(volume of pure ethanol in drink) ÷ (total volume of drink) x 100%
By this formula, water has an ABV of 0%, pure ethanol has an ABV of 100% and all alcoholic drinks lie somewhere in between.
This table shows the typical ABV range of normal commercial drinks. I've not included any 'alcopops'. These are synthesised drinks designed to hook teenagers. They should never have been invented and are not worth discussing further!
The successful ex-pat plonk maker takes account of these norms and does not produce insipid under-strength wines or lethal over-strength beers. The idea, believe it or not, is to turn out a product that people enjoy drinking. There's a radical notion!