22 kilometres. Sealed the whole way. And you'll still cook yourself if you don't pack water and read the wind before you leave Thomson Bay. That's the full Rottnest Island bike loop in one sentence, and most of what follows is just the fine print.
I rode it again last weekend, solo this time, on a hire bike from the shed near the main jetty rather than dragging my own gravel bike over on the ferry. Worth noting up front because it changes the whole experience — the hire fleet is fine for the sealed sections but gets a bit ordinary once you're grinding into a headwind past Salmon Bay with a wire basket rattling on the front.
The Rottnest Island bike loop, numbers first
The full perimeter loop out of Thomson Bay runs roughly 22km if you do the whole thing without side trips, more like 26-28km once you factor in detours to lookouts and the odd wrong turn near the salt lakes. It's almost entirely sealed road, which surprises people who expect gravel. You'll hit maybe two or three short unsealed stretches near the West End car parks, nothing technical, just loose enough that you want your tyre pressure down a touch if you're on anything skinnier than 28c. Road bikes do the loop fine in dry weather. I wouldn't take one out there in a southwesterly with wet gravel underfoot though — that's asking for a low-side on a corner you didn't need to take fast.