Water Intake Calculator
Find your personalised daily hydration target based on your body and lifestyle
Results & Details
// How We Got There
// Suggested Drinking Schedule
// Hydration Facts
How Your Water Target Is Calculated
There is no single universal hydration formula — needs vary significantly based on body size, activity, climate and health status. This calculator uses a weight-based baseline and adds adjustments for each lifestyle factor.
The Formula
Signs of Dehydration
Mild dehydration: thirst, dark urine, dry mouth, headache, difficulty concentrating. Moderate: dizziness, fatigue, reduced urine output. Severe dehydration requires medical attention — symptoms include rapid heartbeat, sunken eyes, and confusion.
Does Coffee Count?
Yes — despite being mildly diuretic, caffeinated drinks like coffee and tea still contribute net fluid to your hydration. The water they contain more than compensates for the diuretic effect in moderate quantities. Alcohol is a stronger diuretic and does not count positively toward hydration goals.
Hydration and Weight Loss
Drinking water before meals can reduce calorie intake. Water is also essential for metabolising stored fat. Some studies suggest that mild dehydration can be mistaken for hunger. Adequate hydration supports both energy levels and exercise performance.
How Much Water You Actually Need (vs the Myths)
Built and verified by Andrius R. · Updated June 2026
Hydration advice is a strange corner of health: the most famous rule has no real scientific source, the official numbers are widely misquoted, and the best gauge costs nothing. Here's what holds up.
The "8 glasses a day" rule has no author
"Eight 8-ounce glasses" (~1.9 L) is repeated everywhere, yet researchers who've hunted for its origin found no study behind it — the most cited trail leads to a 1945 US recommendation that actually said most of the needed water "is contained in prepared foods," a clause history deleted. It's not a dangerous rule, just an invented one. Real needs vary with body size, climate, activity and diet.
What the official guidance actually says
The US National Academy of Medicine's adequate intake is about 3.7 L/day total water for men and 2.7 L for women — but that's total water, including the ~20–30% that arrives in food (fruit, vegetables, soup, yogurt) and every beverage, not 3.7 L of plain drinking water. The common weight-based guideline of 30–35 ml per kg gives a 70 kg person 2.1–2.45 L/day — the approach this calculator uses, adjusted for activity and climate.
When needs genuinely rise
- Exercise: sweat losses of 0.5–2 L per hour are normal. A practical add-on is ~0.5–1 L per hour of activity; for sessions much beyond an hour, weigh yourself before and after — each kg lost ≈ 1 L to replace.
- Heat and altitude: hot, humid weather and elevations above ~2,500 m both increase losses meaningfully.
- Illness: fever, vomiting and diarrhea raise needs sharply — the scenario where dehydration gets dangerous fastest.
- Pregnancy and breastfeeding: guidance adds roughly 0.3 L and 0.7–1 L respectively.
Coffee and tea count toward intake — the diuretic effect of normal caffeine doses is too small to cancel their water content. That one's a myth too.
The best hydration meter is free
Thirst is a decent guide for most healthy adults most of the time, and urine color is the cheap objective check: pale straw = hydrated; dark amber = drink more; completely clear all day = you can ease off. (Caveats: B-vitamin supplements turn urine bright yellow regardless, and thirst sensitivity declines with age, so older adults benefit from drinking on schedule rather than on demand.) Chasing a gallon jug when your urine is already pale adds bathroom trips, not health.
Yes, too much water is a real danger
Drinking far beyond your losses dilutes blood sodium — hyponatremia — which in severe cases causes confusion, seizures and death. It's rare in daily life but a documented killer in endurance events, where well-meaning athletes overdrink plain water for hours while sweating out salt. The marathon-day rule is to drink to thirst, not to a schedule of fear. For everyday life the takeaway is simpler: hydration is a target with a comfortable middle, not a virtue to maximize.
// Morning First
Drink a large glass of water immediately on waking — you lose about 500ml overnight through breathing and sweating.
// Urine Check
Pale straw yellow = well hydrated. Clear = possibly over-hydrated. Dark yellow = drink more immediately.
// Don't Wait for Thirst
Thirst is a late signal of dehydration. By the time you feel thirsty, performance may already be affected.
// Food Counts
Fruits and vegetables provide up to 20% of daily water needs. Cucumber, lettuce and watermelon are over 90% water.