Water Intake Calculator

Water (mL/day) = (weight × 33 + exercise minutes × 8) × climate factor

weight

Body weight in kilograms (kg)

exercise minutes

Minutes of exercise in a day

climate factor

1 in a temperate climate, 1.1 when it is hot or humid

Use it as a starting point, then correct it from what your body tells you — thirst, urine colour and how you feel in training all carry more information than an estimate does. If you are training hard in the heat, the more reliable method is weighing yourself before and after and replacing each kilogram lost with a litre. To see what the exercise itself costs in energy rather than in fluid, the calories burned calculator covers that side.

1. A 70 kg adult, 30 minutes of exercise, temperate climate

   1. Base: 70 × 33 = 2310 mL
   2. Exercise: 30 × 8 = 240 mL
   3. Temperate climate, so no adjustment: 2310 + 240 = 2550 mL of total water
   4. Food supplies about 20%: 2550 × 0.8 = 2040 mL to drink
   5. That is 2.0 litres a day

   The 2550 mL is total water and the 2040 mL is what has to be drunk. Mixing the two up is the usual error, and it runs in the direction of drinking too much — the total already counts the water in your rice, soup and fruit.

2. An 80 kg adult, 45 minutes of exercise, hot and humid

   1. Base: 80 × 33 = 2640 mL
   2. Exercise: 45 × 8 = 360 mL
   3. Hot or humid adds 10%: (2640 + 360) × 1.1 = 3300 mL of total water
   4. Food supplies about 20%: 3300 × 0.8 = 2640 mL to drink
   5. That is 2.6 litres a day

   Two different things are being adjusted for here, and only one of them is well behaved. Body weight and exercise time both scale the estimate in proportion; heat does not, because sweat rate varies several-fold between people and between seasons. The 10% is a disclosed convention, not a measurement.

Two of the three factors are estimates, and one of them is loose. The food share is fixed at 20 percent of total water, which fits a mixed diet and drifts with it: soups, fruit and vegetables push it up, a diet of dry food pushes it down. The exercise allowance of 8 millilitres a minute is an order-of-magnitude figure, because sweat rate varies several-fold between people and between summer and winter. Neither factor covers the extremes — endurance events and hot-weather work bring electrolyte losses this page says nothing about, and children, pregnancy and breastfeeding all have their own recommendations.

How much water should I drink a day?

For an average adult in a temperate climate, somewhere between 1.5 and 2 litres of drinks is the range the public recommendations land in, and this calculator will put you in it for most body weights. Above that, the answer depends on your size, how much you sweat and how much water your food carries. Thirst is a good guide for a healthy adult at rest; it is a poor one during hard exercise, where you can fall behind before you feel it.

Is 8 glasses a day a real rule?

No — it is a round number that spread because it is easy to remember, not because it was measured. The reference intakes are in the same neighbourhood, which is why the rule survives, but they differ by sex, size and climate, and roughly a fifth of the total arrives in food rather than in a glass. Treating 8 glasses as a target to hit rather than a rough centre is harmless for most people and unhelpful for anyone much larger or much smaller than average.

Why does this page show three different numbers?

Because they answer different questions. The Chinese guideline and the US adequate intake are population references — a single figure meant to keep almost everyone out of deficit, which is why neither changes with your body weight. The estimate at the top of the table is built from your weight and your routine, so it moves when you do. Neither kind is wrong, and the gap between them is real information rather than an error to be resolved.

Can I drink too much water?

Yes, though it takes effort. Drinking far more than you lose dilutes the sodium in your blood, a condition called hyponatraemia, and it has occurred in endurance events where people drank on a schedule regardless of thirst. The risk is highest for slow runners in long races who drink a lot of plain water over several hours. Spreading intake across the day and letting thirst have a vote is enough protection for ordinary life.

Do coffee and tea count?

They do, and so does most of what you drink. The idea that caffeinated drinks dehydrate you comes from small short-term studies, and the effect does not hold up at ordinary intakes — the fluid in the drink more than covers the mild diuretic effect. Alcohol is the exception worth separating: it does increase fluid loss, and it is not a good way to rehydrate after exercise.

• Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate — Institute of Medicine — National Academies Press, 2005
• 《中国居民膳食指南（2022）》核心推荐 — 中国营养学会（福建省卫生健康委员会转载）
• Scientific Opinion on Dietary Reference Values for water — EFSA Panel on Dietetic Products, Nutrition, and Allergies — EFSA Journal, 2010