How to Choose a Terrarium Size for Your Pet
Buy for the adult animal, not the juvenile you're bringing home.
Published July 8, 2026
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It's tempting to buy a small enclosure for a small juvenile animal and upgrade later. In practice, that usually means buying two enclosures instead of one, and many species grow faster than expected. Sizing for the adult animal from the start is almost always cheaper and less disruptive.
Floor space vs. height
Ground-dwelling species (many geckos, isopods, some frogs) need floor area more than height. Arboreal species (chameleons, some tree frogs, arboreal geckos) need vertical climbing space more than floor footprint. Buying based on total volume alone can leave you with an enclosure that's the right size on paper but the wrong shape for the animal.
The "gallon" number doesn't tell the whole story
Two tanks with the same gallon rating can have very different footprints depending on height — a tall, narrow 40-gallon tank and a long, low 40-gallon tank suit different animals. Check length × width × height specifically, not just the volume rating on the box.
See terrarium enclosures on Amazon →Leave room for the bioactive setup itself
A drainage layer, substrate depth, plants and hardscape all take up usable space before the animal even moves in. For a bioactive build (see our bioactive terrarium guide), size up slightly from a bare-bottom recommendation to leave room for all of these layers.
When bigger isn't better
Very young or small-bodied animals can sometimes struggle to find food or feel secure in an oversized enclosure. Some keepers start with a well-planted, appropriately sized juvenile setup and upgrade once, rather than starting in full adult-sized housing from day one — check species-specific guidance for what's recommended.
Minimum enclosure sizes vary by species and are debated even among experienced keepers — treat any single number as a starting point and check current care sheets for your species.