There's no single number that fits everyone. A bigger body needs more water than a smaller one, and a day of exercise or heat needs more than a quiet day indoors. The calculator above turns those three things into a simple daily target you can actually follow. Curious what all that water does? Read about the benefits of drinking water for your body.
How the water calculator works
The calculation starts from your body weight: 35 ml of water for every kilogram (about half an ounce per pound). Then it adds a little for how active you are:
- Mostly sitting: +0 ml
- Lightly active (a walk or a workout 1–2 times a week): +250 ml
- Active (exercise 3–5 times a week): +500 ml
- Athlete (hard training most days): +750 ml
The total is rounded to the nearest 50 ml and kept between 1.5 and 4.5 litres, so very low or very high body weights still get a sensible target. On a hot day, add about 250 ml.
Example: someone who weighs 70 kg (154 lb) and is lightly active needs 70 × 35 = 2,450 ml, plus 250 ml, which makes 2.7 litres, or about 11 glasses. It's the same formula Gulp Gulp Ahh uses to set your daily goal. A quick look by body weight:
| Weight | Mostly sitting | Lightly active | Active | Athlete |
|---|---|---|---|---|
| 50 kg (110 lb) | 1.75 L | 2 L | 2.25 L | 2.5 L |
| 60 kg (132 lb) | 2.1 L | 2.35 L | 2.6 L | 2.85 L |
| 70 kg (154 lb) | 2.45 L | 2.7 L | 2.95 L | 3.2 L |
| 80 kg (176 lb) | 2.8 L | 3.05 L | 3.3 L | 3.55 L |
| 90 kg (198 lb) | 3.15 L | 3.4 L | 3.65 L | 3.9 L |
| 100 kg (220 lb) | 3.5 L | 3.75 L | 4 L | 4.25 L |
| 110 kg (243 lb) | 3.85 L | 4.1 L | 4.35 L | 4.5 L |
Why activity and heat matter
Your body loses water all day through breathing, sweat and urine. When you exercise, your muscles make heat and you sweat to get rid of it, so an hour of hard training can easily cost half a litre or more. That's why the activity bonus grows the more you move.
Heat works the same way: on a warm day you sweat even when you sit still. Drink a little extra when it's hot, and more if you're working or exercising outdoors. Sipping before, during and after a workout is better than gulping a lot at the end.
Signs you might need more water
- You feel thirsty, or your mouth and lips feel dry
- Your urine is dark yellow, or you're peeing less often than usual
- You have a headache, feel tired or find it hard to concentrate
- You feel dizzy or light-headed
Pale, straw-coloured urine is usually a good sign that you're drinking enough. Thirst is a useful signal, but don't wait until you're very thirsty, especially on hot days, during exercise and as you get older, when thirst can kick in late.
Food counts too
Water doesn't only come from your glass. On average, roughly a fifth of the water we take in each day comes from food. Fruit and vegetables such as cucumber, lettuce, watermelon, strawberries, oranges and tomatoes are mostly water, and so are soup, yoghurt and milk.
Tea, coffee and other drinks count toward your fluids too. Plain water is still the easiest everyday choice, with no sugar and no calories. So on a day full of fruit, vegetables and soup, landing a little under your target is fine.
Don't overdo it
More isn't always better. Drinking a very large amount of water in a short time can lower the sodium level in your blood (hyponatraemia), which can be harmful. It's rare, but it does happen, for example in drinking challenges or very long endurance events.
The fix is simple: spread your water across the day instead of drinking litres in one go. A glass when you wake up, one with each meal and a few in between works well for most people.
If you have a kidney or heart condition, are pregnant or breastfeeding, or your doctor has told you to limit fluids, ask your doctor how much is right for you before following any calculator.
Frequently asked questions
How many litres of water should I drink a day?
It depends on your weight, how active you are and the weather; for many adults it lands somewhere between 2 and 3 litres. With the formula above, a 60 kg person who doesn't exercise needs about 2.1 litres, and an 80 kg person who works out a few times a week about 3.3 litres.
How much water should I drink per kg of body weight?
A common rule of thumb is about 35 ml per kilogram a day (roughly half an ounce per pound), plus a little extra for exercise and heat. For 70 kg that's 2,450 ml before any activity bonus.
Do tea and coffee count toward my water intake?
Yes. Tea, coffee, milk and other drinks add to your fluids. Caffeine has a mild diuretic effect, but in normal amounts these drinks still hydrate you overall. Plain water remains the best everyday choice.
Can you drink too much water?
Yes, although it's rare. Drinking several litres very quickly can dilute the sodium in your blood. Spread your water through the day and don't force down large amounts at once.
How can I remember to drink enough water?
Keep a bottle within reach, have a glass with every meal and set reminders. Gulp Gulp Ahh sets your goal with this formula and reminds you gently through the day.
This page is general information, not medical advice. If you have a health condition, talk to your doctor about how much water is right for you.
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