"Drink eight glasses of water a day" is the most repeated hydration advice on the planet, and there's almost no science behind it. The real answer depends on how much you weigh, how much you move, and where you live — and once you account for those, your target might be well above or below the famous eight glasses. Here's how to work out your actual number.
Where the "8×8" myth came from
The 8×8 rule — eight 8-ounce glasses (about 2 litres) — is easy to remember and roughly the right order of magnitude, which is exactly why it stuck. But it ignores the two people it's meant to serve: a 55 kg office worker and a 100 kg construction worker have very different needs, and no single glass count fits both. It also double-counts, because you already get a meaningful amount of water from food and other drinks.
The body-weight formula
A far better starting point scales with your size. A widely used estimate is:
Daily water (ml) = body weight (kg) × 30 to 35
So a 70 kg adult needs roughly 70 × 33 ≈ 2,300 ml, or about 2.3 litres, on a normal day. In imperial terms it's close to half your body weight in pounds, drunk as ounces — a 154 lb person aims for around 77 oz. Our water intake calculator does this instantly and adjusts for activity so you don't have to do the arithmetic.
The three things that change your number
Activity. Every hour of hard exercise adds roughly 500–700 ml to replace sweat. If you're tracking workouts, the calories burned calculator is a decent proxy for effort — more calories out generally means more fluid to replace. Endurance training in particular pushes needs up fast; see our guide to heart rate zones for how hard those sessions really are.
Climate and altitude. Heat, humidity and high altitude all increase water loss. A summer day or a hot yoga class can add a litre or more.
Body size and metabolism. Bigger bodies and faster metabolisms run through more water. Your BMR — the energy you burn at rest — tracks loosely with baseline fluid turnover, which is why heavier and more muscular people need more.
What counts toward the total
Good news: it isn't all plain water. Roughly 20% of your daily water comes from food — fruit, vegetables, soup, yoghurt — and tea, coffee and milk all count too. The mild diuretic effect of caffeine is far smaller than the water the drink itself provides, so a coffee is a net gain, not a loss. That's part of why blindly forcing down eight extra glasses on top of everything else is unnecessary.
Can you drink too much?
Yes, though it's rare. Drinking far more than you lose can dilute the sodium in your blood — a dangerous condition called hyponatremia that occasionally affects marathon runners who over-drink plain water. The fix isn't to drink less overall, but to include electrolytes during long, sweaty efforts and to let thirst guide you rather than chugging on a schedule.
The simplest gauge you already have
Forget counting glasses for a moment: urine colour is a reliable real-time readout. Pale straw means you're well hydrated; dark yellow means drink more. Thirst is a lagging signal — by the time you feel it you're already mildly down — so for training days, plan ahead rather than react.
The bottom line
The honest answer to "how much water should I drink?" is: about 30–35 ml per kilogram of body weight, plus roughly 500 ml for every hour you sweat, more in heat. Start there, then let urine colour fine-tune it. Get your personalised target from the water intake calculator, and if you're using hydration as part of a broader fitness push, pair it with your maintenance calories and — if you're cutting — a sensible calorie deficit. Hydration is one of the highest-return, lowest-effort habits in health; you just need the right number, not a folk rule.