Hydration advice is unusually full of confident numbers with nothing behind them. Here’s a target based on your bodyweight and how much you sweat, plus an honest account of how much precision is actually available.
Daily Water Intake
A fluid target for your bodyweight, training load and climate.
Method: about 35 ml per kg of bodyweight, plus roughly 350 ml for every 30 minutes of exercise, adjusted for climate. Roughly a fifth of daily water normally comes from food, and coffee and tea count toward the total.
Estimates only — general education, not medical advice. Talk with your doctor before changing how you eat or train. More tools at whatshouldiweigh.com.
The eight-glasses rule came from nowhere
Eight glasses a day is the most repeated hydration guideline in existence and nobody can find its source. The most credible account traces it to a 1945 US National Research Council note recommending about 2.5 litres of water daily — a recommendation that, in the same sentence, pointed out most of it comes from food.
That qualifier fell away. The number survived.
Actual intake requirements scale with body size, activity, climate and diet. A 120-pound office worker in Vermont and a 220-pound roofer in Arizona do not need the same amount of water, and no single figure serves both.
What the calculator uses
About 35 ml per kg of bodyweight as a baseline, adjusted for:
- Exercise — roughly 350 ml per 30 minutes of activity
- Climate — up to 15% more in heat and humidity, around 5% at altitude
For a 75 kg person doing 45 minutes of training in a temperate climate, that lands near 3.1 litres of total water a day.
Treat it as a starting point. Sweat rates vary enormously between individuals — from under 0.5 to over 2 litres an hour during hard exercise in heat — and no formula can predict yours.
Food and coffee count
Two things most guidelines mention and most people miss.
Roughly 20% of daily water intake comes from food. Fruit and vegetables are largely water; so are soup, yogurt and most cooked grains. Someone eating plenty of produce needs less from the glass than someone living on dry food.
Caffeinated drinks count too. The idea that coffee and tea dehydrate you is largely wrong at normal intakes. Caffeine has a mild diuretic effect, but the fluid in the cup more than compensates. Research on moderate coffee drinking finds no meaningful difference in hydration status compared with water.
So your morning coffee is a hydration source, not a debt.
Better signals than arithmetic
Two indicators beat any calculator.
Thirst. It’s a well-calibrated mechanism. For healthy adults going about ordinary life, drinking when thirsty maintains hydration reliably. The main exceptions are older adults, whose thirst response blunts with age, and endurance athletes during long events.
Urine colour. Pale straw is the target. Dark yellow suggests drinking more; completely clear suggests you’re ahead of what you need.
Neither is precise. Both respond to your actual state, which no formula does.
More is not better
Worth saying, because hydration advice tends to run in one direction.
Drinking far beyond your needs doesn’t confer extra benefit, and at extremes it’s harmful. Exercise-associated hyponatremia — blood sodium diluted by excessive water intake — is a genuine risk in endurance events and has caused deaths in marathon runners who drank heavily at every station.
Practical guidance: during exercise beyond about 90 minutes, or in serious heat, include electrolytes rather than plain water alone. And don’t force intake past comfort in pursuit of a number.
Common questions
How much water should I drink a day? Around 35 ml per kg of bodyweight as a baseline, more with exercise and heat. For most adults that’s 2–3 litres of total water, roughly a fifth of which comes from food.
Does coffee dehydrate me? Not at normal intakes. The water in the cup outweighs caffeine’s mild diuretic effect.
Does drinking more water help weight loss? Modestly, and indirectly. Water before meals slightly reduces intake in some studies, and it substitutes for calorie-containing drinks. It doesn’t burn fat.
What about the half-your-bodyweight-in-ounces rule? It lands in roughly the right territory for average builds — about 30 ml/kg — and it’s easy to remember. It just ignores activity and climate, which are the variables that matter most.
Can I drink too much? Yes. Excessive intake dilutes blood sodium, which is dangerous. It’s rare in ordinary life and a real concern during long endurance events.
The calculators on this site are educational estimates, not medical advice. If you have kidney disease, heart failure, or take medication affecting fluid balance, your intake should be set by your doctor rather than a formula.