Step 4: THIS IS WHAT YOU NEED TO MEASURE
If your current cover fits perfectly, then measure the cover. But, if your cover is waterlogged and swollen or distorted, you should measure your hot tub shell instead to ensure that the new cover is a good fit.
If you measure both your cover and the spa shell you will probably find that your cover is slightly bigger than the shell. Up to a couple of centimeters bigger all round is fine because it makes the cover easier to get on and off.