How Much Water Should You Drink Per Day?

The widely repeated "8 glasses a day" rule is a rough heuristic — not a personalised target. Your actual daily water need depends on your body weight, how active you are, and the climate you live in. A sedentary 60 kg person in a cool climate needs far less than a 90 kg athlete training in summer heat.

The Evidence-Based Formula

Health authorities (including the EFSA and Institute of Medicine) use total fluid intake targets that account for body size. A practical working formula:

Base intake = 30–35 ml per kg of body weight

For a 70 kg adult: 70 × 33 = 2,310 ml (about 2.3 L) as a baseline.

Activity and climate add to this base:

Factor	Additional fluid
Sedentary	+ 0 ml
Lightly active (1–3 days/week)	+ 350 ml
Moderately active (3–5 days/week)	+ 700 ml
Very active (6–7 days/week)	+ 1,000 ml
Athlete / intense daily training	+ 1,500 ml
Hot or humid climate	+ 500 ml
Very hot / outdoor labor	+ 1,000 ml

A 70 kg moderately active adult in a temperate climate: 2,310 + 700 = 3,010 ml (~3 L/day).

Does the "8 Glasses" Rule Hold Up?

The 8 × 8 oz = 2 L/day rule applies roughly to a sedentary adult around 60–70 kg in a cool environment. It significantly underestimates needs for larger or more active people and overestimates for smaller, sedentary individuals.

The origin of this guideline is often traced to a 1945 US Food and Nutrition Board recommendation that noted "most of this quantity is contained in prepared foods" — meaning total fluid from all sources (food and drink combined), not just plain water. The "8 glasses" interpretation lost that caveat over decades of repetition.

Does Coffee Count?

Yes. Despite mild diuretic effects, caffeinated drinks still contribute net fluid. Multiple studies confirm that moderate caffeine intake (up to 400 mg/day, equivalent to 3–4 cups of coffee) does not cause meaningful net fluid loss in regular caffeine consumers. Coffee, tea, and other beverages count toward your daily total.

How to Know If You're Hydrated

The most practical check is urine color:

• Clear to pale yellow — well hydrated
• Pale yellow (like lemonade) — good hydration
• Dark yellow — mildly dehydrated, drink more
• Amber or orange — significantly dehydrated
• Completely clear — possible overhydration; slow down

Other signs of dehydration: headache, fatigue, difficulty concentrating, dry mouth. By the time you feel thirsty, mild dehydration has already begun.

Special Circumstances That Increase Needs

• Illness with fever, vomiting, or diarrhea — fluid and electrolyte replacement is critical.
• Pregnancy — +300 ml/day above baseline.
• Breastfeeding — +700 ml/day above baseline.
• High altitude — above 2,500 m, fluid needs increase due to higher respiration rate.
• High-fiber diet — fiber absorbs water; adequate fluid prevents constipation.

Consult a healthcare provider for personalised guidance in any medical context.

Water from Food

Roughly 20% of daily water intake comes from food. Fruits and vegetables have high water content:

Food	Water content
Cucumber	96%
Lettuce	95%
Celery	95%
Watermelon	92%
Oranges	86%

A diet rich in whole fruits and vegetables provides 400–600 ml of water per day from food alone, which counts toward your total intake target.

Calculate Your Personal Target

Use the Water Intake Calculator to get a personalised daily target based on your weight, activity level, and climate — shown in liters, milliliters, fluid ounces, and number of glasses or bottles.