If you look at the alcohol percentage on beers, it doesn't seem like they're all that different. After all, a 5% lager is just 1% more alcohol than a supposedly lighter beer, right? While that's true, Draft Magazine explains the science of how ABV actually works.
As you'd expect, it has to do with how our bodies process alcohol. Our bodies can process some alcohol continuously, this is known as units of alcohol. We process about one unit of alcohol per hour. So, with that, you can figure out why percentages aren't a good indicator of how drunk you'll get:
I did some maths for you: A 340ml beer of 4% strength contains about 1.4 alcohol units. Let's say you're drinking only one beer per hour — you're probably not, but for simplicity, let's say you are. In that case, your body processes 1.0 units and leaves 0.4 to begin laying down that gentle buzz. Have another beer the next hour, your body handles another unit, and the excess goes to 0.8, and it accumulates from there. The next hour, you're at 1.2 units excess. It's a neat (if oversimplified) way to measure intoxication. Now, a 340ml beer of 5% strength has about 1.8 units. That leaves 0.8 after your hour of your body doing what it does. After another beer and another hour, you're at 1.6. The next hour, you're at 2.4 — that's double the excess alcohol, and it only continues to accumulate.
So, if you've ever wondered why you can pound 10 Hahn Lights at a barbecue but a couple microbrews destroy you, there you go. Head over to Draft Magazine for a more detailed breakdown of all the maths.
Why you still get drunk drinking "session" beers [Draft Magazine]