The famous "8 glasses a day" rule was never based on much — it ignores body size entirely. A 50 kg office worker and a 100 kg construction worker plainly do not need the same amount of water, and this calculator accounts for that.
Enter your weight and activity level and you get a daily water target in liters and glasses, with guidance on when to drink more: hot weather, workouts, illness and high altitude all raise needs. About 20% of daily water typically comes from food, so the target covers total intake including meals.
Part of the Health Calculators collection on CountCrunch.
How the target is calculated
Example: a 70 kg moderately active person gets a base of about 2.1–2.5 liters per day, plus roughly half a liter for a 30-minute run. Sedentary people use the lower end of the range; very active people the higher end.
When you need more water
- Heat and humidity: add 500 ml–1 liter on hot days; sweat losses climb fast above 30°C.
- Exercise: drink 400–800 ml per hour of activity, more in heat.
- Illness: fever, vomiting and diarrhea all increase fluid loss — replace aggressively.
- Pregnancy and breastfeeding: needs rise by roughly 300 ml and 700 ml per day respectively.
- High altitude and air travel: dry air increases invisible water loss.
Signs you are under-hydrated
Dark yellow urine, headache, fatigue, dizziness and dry mouth are the classic early signals. Pale straw-colored urine through the day is the simplest sign you are drinking enough.
Frequently asked questions
Is 8 glasses of water a day accurate?
It is a rough average that happens to suit many adults, but it ignores body size and activity. Smaller sedentary people may need less; larger active people often need more. A weight-based target is more personal.
Does coffee count toward water intake?
Mostly yes. Caffeine is a mild diuretic, but the water in coffee, tea and other drinks still counts toward hydration. Water remains the best default.
Can you drink too much water?
Rarely, but yes — excessive intake in a short time can dilute blood sodium (hyponatremia), mostly a risk in endurance events. Spread intake through the day rather than chugging liters at once.
How do I know if I am drinking enough?
Check your urine color: pale straw means well hydrated, dark yellow means drink more. Steady energy and infrequent headaches are good secondary signs.