What exactly is one unit of alcohol? Why do I get more drunk off a bottle of lager than I do with a bottle of alcopop when they suppose to have the same unit in them? I have never been drunk off alcopops and believe me I've tried!!
Calculator works well but if you can't be bothered just multiply the total volume (in litres) by the ABV% to get the number of units (ie one pint, just over 0.5 litres of standard lager, 5%, is just over 2.5 units.
How quickly the alcohol is absorbed into your bloodstream (and so how quickly you get "drunk") depends on many factors including the type of drink itself.