Sizer
Will your carry-on fit? Sizer answers the question for every airline, every bag, every leg of your trip.
I was searching Google for a carry-on under 21 inches. It returned a 29-inch checked bag, a neck pillow, packing cubes, and a suitcase whose own listing gave three different sets of dimensions in three different places. I tried "EU cabin friendly" and was shown a golf travel case. Somewhere in there I stopped shopping and started taking notes. I discovered that "carry-on" is not a size. The airlines here publish seventeen different limits, from 40 cm to 61 cm on the longest side. And a product listing may or may not be counting the wheels, which are exactly the part a gate agent will measure. Sizer answers what the search box cannot: your bag, your airline, the fare you actually clicked, across a growing list of airlines and bags. It also names the leg of your trip that ruins it, which is never the leg you were worried about.
The problem
Everyone answering this question is trying to sell you a bag. The affiliate roundups copy dimensions off retail listings. The airlines publish the truth and then bury it three clicks into a fare table. Five of them sell a cheapest ticket that includes no cabin bag whatsoever, a detail they are in no rush to lead with. Nobody was answering the specific question, because answering there's no incentive to do so. I built Sizer to show the answer in a way that makes the gap between a brochure figure and a gate visible.
Product decisions
Establish trust. Make it beautiful, but not pretentious. Always show sources. Show links to the airline and bag manufacturers when available. Allow for custom bags. Keep the design clean and functional. Build something useful.
Process
I vibe coded this thing in less than a week. Step 1: Establish clear specs and product vision. Step 2: Establish design direction; generate a round of mockups. Step 3: Write technical specs and implementation plans that refer to mockups, Step 4: Iterate. Rinse. Repeat.
Tech
Astro with Preact islands, Tailwind, ThreeJS for the luggage visualization, deployed static to Cloudflare Workers. The sizing engine is a standalone module with over 600 tests!