Measure the radius and multiply by 2
Measure the circumference and divide by π
Measure the area and use: diameter = 2 × √(area / π)
Measure the chord length if it passes through the center (the chord length is the diameter)
Measure two points on the circle and compute the distance between them if they are the endpoints of a diameter
If given the equation of a circle in standard form (x − h)² + (y − k)² = r², use diameter = 2r
If given the equation of a circle in general form x² + y² + Dx + Ey + F = 0, compute r from the center and constants, then use diameter = 2r