You've heard "drink eight glasses a day" your whole life. It's a fine rule of thumb, but it's not based on your body — and for a lot of people it's either too much or not nearly enough. Your real hydration needs depend on your size, how active you are, and where you live. Here's how to get to a number that actually fits you.
Where the "8 glasses" rule falls short
Eight 8-ounce glasses works out to about 2 litres a day. That's a reasonable average for a moderately active adult in a temperate climate — but "average" hides a lot. A 60 kg office worker and a 95 kg construction worker have very different needs, and a run in summer heat can add a litre or more on its own. Treating one number as universal means some people are chronically under-hydrated while others are just chasing a target that was never theirs.
A better starting point: use your body weight
A widely used estimate is 30–35 ml of water per kilogram of body weight per day. To get your baseline:
- Take your weight in kilograms.
- Multiply by 33 (a middle-of-the-road figure).
- That's your baseline in millilitres.
So a 70 kg person lands around 2,300 ml — a bit above the classic eight glasses. A 55 kg person is closer to 1,800 ml. This is a starting point, not a prescription, but it's already far more personal than a fixed glass count.
Adjust for how you actually live
Activity
Add roughly 350–500 ml for every 30–60 minutes of exercise, more if you sweat heavily. You lose water faster than you notice, and thirst tends to lag behind the actual deficit.
Climate and altitude
Hot, dry, or high-altitude environments increase water loss through sweat and breathing. If you live somewhere warm or you're travelling to elevation, nudge your target upward.
Diet and other drinks
Not all of your intake has to be plain water — fruit, vegetables, soup, milk, and even tea and coffee count toward the total. If your diet is water-rich, you may hit your goal more easily than the number suggests.
Build the habit, not just the number
Knowing your target is the easy part; remembering to drink through a busy day is the hard part. A few things that actually work:
- Anchor it to routines — a glass when you wake up, one with each meal, one mid-afternoon.
- Keep water visible — a bottle on your desk gets sipped; one in the cupboard doesn't.
- Track and get reminders so the goal doesn't quietly slip. An app like Water Reminder calculates a personalized daily target from your weight and gender, logs each glass, and nudges you at smart intervals so you actually reach it.
Signs you've got it about right
You don't need to obsess over the millilitres. A simple, reliable check is the colour of your urine: pale straw usually means you're well hydrated, while darker yellow suggests you need more. Feeling consistently thirsty, tired, or getting headaches in the afternoon can also be a nudge to drink more.
Start with the weight-based estimate, adjust for your activity and climate, and let a routine (and a reminder or two) carry it. Within a week or two, the right amount of water stops being a number you're chasing and just becomes part of your day.
This article is for general information and isn't medical advice. If you have a health condition that affects fluid intake, follow your doctor's guidance.