Water Intake Calculator
Water is involved in virtually every bodily process: regulating temperature, transporting nutrients, flushing waste, lubricating joints, and supporting cognitive function. Even mild dehydration of 1 to 2% of body weight can impair physical and mental performance. Most adults do not drink enough water. The National Academies of Sciences, Engineering, and Medicine (formerly the Institute of Medicine) established Adequate Intake values for total daily water, which form the baseline for this calculator. Because water needs scale with body size, this calculator starts with a weight-based estimate of 35 ml per kg per day as a practical baseline (consistent with the general range derived from population studies), then applies upward adjustments for exercise and hot or humid climate. The result is given in millilitres and in standard 250 ml glasses. Remember that about 20% of your daily water typically comes from food, so your actual drinking target is somewhat below the total daily water figure. This tool provides an estimate; your doctor is the best source of personalised hydration advice.
Formula
Base intake = 35 ml * weight (kg)
Activity adjustment: +0 to +1,050 ml depending on level
Climate adjustment: +0 to +500 ml depending on heat
Total = base + activity + climate (in ml)
Water Intake Calculator: frequently asked questions
How much water should I drink per day?
The National Academies of Sciences, Engineering, and Medicine (formerly the Institute of Medicine) set an Adequate Intake (AI) of 3.7 litres of total water per day for men and 2.7 litres for women. This includes water from all sources: beverages and food. About 20% of water intake typically comes from food. For beverage intake alone, the figures are approximately 3.0 litres (men) and 2.2 litres (women) per day.
Does the '8 glasses a day' rule have scientific backing?
The origin of the 8x8 rule (eight 8-ounce glasses per day, about 1.9 litres) is unclear and it does not have strong scientific backing for universal application. Individual water needs vary substantially by body size, activity level, climate, and diet. The National Academies guidelines, used in this calculator, are more evidence-based and account for some of this variation.
Does exercise increase water needs?
Yes. Exercise increases water loss through sweat. For moderate exercise lasting less than an hour, an additional 0.5 to 1 litre of fluid is typically sufficient. For intense or prolonged exercise (more than 1 hour), fluid needs can be 1 to 2 litres or more per hour of activity. Sports drinks may be appropriate for very long or intense sessions to replace electrolytes.
Does climate affect water needs?
Hot or humid climates increase sweat losses and therefore water needs. People living or exercising in hot environments may need significantly more fluid than those in temperate climates. High-altitude environments also increase respiratory water losses. Air travel (low humidity in aircraft cabins) can increase water needs temporarily.
How do I know if I am drinking enough water?
The colour of your urine is a reliable indicator of hydration status. Pale yellow (like lemonade) indicates good hydration. Dark yellow or amber indicates dehydration. Clear urine may indicate over-hydration. Other signs of adequate hydration include infrequent thirst, moist lips and mouth, and normal energy levels. Thirst itself is a late indicator of dehydration.
Official sources
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.