The radius of any circle is the distance from the exact center of the circle to any edge. This can only be measured if the position of the center of the circle is known.
If the location of the center is not known, the radius can be easily calculated by measuring the diameter of the circle (distance across) and cutting it in half. This is shown by D (diameter) in the illustration. In the illustration A and B are half the length of D.
The first illustration shows Diameter D, and radius measurements A and B. A is the radius inside the circle. B shows the radius of the circle measured outside the circle.
Now, using the radius B approach, we can easily measure the radius of any rounded corner. With a hot tub cover, the position of the center of the circle cannot be known and the diameter can not be measured since we have only one quarter of the circle exposed.
The radius can be determined by measuring to a point OUTSIDE the radius of the corner, as shown. Illustration two shows a photograph of this to go along with the illustration.
If there it is unclear, remember the rule of thumb that a smaller radius dimension is safer than a larger dimension. If the measurement comes out between 6 inches and 7 inches lets say, go with the 6 inch dimension. The smaller the radius the more square the corner will seem.
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