Updated September 2026 A starting estimate from bodyweight, training and climate — then let your body correct it
Where this number comes from
Hydration needs vary enormously with body size, sweat rate, diet, climate and activity — which is why no single figure fits everyone, and why you should treat this output as a starting point rather than a prescription.
We use a bodyweight-based estimate, which scales sensibly across body sizes:
Exercise = +500–700 ml per hour of training
Heat or altitude = ×1.1 to ×1.2
Alongside it we show the European Food Safety Authority’s adequate intake reference — 2.5 litres of total water a day for men and 2.0 litres for women, from all sources. The US Institute of Medicine sets comparable figures at 3.7 and 2.7 litres of total water. Those are population reference values, which is exactly what makes them poor personal targets: they describe what a typical adult consumes, not what you require.
The eight glasses myth
“Eight glasses of water a day” has no identifiable research behind it. The most commonly cited origin is a 1945 US Food and Nutrition Board recommendation suggesting roughly 2.5 litres of water daily for adults — a document that also noted, in a sentence that did not survive into popular memory, that most of this quantity is contained in prepared foods.
That omission is the whole error. Roughly 20 percent of your water intake arrives through food: fruit and vegetables are 80 to 95 percent water, and even bread is around 35 percent. Tea, coffee, milk, juice and soup all count too. Coffee’s diuretic effect is real but modest and does not offset the fluid it delivers — a cup of coffee is a net gain of water.
Better than any calculator: two free signals
- Urine colour. Pale straw is well hydrated. Dark yellow or amber means drink more. Completely clear all day generally means you are drinking more than you need, which is harmless but pointless. Note that B vitamins turn urine bright yellow regardless of hydration.
- Thirst. For healthy adults under normal conditions, thirst is a well-calibrated mechanism and drinking to it is sufficient. The main exceptions are older adults, in whom the thirst signal blunts with age, and prolonged exercise in heat, where sweat losses can outrun the sensation.
If you want a precise figure for training, weigh yourself before and after a session: each kilogram of weight lost is roughly a litre of sweat. Replacing 125 to 150 percent of that loss over the following hours is the usual guidance for rapid rehydration.
Hydration and weight loss
Water is worth attention during a diet for three fairly ordinary reasons, none of them metabolic magic.
- It makes the scale readable. Day-to-day weight swings of one to two kilograms are mostly fluid. Consistent hydration reduces the noise you are trying to see through — relevant if you are tracking a weekly rate of loss.
- Thirst is often mistaken for hunger. A glass of water before deciding you are hungry costs nothing and resolves the ambiguity.
- Higher protein needs more fluid. Nitrogen from protein metabolism is excreted through the kidneys, so if you are eating 2 g/kg or more you should drink accordingly.
What water does not do is meaningfully raise your metabolism. Studies on cold-water thermogenesis find effects in the range of a few calories per glass — real, measurable, and entirely irrelevant to your energy balance.
When more is not better
Overhydration is uncommon but genuinely dangerous. Drinking far more than the kidneys can excrete — roughly 0.8 to 1.0 litres per hour is the usual upper limit — dilutes blood sodium and can cause hyponatraemia, which in severe cases is life-threatening. It is most often reported in endurance athletes who drink heavily during long events while losing sodium in sweat.
For long or hot sessions, include electrolytes rather than water alone. And if you have kidney disease, heart failure, or take medication affecting fluid balance, your fluid intake is a clinical question — speak to your doctor, and read our medical disclaimer.
A worked example
A 78 kg person training for 60 minutes in a temperate climate.
Exercise: 60 min × 10 ml/min = 600 ml
Climate: × 1.0
Total ≈ 3.2 litres of fluid (about 13 glasses)
Then check it against reality. If your urine is pale straw and you are not thirsty, that number is right for you. If it is consistently dark, drink more regardless of what the arithmetic said. The calculator is a starting point; your kidneys are the instrument.
Water intake — FAQ
There is no research behind that figure. It most likely derives from a 1945 recommendation of about 2.5 litres daily, whose accompanying note that most of it comes from food was dropped in retelling. Roughly 20 percent of your water arrives through food, and tea, coffee and milk all count.
No. Caffeine has a mild diuretic effect, but it does not offset the fluid the drink delivers — a cup of coffee is a net gain of water. Regular coffee drinkers also develop partial tolerance to the diuretic effect.
Urine colour is the simplest reliable signal: pale straw means well hydrated, dark yellow means drink more. Completely clear all day means you are drinking more than you need. For healthy adults, drinking to thirst is generally sufficient.
Indirectly. It reduces the day-to-day scale noise that makes progress hard to read, and it resolves the common confusion between thirst and hunger. The metabolic effect of cold water is measurable but worth only a few calories per glass.
Yes, and it is genuinely dangerous. Drinking beyond what the kidneys can excrete — roughly 0.8 to 1.0 litres per hour — dilutes blood sodium and can cause hyponatraemia. It is most often seen in endurance athletes drinking heavily during long events, and is a reason to use electrolytes rather than plain water on long or hot sessions.