Use the formula: diameter = 2 × radius
If you know the radius, multiply it by 2
Use the formula: diameter = circumference ÷ π
If you know the circumference, divide it by 3.14159
Use the formula: diameter = √(4 × area ÷ π)
If you know the area, multiply it by 4, divide by π, then take the square root
Measure straight across the circle through its center
Use a ruler, tape measure, or caliper for direct measurement