Measure your current Hermit Crab's shell with a ruler or tape measurer. Be sure to measure the widest points of the shell OPENING to find the current size. It is recommended to increase your hermit crab's current shell size by 1/8" - 1/4" when purchasing growth changing shells.. It's always a good idea to offer a variety of shell types and sizes to your hermit crabs, and a minimum of 3-5 shells per crab to prevent "shell envy" and aggression. Remember hermit crabs can be quite picky, and "beauty is always in the eye of the shell holder!"