Regardless of the shape your spa, you should measure from lip to lip, or the outside dimension of the acrylic which is the same as lip to lip. After you have a lip to lip measurement you may add up to 1" on both length and width. This will give you a 1/2" of play all around your spa cover.
The skirt is the flap that hangs from the bottom of your cover. The skirt is really just cosmetic but if not measured correctly can cause some problems because if it is too long it will bunch and push your spa cover up. If you have a wood rail measure from the top of the wood rail to the top of your spa acrylic. If anything you want to be a little short rather than long so the skirt does not bunch. If you do not have a wood rail then you want to measure from the top of the spa to the bottom of the spa lip and add 1/2".
As the photo shows the easiest way to measure a radius is with a carpenter square. The dimension needed is from the start of the bend to the imaginary point of the square if the tub was actually square. Rule of thumb is smaller is safer than larger. The smaller the radius the more square the corner of the hot tub cover.
The square hot tub cover is the easiest cover to measure, just measure your outside lip to lip, and add 1" to each dimension. The only other dimension you will need is the skirt length.
The same as the square spa cover, but make sure you request the fold direction you require. Most companies will by default cut the longest of the two sides in half.
Square/Rectangle with Radius Corners
Follow the square and rectangle guide to measure hot tub covers and spa cover and add the radius measurement.
For the round spa cover you only need the lip to lip measurement, if your old cover is measurable just measure across the seam. Remember to add 1" of play. The only other measurement needed is the skirt length.
For the octagon spa cover measure from lip to lip add 1" of play. Also, measure one panel of the octagon from point to point. The only other measurement needed is the skirt length.
Spa cover colors come in Black, Brown, Tan, Sand, Medium Grey, Light Grey, Hunter Green, Light Blue, Navy, Burgundy and Redwood.