I always measure across the shell opening at the widest area (inside to inside of the opening) the best I can. Then order several shells between 1/8" & 1/4" larger. As long as you are close with your measurements you should be fine with ordering up.
You can even take something like a quarter or a bottle cap and measure their diameter then put that against a shell opening to judge the openings. If it goes into the shell you know the opening is bigger. That works much better if the shell doesn't have a tennant though.
The shell sizing chart on here actually works pretty well once you figure it out. It helps ordering shells if the shell size is called out but opening sizes aren't. Great for buying on Etsy or eBay. Find the type of shell and opening size. Go straight up to the black line the over to the shell size. Since this is based on an overall average some openings may be average. The other colors above and below the black line represent where a percentage of the shell openings will range.
http://www.hermitcrabassociation.com/ph ... 24&t=84333
Some crabs will go just a little larger and some will pick shells that you would think are way to big for them. That's why I think it is always good to get a few different shells of each size.
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