You cannot simply convert metres into diameter, because diameter is a physical quantity of an object, where as a metre is merely a unit of physical quantity. Diameter is the length of the line that joins two points of a circle, while passing through the centre of the circle. A metre is the unit of length that the diameter of a circle is measured in. To convert the diameter into the circumference (distance around a circular body), you multiply the diameter by itself, multiply the answer by pi (a unit equating to roughly 3.14), then divide the result by four. Alternatively, if you use the radius (half the diameter - the distance from any point on a circle to its centre), you multiply the radius by itself, then multiply by pi.
