How Much Water Should You Drink a Day? A Science-Based Guide

Why Hydration Matters for Health and Performance

Water accounts for approximately 60% of adult body weight and is involved in virtually every physiological process — from regulating body temperature and transporting nutrients, to lubricating joints, eliminating waste products, and supporting cardiovascular function during exercise.

Unlike macronutrients, the body has no meaningful storage reserve for water. Losses occur continuously through urine, sweat, respiration, and bowel movements — and must be consistently replaced throughout the day to maintain optimal function.

Even mild dehydration has measurable consequences. Research by Cheuvront and Kenefick (2014) found that a fluid deficit equivalent to just 1–2% of body weight — roughly 700 ml to 1.4 L in a 70 kg individual — is sufficient to impair endurance performance by 7–10%, reduce cognitive function, and increase the perceived effort during physical activity. For anyone who trains regularly, maintaining adequate daily water intake is therefore a direct performance variable, not merely a general health recommendation.

Based on: Cheuvront SN, Kenefick RW. "Dehydration: Physiology, Assessment, and Performance Effects." Comprehensive Physiology. 2014;4(1):257–285. PubMed

How Much Water Should You Drink a Day? (Official Guidelines)

Two major scientific bodies have established evidence-based daily water intake recommendations: the European Food Safety Authority (EFSA) and the US Institute of Medicine (IOM). Their figures differ slightly because the IOM figures include water from food — approximately 20% of total daily intake — while EFSA figures refer primarily to water consumed as beverages.

These figures represent baseline recommendations for sedentary adults in temperate climates. Individuals who exercise, live in hot environments, are pregnant or breastfeeding, or have higher body weights will require meaningfully more — as covered in Section 4 below.

Importantly, beverages other than plain water — including coffee, tea, milk, and juice — all contribute to total daily water intake. Despite the common belief that caffeine causes significant dehydration, research indicates that moderate caffeine consumption (up to 400 mg per day) does not meaningfully impair hydration status in habitual consumers.

Based on: EFSA Panel on Dietetic Products, Nutrition, and Allergies. "Scientific Opinion on Dietary Reference Values for water." EFSA Journal. 2010;8(3):1459. EFSA

How to Calculate Your Water Intake by Body Weight

A practical and widely used method for personalising daily water intake is the 35 ml per kilogram of body weight formula. This approach scales intake proportionally to body size, making it more accurate than a flat "glasses per day" recommendation across different body weights.

To use this formula: multiply your body weight in kilograms by 35 to get your baseline daily water intake in millilitres. For Imperial users, divide your weight in pounds by 2.205 to convert to kilograms first.

Example: a 75 kg active adult would target 75 × 35 = 2,625 ml (approximately 2.6 L) per day as a baseline before accounting for exercise or environmental factors.

For a personalised daily water intake target adjusted for your exact body weight, activity level, and climate, use our free Water Intake Calculator — it applies the 35 ml/kg formula automatically and accounts for exercise and environmental adjustments.

Factors That Increase Your Daily Water Needs

The EFSA and IOM baseline figures assume a sedentary adult in a temperate climate. Several common factors significantly increase how much water you should drink per day:

Exercise and Physical Activity

Sweat losses during moderate-intensity exercise typically range from 0.5 to 1.5 litres per hour, and can exceed 2.5 litres per hour in hot conditions at high intensities. The American College of Sports Medicine (ACSM) recommends consuming 400–600 ml in the two hours before exercise, 150–250 ml every 15–20 minutes during exercise, and approximately 1.5 times the fluid lost in sweat after exercise to achieve full rehydration. For a practical exercise hydration protocol, see our guide on hydration during exercise.

Hot or Humid Climate

Ambient temperature and humidity increase passive sweat losses even at rest. Adults in hot climates or who spend significant time outdoors during summer should add 500 ml to 1 L to their daily water intake baseline as a practical adjustment.

Pregnancy and Breastfeeding

EFSA recommends an additional 300 ml per day during pregnancy and an additional 700 ml per day during breastfeeding, above the standard female recommendation of 2.0 L, to account for increased fluid demands.

Illness

Fever, vomiting, and diarrhoea all cause rapid fluid losses that significantly exceed normal daily requirements. During illness, fluid intake should be increased substantially, and electrolyte replacement (oral rehydration salts) may be necessary in cases of prolonged vomiting or diarrhoea.

The "8 Glasses a Day" Myth: What the Science Actually Says

The recommendation to drink eight 8-ounce glasses of water per day — the "8×8 rule" — is one of the most persistently repeated health guidelines, yet it has no clear scientific basis. In a 2002 review published in the American Journal of Physiology, Heinz Valtin systematically searched the medical literature for evidence supporting the "8×8" recommendation and found none.

The actual origin of the guideline is unclear, but it may trace back to a 1945 US Food and Nutrition Board recommendation that stated adults needed approximately 2.5 litres of water daily — but crucially, included the caveat that "most of this quantity is contained in prepared foods." This nuance was dropped as the recommendation spread, transforming a contextual statement into a prescriptive rule.

The more accurate approach — supported by EFSA, IOM, and ACSM — is to use body weight and activity level as the basis for daily water intake targets, monitor hydration through urine colour (pale yellow indicates adequate hydration), and adjust for individual factors including climate, exercise, and health status. Thirst is also a reliable indicator for most healthy adults under normal, non-exercise conditions.