Estimate minimum turtle tank size from shell length and turtle count, using the 10 gallons per inch of shell rule, in gallons and litres.
Rule of thumb: 10 US gallons per inch of shell length, per turtle. Tank length of about 5× shell length is a common guide. Filtration, basking area, UVB and heating matter as much as volume. Not veterinary advice: this is an estimate for planning only. Always confirm care, feeding and health decisions with a qualified veterinarian.
Minimum tank size (gallons)
How to use this tool
Measure the straight carapace length from front to back in inches.
The rule applies per turtle, so groups need proportionally larger tanks.
Both are shown; note these are US gallons.
Add a basking dock, UVB lighting, heating and filtration rated well above the tank volume.
Formula & worked example
A single 4-inch turtle needs at least 10 × 4 = 40 US gallons, about 151 litres, in a tank roughly 20 inches long.
Limitations — read this before you rely on it
The 10-gallons-per-inch rule is a widely used minimum, not an ideal, and active or large species need more swimming room. Water depth, basking access, filtration, temperature and UVB are equally important and are not captured by volume. Not veterinary advice: this is an estimate for planning only. Always confirm care, feeding and health decisions with a qualified veterinarian.
Frequently asked questions
At least 10 US gallons per inch of shell, per turtle, as a minimum — bigger is better.
Yes — many pet turtles grow far larger than buyers expect, so plan for adult size.
Usually at least 1.5–2 times shell length for swimming species, with an easy route to the basking area.
Yes — turtles are messy; use a filter rated for two to three times the tank volume.
A dry basking dock, UVB light, heat source, thermometer and regular partial water changes.