Drinking more water is one of the most repeated pieces of weight-loss advice there is. Some of it is true. Most of it is overstated. And one part of it (what water does to the number on the scale) catches almost everyone out.
The short answer
Drink roughly 30–40 ml per kilogram of body weight a day, plus about 500 ml for each hour of exercise and a little more in hot weather. For most adults that is 2–3 litres from drinks.
There is no separate, higher “weight loss” amount. The water you need to lose weight is the water you need anyway.
| Body weight | Daily water (approx.) | Glasses (250 ml) |
|---|---|---|
| 50 kg | 1.5–2.0 L | 6–8 |
| 60 kg | 1.8–2.4 L | 7–10 |
| 70 kg | 2.1–2.8 L | 8–11 |
| 80 kg | 2.4–3.2 L | 10–13 |
| 90 kg | 2.7–3.6 L | 11–14 |
| 100 kg | 3.0–4.0 L | 12–16 |
Your own figure, adjusted for exercise and climate: water intake calculator.
Tea, coffee, milk and the water in food all count. The idea that caffeine dehydrates you does not survive contact with the evidence at ordinary intakes.
What water actually does for weight loss
1. It can make you eat a little less
This is the best-supported effect. In several controlled trials, people who drank about 500 ml of water 20–30 minutes before meals ate less at those meals and lost modestly more weight over twelve weeks than people who did not. The effect is clearest in middle-aged and older adults.
It is not large. It is a nudge, not a mechanism. But it is real, cheap and easy.
2. It replaces drinks that have calories
This is where water does the most work, and it is usually overlooked because it is not about water at all.
| Drink | Typical serving | Calories |
|---|---|---|
| Chai with 2 tsp sugar | 1 cup | 90–120 |
| Sweet lassi | 1 glass | 250–400 |
| Fruit juice | 1 glass (250 ml) | 110–140 |
| Cold drink / soda | 300 ml | 120–150 |
| Sweetened iced coffee | 1 large | 200–400 |
| Water | any | 0 |
Someone who drinks a juice at breakfast, two sweet chais and a soft drink every day takes in 450–600 calories from drinks alone. Swapping even half of those for water is a larger deficit than most diet changes. See calories in alcoholic drinks for the other big category.
3. It is sometimes mistaken for hunger
Mild thirst and mild hunger feel alike, especially in the afternoon. A glass of water and ten minutes will sometimes settle what felt like a craving. Sometimes it will not, and then you were hungry. Both are useful to know.
What water does not do
- It does not “flush out fat”. Fat leaves the body mostly as carbon dioxide you breathe out, and a little as water. Drinking more water does not speed this up.
- It does not meaningfully boost metabolism. Drinking cold water makes your body spend a few calories warming it, somewhere around 5–10 calories a glass at most. Eight glasses is less than a biscuit.
- Warm water, lemon water, jeera water and ajwain water are all fine to drink and none of them burns fat. Their benefit, if any, is that they replace something with sugar in it.
- Drinking far beyond thirst does not help and, in extreme amounts over short periods, can be dangerous.
Why water makes the scale lie
This is the part that matters most for anyone weighing themselves.
Your body holds a variable amount of water, and that amount changes by one to two kilograms from day to day for reasons that have nothing to do with fat:
- Salt. A salty meal (a restaurant dinner, pickles, namkeen, a pizza) makes you retain water to keep sodium balanced. The next morning's weight can be 0.5–1.5 kg higher.
- Carbohydrates. Each gram of stored glycogen holds roughly three grams of water. A high-carb day refills glycogen and adds weight; a low-carb day empties it and removes weight. This is most of what people lose in the first week of any diet.
- Hard exercise. Muscles hold extra water while they repair, typically for two to four days after an unusually hard session.
- Hormonal cycles. Many women see 1–2 kg of water weight in the days before a period.
- Not drinking enough. Mild dehydration can cause the body to hold onto water once you drink normally again.
None of this is fat. All of it shows up on the scale.
What to do: weigh at the same time each morning, use the weekly average, and judge progress over three weeks rather than three days. Why your weight fluctuates daily goes into this in more detail.
Does drinking more water reduce water retention?
Somewhat, yes, counter-intuitive as it sounds. The body retains water more readily when intake is low and sodium is high. Drinking normally and keeping salt reasonable usually lets excess water go within a day or two.
It will not make water weight disappear overnight, and it is not a reason to force down four litres after a restaurant meal. Drink to thirst, and give it 48 hours.
A simple routine
- A glass on waking. Easy to remember, and it gets the first 250 ml in before tea.
- A glass before each main meal. The one habit with evidence behind it.
- Water as the default drink. Tea and coffee are fine; sugary drinks are the ones to swap.
- More on training days and hot days. Roughly 500 ml per hour of exercise.
- Check the colour. Pale straw is fine. Dark yellow means drink more.
Logging
BetterCal does not track water, deliberately. Logging every glass is exactly the sort of small chore that makes people give up tracking altogether, and it does not change your calorie numbers.
What it does track is everything with calories in it, including the drinks. “Two chais and a sweet lassi” is one sentence and is logged with the sugar counted.
Log a meal the way you would write it down. BetterCal turns a plain sentence, spoken or typed, into calories and macros. Free to start.
Download BetterCal on the App Store · Get BetterCal on Google Play
Frequently asked questions
How much water should I drink a day to lose weight?
The same as you need for general health: roughly 30–40 ml per kilogram of body weight, which is 2–3 litres for most adults, plus extra for exercise and heat. There is no special weight-loss amount.
Does drinking water help burn belly fat?
No. Water does not burn fat from any part of the body. It can help you eat slightly less, and swapping sugary drinks for it reduces calories, which leads to fat loss overall, but you cannot choose where that fat comes from.
Is it okay to drink water right after a meal?
Yes. The idea that water after meals dilutes digestion or causes weight gain has no good evidence behind it. Drink whenever you are thirsty.
Why did I gain 1 kg overnight?
Almost certainly water, not fat. Gaining a kilogram of fat takes a surplus of around 7,700 calories. A salty or high-carbohydrate meal can add a kilogram of water weight overnight, and it usually goes within two or three days.
Does warm water with lemon help weight loss?
Not directly. It is a pleasant, calorie-free drink, and if it replaces a sweet morning chai or juice it helps through that swap. The lemon and the temperature do nothing special for fat loss.
Related: water intake calculator · why you're not losing weight in a calorie deficit · what is a calorie deficit
