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 = 2 × square root of (area ÷ π)
If you know the area, divide it by π, take the square root, then multiply by 2
Measure straight across the circle through the center if using a ruler or tape measure