❤️ Health
How Much Water Should You Drink a Day?
The old 'eight glasses' rule is a rough guess. Here's how daily water needs really scale with body weight and exercise — and how to estimate a sensible personal target.
Updated 14 September 2026 · 2 min read
"Drink eight glasses of water a day" is the advice everyone has heard — and it's a reasonable ballpark, but it ignores the two things that matter most: how big you are and how much you sweat. A 55 kg office worker and a 95 kg runner have genuinely different needs. Here's a more sensible way to estimate yours.
Start from your body weight
A widely used starting point is roughly 35 millilitres of water per kilogram of body weight per day. That already personalises the target far better than a fixed number of glasses:
The water intake calculator does this for you in kilograms or pounds, and shows the result in litres, millilitres, cups and fluid ounces so you can track it however you prefer.
Add for exercise
Every session of exercise costs you water through sweat and faster breathing, so the calculator adds an allowance on top of the baseline — about 350 mL for each 30 minutes of moderate activity. A 70 kg person who trains for an hour would land nearer 3.1 litres than 2.45. On a hot day, or during hard or long exercise, you'll need more still.
If you're tracking exercise for weight or fitness goals too, our guide to TDEE and calories explained covers how activity feeds into your energy needs the same way it feeds into hydration.
What the estimate leaves out
A formula can only ever be a starting point. Real needs shift with several things it can't see:
- Climate and altitude — heat, humidity and high altitude all raise how much you lose.
- Food and other drinks — roughly a fifth of most people's water comes from food, and tea, coffee, milk and juice all count toward the total.
- Health and life stage — illness with fever, pregnancy and breastfeeding all increase needs; some medical conditions require a limit set by a doctor.
Treat the calculator's figure as a sensible daily aim, not a strict quota — spread it through the day rather than gulping it all at once, and adjust up when it's hot or you've trained hard. For the bigger picture of body-weight targets, the BMI calculator and our note on what BMI tells you are a useful companion.
Frequently asked questions
- Is drinking eight glasses of water a day accurate?
- It's a rough average, not a rule. Eight 250 ml glasses is about two litres, which suits a smaller, less active person reasonably well but underestimates the needs of someone larger or exercising hard. Scaling to your body weight and activity, as this calculator does, gives a more personal figure.
- Do tea, coffee and food count toward my water intake?
- Yes. Almost all drinks — including tea and coffee — count toward your daily fluid, and food provides roughly a fifth of it. The calculator estimates total water needs, so you don't have to hit the figure from plain water alone.
- Can you drink too much water?
- Rarely, but yes — drinking very large amounts in a short time can dilute the sodium in your blood, which is dangerous. This is uncommon in everyday life. Spreading your intake through the day and drinking to thirst keeps you well clear of it.