Water Intake Calculator
How much water you should drink a day.
Show detailsHide details
Hydration needs vary with body size, activity, and climate. This calculator estimates a sensible daily water target in litres and cups — accounting for exercise and hot or cold weather — so you can drink to your actual needs rather than a one-size-fits-all figure.
Loading calculator...
Water Intake — The Quick Answer
A common rule is about 35 ml of water per kilogram of body weight, plus extra for exercise and hot weather. For a 70 kg adult, that's roughly 2.5 litres a day before exercise.
Shortcut
Water (L/day) ≈ weight (kg) × 0.035 + 0.35 L per 30 min exercise
Rough daily water targets by weight
| Body weight | Base water/day | With 1 hr exercise |
|---|---|---|
| 50 kg | 1.75 L | 2.45 L |
| 60 kg | 2.10 L | 2.80 L |
| 70 kg | 2.45 L | 3.15 L |
| 80 kg | 2.80 L | 3.50 L |
| 90 kg | 3.15 L | 3.85 L |
This counts drinking water. Food — especially fruit and vegetables — also contributes water, so the drinking target can be a little lower for people with a produce-rich diet.
How It Works
The Formula
Weight-based hydration
Formula
Water (L) = (weight × 0.035 + exercise × 0.0117) × climate factor
Variables
Body weight
In kilograms. Larger bodies need more water — roughly 35 ml per kg is a common baseline.
Activity
Each 30 minutes of exercise adds about 0.35 L to replace sweat losses.
Environment
Hot, humid weather increases sweat losses; the calculator adds about 15% in those conditions.
Note: The '8 glasses a day' advice is a rough guideline, not a scientific rule. Actual needs vary widely — this estimate tailors the figure to you.
Worked Example: A 70 kg Adult Who Exercises 30 Minutes
Weight, activity, and climate combined
Multiply weight by 0.035 L
70 × 0.035 = 2.45 L base
Add ~0.35 L per 30 min of exercise
+0.35 L = 2.8 L
Adjust for climate
Hot or humid adds roughly 15%
Convert to glasses
≈ 11 glasses of 250 ml
Reference Guide
| Body weight | Base water/day | With 1 hr exercise |
|---|---|---|
| 50 kg | 1.75 L | 2.45 L |
| 60 kg | 2.10 L | 2.80 L |
| 70 kg | 2.45 L | 3.15 L |
| 80 kg | 2.80 L | 3.50 L |
| 90 kg | 3.15 L | 3.85 L |
How to Tell If You're Drinking Enough
A better guide than counting glasses
The simplest everyday test: pale straw-coloured urine suggests good hydration, dark yellow means drink more. Combined with rarely feeling strongly thirsty, it tells you most of what you need without tracking millilitres.
Best for: A quick, reliable daily hydration check
Despite caffeine's mild diuretic effect, tea and coffee still contribute to hydration, and water-rich foods (fruit, vegetables, soups) add a meaningful share. 'Water intake' doesn't mean plain water only — though plain water is the ideal, calorie-free option.
Best for: Counting all fluids, not just water
Needs rise with exercise, heat, illness, pregnancy, and breastfeeding — but drinking excessive amounts quickly can dilute blood sodium (hyponatraemia), a real risk in extreme endurance events. For everyday life, drink to thirst and urine colour. This is general information, not medical advice.
Best for: Avoiding both under- and over-drinking
How Much Water Do You Really Need?
The familiar 'eight glasses a day' is a handy slogan with no strong scientific basis — real needs vary with body size, activity, climate, and diet. A reasonable evidence-informed estimate is around 35 ml per kilogram of body weight, plus roughly 0.35 L per 30 minutes of exercise, with more in hot weather. For a 70 kg adult that lands near 2.5 litres before activity. This calculator tailors the figure to you rather than using one number for everyone. Remember that tea, coffee, and water-rich foods all count toward your total, and that the most practical check on your hydration is simply the colour of your urine.
Key Features
💡 Pro Tips
- →Keep a bottle in sight — visibility is one of the most effective nudges to drink more consistently through the day.
- →In hot weather or hard exercise, sip regularly rather than gulping a large amount at once.
- →If your urine is consistently dark despite drinking, or you feel dizzy, that's worth attention beyond a calculator.
- →Count tea, coffee, and milk toward your total — they hydrate too; plain water is simply the calorie-free default.
Common Mistakes
Treating '8 glasses' as a universal rule
It's a slogan, not science. A smaller, sedentary person needs less; a larger, active person in heat needs more. Tailor the amount to your body and conditions.
Assuming coffee and tea don't count
For habitual drinkers, the fluid in tea and coffee still contributes to hydration despite caffeine's mild diuretic effect. They count toward your daily total.
Force-drinking far beyond thirst
Drinking excessive amounts in a short time can dangerously dilute blood sodium. For everyday life, thirst and urine colour are better guides than hitting an arbitrary high quota.
Research & Citations
Sources and standards referenced for the factual claims on this page
- [1]
US National Academies of Sciences, Engineering, and Medicine (2005). Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press.
Reference intakes for total water from all sources.
View source - [2]
European Food Safety Authority (EFSA) (2010). Scientific Opinion on Dietary Reference Values for water. EFSA Journal, 8(3).
Adequate water intake guidance for adults.
View source
This calculator is a reference tool, not a substitute for professional advice. Verify important health, financial, legal, or safety decisions with a suitably qualified professional.
Last updated: October 7, 2026
Creators
Computer Scientist & Developer Tools Author
Reviewers
Technical Reviewer
Table of Contents
Quick Facts
Related Tools
Privacy Guaranteed
Your data never leaves your browser. All calculations are 100% private.
Frequently Asked Questions
How much water should I drink a day?
It depends on your size, activity, and climate. A common estimate is 35 ml per kg of body weight — about 2.5 litres for a 70 kg adult — plus extra during exercise and in hot weather. This calculator gives a personalised figure.
Is 8 glasses of water a day enough?
It's a reasonable ballpark for many people but not a fixed rule. Someone smaller and sedentary may need less; a larger, active person in a hot climate may need considerably more. Tailor it to your own body and conditions.
Does tea and coffee count towards water intake?
Yes, largely. Despite the mild diuretic effect of caffeine, the fluid in tea and coffee still contributes to hydration. Water is ideal, but most drinks count towards your daily total.
How do I know if I'm dehydrated?
Early signs include thirst, dark-yellow urine, tiredness, and headaches. Pale straw-coloured urine and rarely feeling thirsty are good signs you're well hydrated. Persistent dark urine or dizziness warrants attention.