If r is given ...then D is twice of radius.
2 x r = D
Where r is radius of the circle,
and D is the Diameter of the circle...
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great answer bro
thank u ...dude'
welcome bro

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It is the longest chord of the circle which passes through the center of the circle .
you can measure the straight line distance from the center to any point on the circumference of the circle and then you can double the value to get the distance of the diameter. 
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