Hermit Crab Tank Calculator
Use this free hermit crab tank calculator to check hermit crab tank size requirements for your colony: the minimum tank gallons, substrate depth for molting, water dish sizing, and how many extra shells to provide — all computed instantly from widely used hermit crab care guidelines.
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These are widely used hermit crab care guidelines (10 gallons per crab, deep moltable substrate). Results are minimums — a bigger tank is always better for your crabs.
How to use this calculator
Enter the number of hermit crabs you keep (or plan to keep). The calculator applies the standard 10-gallons-per-crab rule to find your minimum tank size, then suggests a commonly available tank size that meets it.
Optionally measure the opening of your largest crab's shell in inches. This drives the substrate depth calculation — hermit crabs need at least 6 inches of substrate, or three times their largest shell opening, whichever is deeper. It also sizes the water dishes: each dish should be deep enough for the largest crab to fully submerge.
Your results update instantly as you type. Use the tank suggestion, substrate depth, dish sizing, and extra-shell count as your shopping list, and keep the heat and humidity targets in mind when setting up heating and a lid.
How it works
The calculator uses widely cited hermit crab care guidelines. Tank size follows the 10-gallons-per-crab rule: minimum gallons = number of crabs × 10. For example, 3 crabs need a minimum 30-gallon tank, for which a standard 30-gallon or a 29-gallon breeder is the usual suggestion.
Substrate needs to be deep enough for crabs to bury themselves completely while molting: depth = max(6 inches, 3 × the largest shell opening). Shallow substrate is one of the most common causes of failed molts in pet hermit crabs.
Water requires two dishes — one freshwater, one saltwater — each deep enough for the largest crab to submerge (about 1.5 × the largest shell opening). Extra shells follow the 3–5 shells per crab guideline, offered in natural (never painted) shells slightly larger than the crab's current one. The 75–85°F temperature and 70–80% humidity targets are the standard ranges used to keep the tank's air moist enough for the crabs' modified gills.
These are general care guidelines used across the hermit crab hobby, not veterinary advice. Larger tanks, deeper substrate, and more shells than the minimums are always safe upgrades.
Frequently asked questions
How big of a tank do I need for 2 hermit crabs?
Two hermit crabs need a minimum 20-gallon tank, following the widely used guideline of 10 gallons per crab. A standard 20-gallon long is a popular choice because its larger footprint gives the crabs more floor space for shells, dishes, and climbing.
Can hermit crabs live together?
Yes, hermit crabs are social animals and do best in groups of three or more. A colony needs enough space (10 gallons per crab), plenty of extra empty shells, and separate fresh and saltwater dishes to prevent fights over resources.
How deep should the substrate be in a hermit crab tank?
Substrate should be at least 6 inches deep, or three times the length of your largest crab's shell opening — whichever is deeper. Hermit crabs burrow into the substrate to molt, and too-shallow substrate can cause stress, failed molts, and even death.
Are painted shells or natural shells better for hermit crabs?
Natural, unpainted shells are far better. Painted shells can chip and contain chemicals that are toxic to hermit crabs, and many crabs refuse to change into painted shells at all. Offer 3 to 5 natural extra shells per crab in assorted sizes slightly larger than their current shell.