Measure Along the Flight Path
A Gouldian finch exercises by flying flat and fast from one end of its cage to the other, so the dimension that decides welfare is length. For a pair, start at about 30 inches long, 18 inches deep, and 18 inches tall, and go longer before you go taller if you are choosing between cages. A tall, narrow cage sold for its looks gives a Gouldian little more than fluttering between perches, and these are not birds that climb bars like parrots to use vertical space. In a longer cage the birds can build real flight speed, land cleanly, and keep the muscle tone that supports their health.
Bar Spacing for a Very Small Head
Gouldians are slender, small-headed finches, and spacing that suits a budgie can be an escape route or a trap for them. Keep bars no more than about half an inch apart, and check the gaps around doors and tray slots, which are often wider than the main panels. Horizontal bars on at least some sides give the birds footholds for short climbs to feeders, though the main travel is flight. Avoid decorative cages with uneven spacing that narrows and widens, because a finch exploring a widening gap can wedge at the neck.
A Layout With a Quiet End
Gouldians are shyer and less forgiving of disturbance than zebra finches, so arrange the cage to include refuge. Put perches at each end to encourage the full flight, hang food and water where droppings from the favourite top perch will not fall into them, and screen or furnish one end with a little cover so a startled bird has somewhere to settle out of view. Floor space matters too: Gouldians will forage on a clean cage floor, so a grate that keeps them off dropped food is less suitable than for some species, and paper changed frequently keeps floor feeding hygienic.
Company Changes the Calculation
The minimum above assumes a compatible pair. Gouldians are often kept in a small same-species group or a peaceful mixed finch aviary, and every added bird needs flight room and escape distance, not just another perch. Crowded Gouldians bicker at feeders and the weakest bird gets displaced from food, which can look like a sudden illness when it is really access. Provide more than one feeding point in a group cage and enough length for a chased bird to break line of sight. If breeding is planned, the pair needs still more room, plus a nest site they can defend without stress.
A Setup Worth Writing Down
Once you have tuned a cage that keeps Gouldians calm, flying, and feeding well, the details are worth preserving: exact length, which end the birds roost in, feeder positions, and companions they live peacefully beside. A new keeper guessing at those details can unsettle birds that dislike change. Recording the working arrangement takes minutes and spares the finches a stressful trial-and-error move. That record travels best with the birds themselves, and KinsLe's Transferrable Profiles do exactly that, passing a Gouldian's housing, companions, and daily routine on intact whenever the flock changes home.