One of the difficult things for many people, is trying to measure the radius of a rounded corner. There are numerous ways one my go about figuring this measurement, but here are the easiest we have found so far.
If your existing cover fits properly, set it upside-down on top of a piece of newspaper. Line up the edges of the newspaper with the edges of the cover. The distance from the corner of the newspaper, to where the cover completely covers the paper is the radius of the corner.
You can also use a framing square to measure the radius of your spa corners. Set the square against the edge of the spa as shown below. The radius of the rounded corner is the distance from the inside corner of the square, to where the cover just touches the square.
The diagram below shows the pitfalls with miss-measuring a rounded corner. The green line represents a measurement of too large a radius, while the red line represents a measurement too small. As you can see, with too large a radius the cover wont completely cover the spa. Too small a radius and the cover simply overhangs a bit. If in doubt always go with a slightly smaller measurement. The cover may be slightly large and overhang the corner a bit, but it will cover the entire spa…… and no one but you will ever know.