how do make the radius of the circle responsive in canvas element?

I have a canvas element in which i have drawn a circle. How do i give the responsiveness to the canvas element. If the width of the browser changes i need to change some amount of radius, if the height of the browser changes i need to change the some amount in radius. if both both changes i need to changes both in some amount. For that how i need to calculate the changing ratio?

        var canvasWidthRatio = currentDivWidth / initialDivWidth;
        var canvasHeightRatio = currentHeight / initialDivHeight;
        radius=(some thing i need here).

please help me...