The short answer for a painted turtle
Plan on a 75- to 100-gallon tank, or an indoor stock tank of similar volume, for one adult painted turtle. Most adults reach 4 to 7 inches of shell length, and the working guide of about 10 gallons of water per inch of shell puts a full-grown turtle well past the small tanks often sold for hatchlings. A long, low footprint matters more than height, because painted turtles swim horizontally, patrol the bottom, and need a dry dock they can haul out onto completely to bask and dry their shells.
Why painted turtles outgrow small tanks so fast
A hatchling painted turtle looks tiny in a 20-gallon tank, but it grows steadily and spends most of its day moving through water. Cramped swimming space means weak exercise, faster fouling, and a turtle that spends its time bumping glass instead of diving, foraging, and surfacing naturally. Water quality also collapses quickly in small volumes, because painted turtles eat in the water and produce waste there too. Starting closer to adult size saves you from buying two or three replacement tanks and keeps filtration, heating, and the basking area proportioned correctly from the beginning of adult life.
Laying out swimming water and a real basking dock
Fill the tank deep enough for genuine swimming, generally at least as deep as the shell is wide so the turtle can right itself if it flips, and deeper as it grows. Reserve one end for a basking platform that sits fully above the waterline, big enough for the whole shell, with a ramp the turtle can grip. Leave open water between the dock and the far end rather than filling the surface with decorations. A secure screen lid is essential, since painted turtles climb better than most new keepers expect, especially near filter returns and dock corners.
Floor space and placement in your home
A 100-gallon setup is heavy once filled, so choose the final spot before adding water and confirm the stand and floor can carry the combined weight of tank, water, and substrate. Keep the tank away from direct window sun, which can overheat the water and drive algae, and leave working room above it for lamps and filter maintenance. If two painted turtles will share the enclosure, add substantial extra volume and a second basking spot, and be ready to separate them if one monopolizes the dock or nips at the other during feeding.
Signs the enclosure has become too small
Watch for a turtle that paces the glass in repetitive laps, struggles to turn without scraping the sides, or can no longer submerge fully with room to spare above its shell. A basking dock the turtle overhangs on every side, or water that fouls within days despite a filter rated above the tank size, also points to outgrown quarters. Growth is gradual, so measure shell length a few times and compare it honestly against the swimming room left. Upgrading before these signs become constant is far kinder than waiting for shell or skin problems to appear.
Planning the adult setup from day one
Sketch the adult enclosure before the hatchling arrives: tank length, dock position, filter footprint, and the stand it all sits on. That single plan prevents the common pattern of emergency upgrades in undersized temporary tubs. Share the finished dimensions with anyone who feeds or cleans for the turtle, so a helper never assumes a smaller holding tank is acceptable for more than a brief cleaning window. Keeping those measurements on the turtle's profile through KinsLe Family Sharing means the whole household works from one agreed adult setup instead of guessing.