There are several methods to measure the corner radius. The easiest is to place a framing square against the corner of the spa. Read the number where the acrylic touches the framing square. This measurement is the radius.
See figure 1.

When measuring a radius you want to round down to a slightly smaller size. If a cover is made with too large of a radius, it will not fit. If the radius is slightly smaller, the cover will fit and insulate just as well. The radius measurement may also be obtained by using 2 yard sticks or any straight edge and a tape measure.
See Figure 2

You can also download our corner radius chart by clicking here. If the file does not start to download, right click on it and select "save as". You can then print this page and hold it up to your spa to tell you what the radius is. Make sure when you print this not to shrink or expand it at all.

Measuring Radius with a Framing Square Measureing Radius with a Leval and Tape Measure