R (Corner Radius): Corners labeled "R" in the diagram may or may not be rounded. If the corners are rounded, hold a carpenters square to the corner of your existing cover or hot tub. Measure from the inside corner of the square - to where the square first touches the cover or hot tub.
It is important your cover isn't too small, but is fine to be a little big, so always round DOWN to the nearest inch when determining the radius. A smaller corner radius will mean less of the corner will be cut off and will result in a larger cover.

Skirt Length: Measure the skirt from the bottom of your cover to the end of your skirt (as pictured in the diagram). If you don't have an existing cover to measure, note that the skirt is meant to cover the acrylic lip on your spa, so you can measure the width of your spa lip. (Measure in inches)

Straps Length: Measure from the bottom of the skirt to the end of the locking clip (include the clip in your measurement). Your new cover will come with the male and female sides of the locking clip. (Measure in inches)