If you have no other experience, then go by the manufacturer's suggested sizing for that model of bike you are looking at. If you look at another model, then find the suggested sizing for it. As the recommended sizing sometimes is not the same if the other model is different enough in some aspect of it's geometry or the other things put on the bike such as bars and stem.

If you go to a bike shop, the sales person will have a good idea what size bike you should look at just by judging your height and maybe leg length. You'll fit a size or two up and down without big issues. At least that's true for the majority of people. Only your experience with bikes will tell you if you are in that majority or not.

There really isn't a perfect size that you can calculate or guess at. It really just comes down to trying them out and finding what you like. Understand that different bikes give different positions for the rider. Some you sit almost upright. Some you are in a very aggressive and aerodynamic position. Also know what kind of riding you will be doing with the bike. Try to get one that satisfies the riding you intend to do the most. IE: off road get a mountain bike. Paved roads and paved trails get a road bike. Gravel roads, get a gravel bike. Slow leisurely rides get a cruiser style bike.



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