If you know the radius: diameter = 2 × radius
If you know the circumference: diameter = circumference ÷ π
If you know the area: diameter = 2 × √(area ÷ π)
If you know the chord and sagitta: diameter = (chord² ÷ (8 × sagitta)) + (sagitta ÷ 2)
If you know the circle’s equation in standard form: diameter = 2 × √(radius²)
If you know the sphere’s volume: diameter = 2 × ∛((3 × volume) ÷ (4 × π))
If you know the sphere’s surface area: diameter = √(surface area ÷ π) × 2
If you know the rectangle’s diagonal and one side for a circle-like cross section: use the given dimensions to solve for the circle’s diameter
Use π ≈ 3.14159 when needed