If you are measuring the actual hot tub or spa instead, BeyondNice then recommends measuring the outer lip of the acrylic. You can do this by measuring from the top of the acrylic down to the bottom of the acrylic and then rounding up about ½ inch. This will ensure that the entire amount of the acrylic is protected.

BeyondNice often finds that clients want to order the skirt longer than necessary. Clients want to do this for a variety of difference reasons, one is believing that a longer skirt will provide additional insulation for the cover and help keep the heat in. As far as ordering a longer skirt for more insulation, that would seem like it would work. What actually happens if you order a longer skirt than necessary is, the skirt gets caught between the cover and the hot tub and breaks the seal allowing heat to escape causing the opposite effect you were seeking.

Another common problem when ordering your skirt too long is that if you have a cabinet or wooden ledge, the skirt will bunch up and could also get caught in between the top and spa. At this point you would actually be losing heat and not keeping heat in. To avoid the skirt from getting caught and breaking the seal, BeyondNice encourages our clients to order a cover with split corner skirts and order a skirt that is about ½ longer than the acrylic lip to ensure both protection and insulation.