Hydration Calculator
Personalized daily water intake based on body weight, activity, climate, coffee and alcohol.
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The '8 glasses a day' rule has no scientific basis — it was invented in 1945 and has never been validated in a clinical trial. Actual water needs depend on body weight, activity level, climate, how much coffee or alcohol you drink, and the water content of your food. This calculator uses those inputs to give you a personalised daily target that reflects your real physiology, not a one-size-fits-all myth.
Example
Activity: 45min exercise
Calculation
Base = Body weight (kg) × 35ml Exercise add = Duration (h) × 500ml Heat add = 0–1,000ml based on climate Coffee net = −75ml per cup (coffee content minus mild diuretic effect) Alcohol add = +250ml per standard drink Total = Base + adjustments
Why Your Needs Vary So Much
Body size is the biggest driver — larger bodies have more cells to hydrate. Exercise loses 0.5–2L/hour through sweat depending on intensity and heat. Hot weather adds 500ml–1L/day. Coffee and tea count toward fluid intake (mild diuretic effect is overblown); alcohol genuinely dehydrates and requires offsetting. Food contributes 20–30% of daily fluid in a diet rich in fruits and vegetables.
Frequently Asked Questions
Does coffee dehydrate you?
Mildly. At typical doses (1–3 cups/day), the diuretic effect is minimal and the water content largely offsets it. At 5+ cups it becomes meaningful.
Is sparkling water as hydrating as still?
Yes — carbonation doesn't affect hydration. Sparkling water counts the same as still water.
How do I know if I'm drinking enough?
Urine colour is the most practical indicator: pale yellow = good. Dark yellow = drink more. Colourless = drinking too much.
Can I drink too much water?
Hyponatremia (water intoxication) is rare but real, mainly in endurance athletes who drink enormous amounts without electrolytes. For most people, thirst is a reliable guide.