Should I measure just the spa (acrylic), or should I include the wood rim?
A round or circular tub is one of the easiest shapes to measure. Simply stretch your measuring tape across, measuring from the acrylic, from lip to lip. This is the length across the tub, going directly through the center.
As long as the cover forms a good seal over the water, you’ll be conserving energy and protecting your tub. Don’t be too worried about taking perfect measurements—Add an extra inch of play, just to be certain there is enough coverage.
Also take measurements for the cover skirt or flap. You’ll want to make sure it hangs straight down and extends a good 3” below the seam, enough to cover the spa lip or the spa cabinet (again, view the figure below). If the skirt is too short, it can buckle and fold, allowing precious heat to escape, so again, it is fine to add a couple of inches to make sure your new or replacement cover will lie flat.