How far down is it safe to open a car window for a dog?

A narrow gap lets air circulate and gives a dog's nose access to outside scents, which many dogs find genuinely enjoyable, without leaving enough room for the head or shoulders to pass through even if the dog leans hard against it. This balance is harder to achieve than it sounds, since dogs are often more flexible and determined than owners expect, so testing the gap with your specific dog present, engine off, is worth doing once.

A window left too far down becomes a real escape route the moment the car stops at a light, in traffic, or at a rest area, particularly if something exciting like another dog or a squirrel appears outside. This risk exists even with an otherwise well-behaved dog, since a strong enough trigger can override normal training in the moment.

Many modern cars include a window lock or child-safety mode that prevents rear windows from being operated by anything other than the driver's controls, which stops a dog from nudging a window switch and opening it further by accident. Combining a small, fixed gap with this lock feature covers both the accidental and the impulsive escape scenarios.

Older cars with manual window winders avoid the accidental-button-press scenario entirely, but they also mean a fixed gap needs setting once before the drive rather than being adjustable on the move, so decide on the gap before setting off rather than during the journey.