Interesting this. When I started, I measured what all the cups and mugs I normally use held as a normal serving. I then tracked the drinks I had, including black coffee as water (is that correct?) Anyway, I was sure that I drank very near the recommended minimum of 4 pints each day anyway so was not worried.
I set up a spreadsheet on my phone (OK, I am a bloke!!!) to track exactly what I was drinking and was shocked to find that I was no where near the 4 pints, in fact, I was only just over two.
So by tracking exactly what I was drinking I soon got a feel for what 4 pints was and now I have no trouble drinking 5 pints per day. However, I don't like water that much and we bought a water filter jug in the hope of making it taste better, which it does. I use the same filtered water kept in the fridge to make the shakes nice and cold, just how I like them.
I would say though that if you are guessing your intake, you may be a lot less or a lot more than you think, you don't have to be as 'geeky' as me with my spreadsheet but keep an accurate track, you may be surprised.
(Aimed at newcomers, not trying to teach granny how to suck eggs here)