This calculator builds a daily water target from your body weight, then adjusts it for how long and how hard you exercised, the climate you are in, and whether you are pregnant or breastfeeding. It reports the result as plain water to drink, as cups, and as refills of your own bottle — plus the total including the roughly 20% of fluid most people get from food, which is the figure official guidelines actually quote. A simple hourly schedule breaks the target into manageable amounts across the day. See the TDEE calculator for the energy side of the same training load.
Water Intake Calculator
A daily water target adjusted for your weight, training and climate — in litres, cups and actual bottles.
Drink per day
—
In cups (250 ml)—
Your bottles—
Total fluid incl. food—
From food (~20%)—
Where it comes from
A simple schedule
Embed this calculator on your site — free
How much water you actually need
There is no single correct number, which is why the famous "eight glasses a day" advice has no real evidence behind it — it appears to trace back to a 1945 recommendation that also noted most of that water comes from food, a line that got dropped in the retelling. Requirements scale with body size, how much you sweat, the temperature you live in, and your diet.
The reference figures worth knowing come from the US Institute of Medicine, which sets adequate total water intake at about 3.7 litres a day for men and 2.7 litres for women. Crucially those totals include water from food and all other drinks. Roughly 20% of intake typically comes from food — fruit, vegetables, soup, yoghurt — so the amount you need to actually drink is lower than the headline figure. This calculator shows both.
The weight-based baseline
A practical starting point is 30–35 ml of water per kilogram of body weight, which is the basis used here. For a 70 kg adult that is about 2.1–2.5 litres. The common imperial version — drink half your body weight in pounds as fluid ounces — works out to roughly the same place. Both are approximations, but they scale sensibly with body size in a way that a flat "two litres for everyone" does not.
Exercise and climate adjustments
Sweat rates vary hugely between individuals, from under half a litre to over two litres an hour. As a working figure, add 350–700 ml for every 30 minutes of exercise depending on intensity. Heat and humidity raise the requirement further, and so does altitude — dry air and increased respiration at elevation cause fluid loss you may not notice because you do not feel wet.
If you want a personal number rather than an estimate, weigh yourself before and after a training session. Each kilogram lost is roughly a litre of sweat, and you should replace 1.2–1.5 times that over the following hours.
Reading your own hydration
Urine colour is the most practical everyday check: pale straw is well hydrated, dark amber means drink more, and completely clear may mean you are overdoing it. Thirst is a reasonable guide for healthy adults, though it lags behind actual need during hard exercise and becomes less reliable with age.
You can drink too much
Hyponatraemia — dangerously diluted blood sodium — is rare but genuinely serious, and it happens most often in endurance athletes who drink large volumes of plain water over several hours. If you are exercising hard for more than about 90 minutes, or sweating heavily in the heat, replace electrolytes as well as fluid. Do not force water far beyond thirst on the assumption that more is always better.
Kidney disease, heart failure and some medications change fluid requirements substantially, sometimes in the direction of restriction. Any calculator output is general guidance for healthy adults, not a clinical prescription — if you have a condition that affects fluid balance, follow your doctor's advice over this.
Related calculators
Frequently asked questions
Is eight glasses of water a day right?
It is a rough approximation that happens to land near many people's needs, but it has no strong evidence base and ignores body size, activity and climate. Eight 250 ml glasses is two litres, which suits an average sedentary adult in a temperate climate and undershoots for anyone training hard or living somewhere hot.
Do tea, coffee and other drinks count?
Yes. Despite the persistent myth, moderate caffeine does not dehydrate you — the diuretic effect is smaller than the fluid volume the drink provides, so a cup of coffee is a net gain. Milk, juice, and soft drinks all count toward fluid intake too, though for reasons of sugar and calories water remains the sensible default.
How much extra should I drink when exercising?
Roughly 350–700 ml per 30 minutes, scaled to intensity and heat. For a personal figure, weigh yourself before and after a session — each kilogram lost is about a litre of sweat, and you should replace 1.2–1.5 times that over the following few hours.
How do I know if I am drinking enough?
Check your urine colour. Pale straw means you are well hydrated, dark amber means drink more, and persistently colourless may mean you are drinking more than you need. Thirst is a decent guide for healthy adults, though it lags during hard exercise.
Can you drink too much water?
Yes. Drinking very large volumes quickly can dilute blood sodium, a condition called hyponatraemia, which is rare but potentially serious. It occurs mainly in endurance athletes drinking heavily over several hours. If you are exercising for more than 90 minutes, take in electrolytes as well as water.
Does water intake need to change with age?
The thirst signal weakens with age, so older adults often drink less than they need without feeling thirsty. The requirement itself does not rise much, but the case for drinking to a schedule rather than to thirst gets stronger. Kidney function and medications also matter more with age.