How to Measure Your Hermit Crab Shell Opening

Getting the correct size is critical. Even a difference of 2–3 mm can determine whether a shell fits properly.

Many sizing issues happen because the opening is measured too deep inside the shell. This results in a smaller number and shells that are too small for your crab.

Measure the Opening at the Widest Point of the Lip

• Place your calipers across the shell opening
• Measure from inside edge to inside edge
• Measure at the widest part of the opening
• Keep the caliper tips right at the rim

Do not push the calipers deep into the shell cavity.

The opening narrows as you go deeper inside the shell. Measuring too far in can reduce your measurement by several millimeters and result in ordering the wrong size.

Correct vs Incorrect Measurement

Correct measurement:
Calipers sit directly at the opening lip, touching the inside edges only.

Incorrect measurement:
Calipers are inserted deep inside the shell where the opening narrows.

Measuring too deep can reduce your size by 3–6 mm

Why Proper Sizing Matters

Hermit crabs choose shells based on internal space and opening width.

If the opening is too small:
• The crab cannot fully retreat
• The shell may be rejected
• The crab may experience stress

If the opening is measured correctly:
• The crab can seal itself properly
• There is proper internal space
• Shell switching success increases

We Recommend Measuring Twice

Measure your crab’s current shell opening carefully. Then consider ordering one size up as well.

Many hermit crabs prefer a slightly larger opening than what they are currently wearing.

When in Doubt, Size Up Slightly

If your measurement falls between sizes, we recommend choosing the larger opening. A slightly larger shell is typically preferred over one that is too tight.