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Measure Shell Openings

It is important to choose shells with the correct opening size for your hermit crabs. The opening size you choose should be 1/8" bigger than your hermit crab's big claw. Hermit crabs use their biggest claw as a trap door. Their claw should be recessed just inside the opening. If their big claw is down inside, the shell may be big enough or possibly a little too big depending on how deep the claw is in the opening. If the claw is too big to recede within the opening, your crab definitely needs a larger shell. A good rule of thumb is to buy shells with an opening that's 1/8" larger than your hermit crab's big claw.

hermit crab shell size    Ruler measurements.

How we measure the openings of our shells.

Measure a Round Opening

Round Opening seashells are measured side to side.

Round Opening seashells are measured side to side as shown.

Measure an Oval Opening

Oval opening seashells are measured by picturing an oval.

Oval opening sea shells are measured by picturing an oval within the opening, not measuring all the way to the crease where the hermit crab won't fit.

Give your hermit crab a little variety! If you present your crab with a few properly sized shells to choose from he or she is much more likely to find a comfortable match. You should have at least three shells for every crab in your tank. This will also help to prevent ugly shell fights.


Hermit crabs change shells on their own terms! Under no circumstances should you try to force your crab to switch shells. Hermit crabs will literally let themselves be ripped apart before letting go of their shell.

Author: Amy Ferguson