How Much Water Should I Drink a Day?
You've heard “8 glasses,” “2 liters,” “a gallon.” Here's the part most advice skips: the answer depends on your body. Find yours below.
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The Short Answer
If you want one sentence to remember: most healthy adults do well on 2 to 3.5 liters of total fluid per day, and where you fall in that range is set mainly by your body weight, then by how much you move and how hot your environment is. European (EFSA) and U.S. (IOM) authorities converge on this range using exactly those variables — which is why our calculator asks for them and nothing else. If you're 60 kg with a desk job, you'll land near the bottom; if you're 90 kg and training outdoors in summer, you'll push past four liters.
Why There's No Single Magic Number
The famous “eight 8-ounce glasses” rule traces back to a 1945 nutrition recommendation whose very next line — “most of this quantity is contained in prepared foods” — got dropped somewhere in popular retelling. Your actual need moves constantly: an hour of sweaty basketball can add a full liter; a day of air-conditioned rest needs less than a garden workday in August. Kidneys, thirst signals and urine color give you real-time feedback, while your calculated target gives you the starting anchor. Want to go deeper? Read about the best times to drink water or browse the weight-by-weight chart.
Frequently Asked Questions
How much water should I drink a day?
For most healthy adults the answer lands between 2 and 3.5 liters of total fluid a day, but the useful answer is personal: take 35 ml × your body weight in kg, add 500 ml for every 30 minutes of exercise and 500 ml more in hot climates. The calculator above runs this for you in seconds.
Is it true that thirst means I'm already dehydrated?
Thirst is an early warning, not an alarm bell. By the time you feel thirsty you've typically lost 1–2% of body water — mild dehydration that's easily corrected by drinking normally. For daily life, drinking to thirst plus with meals works well; use the calculator to set a sensible backdrop.
Do I need to drink more when I'm sick?
Yes — fever, vomiting and diarrhea all increase fluid losses, and congestion makes mouth-breathing dry you out faster. Add a few hundred ml to your normal target and consider oral rehydration fluids if losses are severe. See our guide to the signs of dehydration for warning symptoms.
Related Tools & Guides
Daily Water Intake Guide
What 'daily water intake' really covers — food, coffee and all.
Water Intake During Pregnancy
Hydration targets tuned for expecting mothers.
Water Intake for Weight Loss
How much water supports a calorie-deficit lifestyle.
Kids' Water Intake
Age-by-age hydration charts for children.
Water Intake for Runners
Hydration math for training days and race days.
Water Intake for the Gym
Lift, sweat, repeat — drink the right amount.