How much water should I drink a day? The maths by body weight

How much water do adults need?

The most useful reference comes from the European Food Safety Authority (EFSA): an adequate total water intake of about 2.0 litres a day for women and 2.5 litres for men. "Total" is the key word. It counts water from every drink and from food, not just glasses of plain water.

That's why the familiar "eight glasses a day" works as a rough reminder rather than a rule. Your real need moves with your size, the weather, how much you move and what you eat. On an ordinary day, thirst is a reasonable guide for most healthy adults, and a steady habit makes the rest easy. A handy everyday check: if your urine is pale yellow, you're usually drinking enough; if it's dark, have a glass of water.

Other drinks count too. The NHS counts tea, coffee and lower-fat milk towards your daily fluids. Water is still the simplest everyday choice: no calories and no sugar.

Water intake calculator by weight

Body weight is the simplest way to scale a daily target, and it's how Doydum does it:

Daily water = 33 ml × your weight in kg, rounded to the nearest 50 ml, and never less than 1.5 L.

Below about 45 kg the formula would dip under 1.5 L, so the target stays at 1.5 L. Some calculators also ask for your height or age; Doydum's formula uses weight only, and unlike EFSA's two reference values it doesn't look at sex either. It's an estimate from a simple formula, not a prescription, so treat it as a starting line and adjust for heat and activity. Work out yours here:

How many glasses of water a day is that?

Litres are hard to picture, so here's the same target in everyday containers, using a 250 ml glass and a 500 ml bottle:

• 1.5 L (the minimum): 6 glasses or 3 bottles.
• 2.0 L (60 kg): 8 glasses or 4 bottles.
• 2.3 L (70 kg): about 9 glasses or 4.5 bottles.
• 2.65 L (80 kg): about 10.5 glasses or 5.5 bottles.
• 2.95 L (90 kg): about 12 glasses or 6 bottles.

Spread over roughly 16 waking hours, even 3 litres is less than a glass an hour. We count in 250 ml glasses because that's what the "+ glass" button on Doydum's widget adds. If your glasses at home are smaller, you'll simply need a few more of them.

Does water from food count?

It does, and it's already built into EFSA's totals. A fair share of your daily water arrives on your plate rather than in your glass:

• Fruit and vegetables such as watermelon, cucumber, tomatoes and oranges are mostly water.
• Soups, stews and dishes cooked in plenty of liquid, from red lentil soup to bean stews.
• Yogurt, ayran and milk.

That's one reason two people with the same weight can need different amounts from drinks. If your meals are full of fruit, vegetables and soup, your glass has less work to do. Even a Turkish breakfast of tomatoes, cucumbers and olives with a glass of tea brings more water than it seems. Doydum's target is a simple daily goal for what you drink, so treat it as a helpful line, not a test.

When should you drink more water?

• Hot days. You sweat more in summer heat, even when you hardly notice it. Start the day with a glass and keep a bottle within reach.
• Exercise and long walks. Drink before, during and after. Long walks are also where big step counts come from, as our guide to 10,000 steps a day explains.
• After a salty meal or alcohol. A glass of water alongside is an easy habit to build.
• Being unwell. With a fever, vomiting or diarrhoea you lose fluid more quickly. Ask a pharmacist or doctor for advice, especially for children and older relatives.
• Pregnancy and breastfeeding. Needs go up, so follow the advice of your doctor or midwife.

And one case for less: if a doctor has given you a fluid limit, for example because of a heart or kidney condition, follow it instead of any formula, ours included.

How much water should I drink to lose weight?

Honest answer: there's no special amount. Water doesn't melt anything away, and extra litres won't change your energy balance on their own. Your normal daily target is enough.

Where water genuinely helps is in what it replaces. It has no calories, while sweet drinks add up quietly: cola has about 42 kcal per 100 g, and orange juice is in the same range at about 45 kcal per 100 g. Swapping some of those for water, or for unsweetened tea, lowers the calories you drink without changing anything on your plate. Swapping a single 330 ml can of cola for water means about 140 kcal less in your glass.

If you're working on a calorie deficit, our guide to calorie deficits explains how Doydum sizes one.

Water myths worth letting go

• "Everyone needs exactly eight glasses." Needs vary with size, heat, activity and food. Eight glasses is a reminder, not a measurement.
• "Tea and coffee don't count." According to the NHS, they do count towards your fluids.
• "The more, the better." Past your needs, extra water brings no extra benefit, and very large amounts in a short time can upset your body's salt balance. Spread it through the day.
• "Water melts fat." It doesn't. Water simply has no calories, which is useful enough.
• "If you feel thirsty, it's already too late." For most healthy adults, thirst is an early, useful signal rather than an emergency. Just drink when it shows up.
• "Water first thing in the morning works wonders." The clock doesn't matter much. A morning glass is a nice habit, nothing more; what counts is the day's total.

We dig into more of these in water myths.

How to drink more water without thinking about it

• Tie it to things you already do. A glass when you wake up, one with each meal, one when you get home.
• Keep it in sight. A bottle on your desk or in your bag gets finished; one left in the kitchen gets forgotten.
• Give it some taste. Lemon, mint, cucumber slices or a cold sparkling water all count. Cold or warm: pick whichever you'll actually drink.
• Let Doy keep score. In Doydum, Doy's body is the water meter: he fills up as you drink, all the way to your target, which is a lot more fun to watch than a progress bar.

You can see how that looks on our home page.

Questions people ask