Small beer and ale was pretty dilute. Somewhere between 0.5-3% alcohol. In modern terms somewhere from non-alocholic beer to light beer range. Also there's a lot of myths about this, medieval people did also drink water, they knew to boil it or get it from fast running streams/deep wells, even if they didn't understand why. It wasn't just that beer was safe, they liked drinking beer for the same reasons we do. The taste and getting a little buzzed. Also, it was a way to store excess calories that would otherwise go bad. Similarly with wine/grapes.