HOT TUB COVER MEASURING TIPS
When measuring the spa for a new spa cover, there is one important thing to remember
IT HAS TO BE BIG ENOUGH TO FIT!
A bit too big is much better than too small
Hot tub covers can fit in one of two ways. The most popular is a cover that fits only over the acrylic shell (in a lot of hot tubs, the acrylic shell comes out further than the wood or plastic skirt) and in this case, you must measure from the outside edge of the shell. However, if your hot tub resembles the diagram below, with a wood or plastic sill on the outside of the acrylic shell, and you would like the new lid to cover both the shell and the sill, then you must measure from the outside of the wood skirt. The cover will perform equally well either way, as long as it is big enough to fit completely over the spa shell.
This is the height of the acrylic lip and will determine the cover´s skirt length if you want your cover to to sit on top of a sill. Otherwise you may choose any skirt length up to 25.5 cm so that it will easily sit over the edges of the spa with a little room to spare.
Outside dimension of the acrylic shell. This is the normal measurement to specify unless you have a sill and prefer your cover to fully enclose it.
Outside dimension of the hot tub cabinet. You should use this measurement if you have a sill and prefer your cover to fully enclose it.
More measuring tips:
Above is an illustration of the best way to measure the radius.
Measure the distance from the imaginary corner, A, to the point at which the curvature stops, B.