the chord is D:
[tex]R^2=\left(\dfrac{C}{2}\right)^2+D^2\\\\\boxed{R=\sqrt{\left(\dfrac{C}{2}\right)^2+D^2}}[/tex]

Knowing R, you have the diameter.

I hope I have helped!

How to find the diameter of a circle, knowing the chord and the distance from the center of the circle to the chord?