The one rule behind every spacing figure
Bar spacing is right when your bird cannot fit its head between the bars. A bird who can push its head through but not withdraw it cleanly, or who panics halfway, can injure or break its neck thrashing to get free, and even a bird who slips through entirely is then loose in a room that was not made safe. That single test explains every recommended figure: spacing scales with head size, which is why a cage marketed for a cockatiel can be dangerous for a budgie even though both are small parrots.
Working figures by bird size
As a practical guide, finches and budgies need spacing around half an inch, and budgies are safest at the tighter end of that range because determined birds test gaps. Cockatiels, lovebirds, and parrotlets sit around half to five-eighths of an inch. Small conures and Quakers generally need five-eighths to three-quarters, medium parrots such as African greys around three-quarters to an inch, and large macaws and cockatoos one to one and a half inches, where bar strength matters as much as the gap. Treat any cage at the wide edge of your bird's range with suspicion, and measure the real spacing with a ruler rather than trusting a product listing, because advertised figures are often rounded.
The other half of the problem: bars a big beak can defeat
Spacing that is safe against trapping can still fail a large parrot if the bars themselves are flimsy. Macaws and cockatoos flex thin bars apart with their beaks, widening a gap until a head or whole body fits, and they learn quickly which bar gives. For strong-beaked birds, thick welded bars and secure door latches matter as much as the measured gap. Small birds face the opposite version: horizontal bars on a budgie or cockatiel cage are a climbing asset, while vertical-only bars give little feet nothing to grip, so orientation is worth checking alongside spacing when you shop.
Checking a cage you already own
Measure gaps in several places, including the door surround and the corners, because spacing is often uneven where panels join, and check any decorative scrollwork or widened sections a head could reach. Watch your bird for a week in a new cage: repeated attempts to push the head at one spot mean the gap is tempting fate even if it has not yet trapped. Cover or replace any section where the head passes more than partway. Never solve wide spacing with a second cage inside or loose netting fitted by hand, which creates new trapping points; the fix is a cage built for the species.
Why this decision follows the bird, not the cage
Owners replace cages after moves, damage, or upgrades, and spacing is the specification most often guessed wrong in the replacement. Keeping your bird's species, head-safe spacing, and cage dimensions noted together means the next purchase starts from the right figures instead of a showroom guess. KinsLe Pet Pass works as that ready reference because your bird's verified ID card holds its identity and your contact details in one place you can show anywhere, alongside the profile where its cage measurements belong.