What this calculator does
The usual guideline for aquatic turtles is ten gallons of water per inch of shell length, so a six inch turtle needs about 60 gallons. Each extra turtle adds half the base requirement again rather than doubling it, since they share swimming space even though they need more of it.
These numbers surprise people, and they are the most common reason pet turtles end up in tanks far too small. A red-eared slider bought at two inches reaches eight to twelve inches as an adult female, which takes the requirement from 20 gallons to well over 100. The tank to buy is the one the adult needs.
The formula
The base volume is the shell length in inches multiplied by ten gallons. Each turtle beyond the first adds 50% of that base figure. The result converts to litres. This is a rule of thumb for water volume only, and it takes no account of the basking area, filtration or land space that a turtle setup also requires.
| Term | Meaning |
|---|---|
| Shell length | Straight carapace length, measured in a straight line rather than over the curve. |
| Ten gallons per inch | The common guideline for aquatic turtle water volume. |
| Basking area | Dry space under a heat and UVB lamp, essential for aquatic turtles and not included in these figures. |
The inputs explained
| Field | What to enter |
|---|---|
| Shell length (in) | Straight carapace length in inches. Use the expected adult length rather than the current one when buying a tank. |
| Number of turtles | How many turtles share the tank. |
When to use it
Buying a tank for a young turtle
Sizing for the current shell length means replacing the tank within a year or two. Sizing for the adult is cheaper and far better for the animal.
Adding a second turtle
The additional requirement is half the base rather than a full doubling, but it still means a substantially larger tank and it is worth checking the current one can take it.
Checking an existing setup
Comparing what a turtle has against what the guideline suggests is usually the prompt for an upgrade, since undersized tanks are the norm rather than the exception.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What tank volume does each shell length need?
A range of shell lengths from a juvenile to a full-grown adult.
| Shell length | Minimum tank volume | In litres | Rule used |
|---|---|---|---|
| 4 in | 40 gallons | 151 L | 10 gallons per inch of shell length for the first turtle, plus 50% more per additional turtle |
| 6 in | 60 gallons | 227 L | 10 gallons per inch of shell length for the first turtle, plus 50% more per additional turtle |
| 8 in | 80 gallons | 303 L | 10 gallons per inch of shell length for the first turtle, plus 50% more per additional turtle |
| 10 in | 100 gallons | 379 L | 10 gallons per inch of shell length for the first turtle, plus 50% more per additional turtle |
How much more does each additional turtle need?
A six inch turtle with additional turtles sharing the same tank.
| Turtles | Minimum tank volume | In litres | Per turtle, on average |
|---|---|---|---|
| 1 | 60 gallons | 227 L | 60 gallons |
| 2 | 90 gallons | 341 L | 45 gallons |
| 3 | 120 gallons | 454 L | 40 gallons |
Questions
How big a tank does a red-eared slider need?
Adult females commonly reach 10 to 12 inches, which puts the requirement at 100 to 120 gallons. Males stay smaller at around 8 to 10 inches. Buying for the hatchling size is the most common mistake made with this species.
Does the ten gallons per inch rule include the basking area?
No, it covers water volume only. Aquatic turtles need a dry basking platform under both a heat lamp and a UVB source, and that space is additional to the water. A tank filled to the guideline volume with no basking area is not an adequate setup.
Can two turtles share a tank?
Sometimes, with enough space, but it is not automatic. Turtles can be territorial and males in particular may harass tank mates. Enough volume reduces the risk without eliminating it, and separate basking spots help.
Why do turtles need so much more water than fish?
Because they are messy. Turtles produce a great deal of waste for their size and eat protein-rich food that fouls water quickly, so volume is doing dilution work as much as swimming-space work. Filtration rated well above the tank volume is standard practice.
For a freshwater fish tank instead, see the fish tank stocking calculator. For rabbit housing, see the rabbit cage size calculator.