Daily target
2.8L
total fluid, food and drinks included
In glasses
12
240 ml cups
In fluid ounces
96
US fl oz
Where the target comes from
Baseline only
2.5 L
33 ml per kg of body weight
Per waking hour
177 ml
spread across ~16 hours
How the Water Intake Calculator Works
The formula
The baseline is 33 ml per kilogram of body weight, sitting in the middle of the widely cited 30-35 ml/kg range for healthy adults. Exercise adds 350 ml per 30 minutes, and a hot or humid climate lifts the baseline by 15%. The uplift applies to the baseline only — exercise fluid is counted separately and would otherwise be inflated twice.
Why scale by weight?
Because "eight glasses a day" ignores the person drinking them. Eight 240 ml glasses is about 1.9 litres — a reasonable average, but a 55 kg office worker and a 95 kg runner have very different needs. Scaling by body weight gets you materially closer with barely more effort.
This is total fluid, not glasses of water
The target covers everything you take in. Tea and coffee count despite their mild diuretic effect, and food typically supplies around 20% of daily fluid — more if you eat plenty of fruit, vegetables and soup. You do not need to drink the whole figure from a glass.
Treat it as a starting point
Illness, pregnancy, breastfeeding, altitude and certain medications all shift requirements, and some medical conditions call for restricted fluid intake. Urine colour is the simplest day-to-day check — pale straw is the goal. And more is not always better: drinking far beyond what your kidneys can excrete dilutes blood sodium, which is genuinely dangerous, if rare outside endurance events.
Frequently asked questions
How much water should I drink a day?
A common guideline for healthy adults is 30-35 ml of fluid per kilogram of body weight per day, which this calculator applies at 33 ml/kg. For an 80 kg person that is roughly 2.6 litres. Exercise, heat, illness and pregnancy all push the figure up, so treat it as a starting point rather than a precise prescription.
Is the '8 glasses a day' rule true?
It is a rough approximation with no strong scientific basis. Eight 240 ml glasses is about 1.9 litres, which is in the right ballpark for an average adult but ignores body size and activity entirely. A 55 kg desk worker and a 95 kg runner have very different needs — scaling by weight is a better starting point.
Does coffee, tea or food count towards my intake?
Yes. Tea and coffee are mostly water and count towards your total despite their mild diuretic effect. Food typically contributes around 20% of daily fluid intake, more if you eat a lot of fruit, vegetables and soup. The target here is total fluid, not water drunk from a glass.
How much extra should I drink when exercising?
Roughly 350-500 ml for every 30 minutes of exercise, which this calculator adds at 350 ml per 30 minutes. Actual sweat losses vary widely with intensity, heat and individual physiology — endurance athletes often weigh themselves before and after training, since each kilogram lost is about a litre of fluid to replace.
Can I drink too much water?
Yes, though it is rare. Drinking far more than your kidneys can excrete dilutes blood sodium, a condition called hyponatraemia, which is dangerous and occasionally fatal. It mostly affects endurance athletes who overdrink during long events. Spreading intake through the day rather than forcing large volumes at once avoids it.
How do I know if I'm properly hydrated?
Urine colour is the simplest everyday check: pale straw suggests you are well hydrated, while dark amber suggests you need more. Thirst is a reasonable guide for most healthy adults, though it becomes less reliable with age. Any calculated target is an estimate — your body's signals are the better feedback.