Daily water by body weight
| Body weight | Approx. daily water | In cups (240 ml) |
|---|---|---|
| 50 kg (110 lb) | ~1.8 litres | ~7–8 |
| 60 kg (132 lb) | ~2.1 litres | ~9 |
| 70 kg (154 lb) | ~2.5 litres | ~10 |
| 80 kg (176 lb) | ~2.8 litres | ~12 |
| 90 kg (198 lb) | ~3.2 litres | ~13 |
| 100 kg (220 lb) | ~3.5 litres | ~15 |
Read this before using the numbers
The 35 ml-per-kilogram figure is a widely repeated heuristic, not a validated clinical guideline, and the honest position is that no single formula fits everyone. The US National Academies took a different approach entirely: rather than prescribe a target, they reported observed adequate intakes of roughly 2.7 litres a day of total water for women and 3.7 litres for men — and stressed that the great majority of healthy people meet their needs perfectly well by drinking to thirst.
Two things in that are worth holding onto. First, those figures are total water, which includes everything you drink and the water contained in what you eat — typically about a fifth of intake, and much more if you eat a lot of fruit, vegetables, soup, or yoghurt. Second, they are observations of what healthy people do, not thresholds you fail by missing.
Where the eight-glasses rule came from
It is not from research. The most commonly traced origin is a 1945 US Food and Nutrition Board recommendation of about 2.5 litres of water a day — which, in the very next sentence, noted that most of this quantity is contained in prepared foods. The qualifier dropped away, the number survived, and eight glasses of additional water became folk wisdom. There is no good evidence that healthy adults with normal thirst need to hit a fixed count of glasses.
What actually changes your needs
- Exercise. Add roughly 350–700 ml per hour of training, more in heat. Sweat rates vary enormously between individuals — from under half a litre to well over two litres an hour — so this is one of the widest ranges on the list.
- Heat and humidity. Both raise sweat losses substantially. Humidity matters because sweat that cannot evaporate does not cool you, so you keep producing more of it.
- Altitude. Increases fluid loss through faster breathing and increased urination.
- Illness. Fever, vomiting, and diarrhoea raise requirements sharply, and this is when dehydration becomes genuinely dangerous rather than merely uncomfortable.
- Pregnancy and breastfeeding. Both raise needs — breastfeeding notably so.
- Medication and medical conditions. Diuretics increase losses. Some kidney and heart conditions require fluid restriction, in which case a chart like this does not apply to you at all and your clinician's advice does.
The check that beats counting
Urine colour is a decent, free proxy for hydration status and requires no arithmetic. Pale straw suggests you are well hydrated; dark yellow or amber suggests you should drink more. Two caveats: B vitamins turn urine bright yellow regardless of hydration, and first-morning urine is normally darker than the rest of the day. Thirst is also a more reliable signal than it is usually given credit for — in healthy adults with access to fluid, it works.
Signs that you have gone too far the other way matter as well. Consistently clear, colourless urine with very frequent urination usually just means you are drinking more than you need. Genuine overhydration — hyponatraemia, where blood sodium drops dangerously — is rare but real, and occurs mostly in endurance athletes who drink large volumes of plain water over many hours. It is a reminder that more is not automatically better.
Does drinking water help weight loss?
Modestly, and mostly through indirect routes. Water has no calories, so replacing sugary drinks with it removes intake without requiring any change in food. Drinking before a meal can slightly increase fullness. And mild dehydration is sometimes misread as hunger. None of these is a fat-loss mechanism in itself, and water does not meaningfully "boost metabolism" — but the substitution effect alone can be worth several hundred calories a day for someone who drinks a lot of soft drinks.
Making it easier
Most people who struggle with hydration are not lacking motivation; they lack a cue. Attaching a glass of water to things you already do reliably — waking up, each meal, arriving at your desk, before a workout — works better than intending to drink more. A visible bottle on the desk beats one in a bag. And if plain water bores you, tea, coffee, milk, and sparkling water all count toward total fluid; the mild diuretic effect of caffeine does not cancel out the fluid the drink contains. See how to drink more water every day for practical habits, and signs of dehydration for what to watch for.