Width
Movement space matters.
Updated
Bird guides
A cockatiel needs a roomy rectangular cage with safe bar spacing, enough width to move and flap, space for natural perches, bowls, and toys, and a layout that keeps the long tail from constantly hitting everything.
Cockatiels are not large parrots, but they still need real movement space.

Cockatiel Questions
A cockatiel needs a roomy rectangular cage with safe bar spacing, enough width to move and flap, space for natural perches, bowls, and toys, and a layout that keeps the long tail from constantly hitting everything.
Use the full species care guide.
Use the hub for nearby questions after this answer.
Use supplies after the care plan is clear, not before.
Pick gear that makes the daily routine easier to repeat.
Movement space matters.
Long feathers need clearance.
Prevent head traps.
Do not overcrowd.
Daily access should be easy.
Room safety still matters.
Choose the largest safe rectangular cage you can maintain, with width and layout prioritized over decorative height.
Cockatiels need room to turn, stretch, and move without tail feathers dragging through bowls or bars.
Spacing should be narrow enough that the cockatiel cannot get its head stuck and doors should latch securely.
Perches and toys should not fill all the open space. Leave movement paths and safe landing spots.
Buy for daily living, not for the smallest cage a cockatiel can fit inside.
Width is usually more useful than narrow height because movement space matters.
Many starter cages are too small for daily life.
Many cockatiel cages use around half-inch spacing, but check the exact bird and every opening.
Most benefit from safe supervised time outside the cage when the room is prepared.
Use these after the care plan is clear. Match size and materials to the bird you actually keep.
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Start with safe space, ventilation, bar spacing, and room for natural perches.

Varied perch diameters support normal feet better than one smooth dowel.

Separate clean food and water dishes that are easy to wash every day.

Plain paper makes droppings easier to monitor without scented products.