How to Measure a Hot Tub Spa Cover the Right Way Ordering a replacement spa cover should not involve guesswork. The #1 reason customers receive the wrong cover size is simple: They measure the old cover instead of the spa shell. This guide will show you exactly how to measure your hot tub correctly so your new cover fits perfectly the first time. Step 1: Measure the Spa Shell — NOT the Old Cover Over time, foam cores absorb moisture and expand. Covers sag. Vinyl stretches. If you measure …