💧 How Much Water Should You Drink? The Simple Formula That Beats '8 Glasses a Day'

Why "8 glasses" fails

Eight glasses sounds precise, but it is one fixed number for everybody: 8 × 250 ml = 2.00 litres. Compare that with what the weight-based formula gives. A 60 kg desk worker who is sedentary needs about 1.98 litres — the rule fits, by luck. A 90 kg person in a hot climate needs about 3.97 litres — the rule gives barely half. The same advice cannot be right for both people, because it ignores the two things that matter most: body size and how much you sweat.

It also ignores where water comes from. Roughly a fifth of daily water typically arrives in food — soups, fruit, vegetables and other moist foods — so the amount you need to drink is a little less than your total water need. A slogan cannot handle any of this. A simple formula can.

The 33 ml per kilogram formula

Our Water Intake Calculator uses a straightforward method, and you can do it by hand in three steps:

1. Take your weight in kilograms and multiply by 0.033. That is your base in litres (33 ml per kilogram).
2. Add an activity bonus: sedentary +0 litres, moderately active +0.35 litres, very active +0.70 litres, hot climate or heavy sweating +1.00 litre.
3. Read the result in litres, then divide by 0.25 if you think in 250 ml glasses.

Three worked examples, exactly as the calculator gives them:

• 60 kg, sedentary: 60 × 0.033 = 1.98 litres — 8 glasses, 67 fl oz.
• 70 kg, moderately active: 2.31 + 0.35 = 2.66 litres — 11 glasses, 90 fl oz.
• 90 kg, hot climate: 2.97 + 1.00 = 3.97 litres — 16 glasses, 134 fl oz.

Run your own numbers with the Water Intake Calculator — enter your weight, pick the activity level closest to your day, and it gives your target in litres, glasses and ounces. That one number is the rest of this guide made personal.

Adjusting for exercise, heat and illness

The activity bonus already covers an ordinary active day, but a few situations deserve extra attention. On a hard training day, sip before you start, drink during the session, and top up afterwards rather than trying to catch up in one huge gulp at night. In hot weather, do not wait until you feel thirsty — thirst lags behind actual loss, especially when you are busy or outdoors. Take the hot-climate bonus as your starting point and add more if you are sweating heavily for hours.

Illness changes the maths too: fever, vomiting and diarrhoea all lose fluid, so you will need more than usual, ideally with electrolytes if you are losing salts as well as water. If you cannot keep fluids down, or you feel dizzy, confused or very unwell, get medical advice promptly rather than relying on a calculator. The same caution applies if you have a kidney or heart condition, or you take water tablets (diuretics) — ask your clinician for a personal target instead of following a general formula.

A system that actually gets the water drunk

Knowing your number is easy; drinking it is the hard part. The trick is to turn the target into a visible, countable routine. If your target is 2.66 litres, that is roughly two and two-thirds of a 1-litre bottle: fill the bottle in the morning, finish it, refill it, and you are nearly there before dinner. A glass when you wake up, a glass with each meal, and a bottle at your desk covers most targets without any willpower.

Three habits make it stick. First, keep water where you can see it — a bottle out of sight is a bottle never drunk. Second, tie drinking to something you already do: a glass with your morning coffee, one when you sit down to work, one before each meal. Third, if plain water bores you, add lemon, cucumber or mint rather than switching to sugary drinks. And count in the calculator's glasses: "11 glasses" is a target you can tick off through the day, while "2.66 litres" is just a number.

How to tell if you are drinking enough — and the signs of too much

You do not need a lab test. The simplest check is urine colour: pale straw generally means you are well hydrated, while dark yellow, especially with a headache, tiredness or a dry mouth, means drink more. Thirst is a useful signal too, but treat it as a late warning in heat or exercise, not your only guide — by the time you feel very thirsty, you are already behind.

Yes, you can also drink too much. Forcing litres beyond thirst and your target can dilute blood sodium, a condition called hyponatraemia — rare, but serious. Warning signs are constantly clear urine, needing the toilet every hour, bloating, nausea or headache after drinking a great deal. It mostly affects endurance athletes who drink huge volumes of plain water without replacing salts, which is why sports drinks exist for very long sessions. For most people the safe zone is simple: your calculated target, plus thirst, minus any urge to force extra gallons.

What counts towards your total

Water is the best choice, but it is not the only thing that counts. Tea and coffee contribute almost all of their volume — caffeine is only mildly diuretic, so a cup of tea is not "minus one glass" — though water should still be your main drink. Milk, soups, fruit and vegetables all count, which is why about a fifth of daily water comes from food. Sugary drinks and fizzy drinks add calories without helping hydration, and alcohol works against it: a glass of water alongside each alcoholic drink is a sensible habit.

Calculate my daily water target →

The verdict

Stop aiming at eight glasses. Weigh yourself, multiply by 0.033, add the bonus for your activity, and you have a target built for your body — 1.98 litres for a sedentary 60 kg adult, 2.66 litres for a moderately active 70 kg adult, 3.97 litres for a 90 kg adult in the heat. Spread it through the day, check for pale-straw urine, and adjust up on hot or hard-training days. Once hydration is sorted, pair it with the bigger health picture: your Calorie Calculator target for energy, your BMI Calculator result for weight range, and your Body Fat Calculator estimate for body composition.