How Much Water Should You Drink a Day?

The Official Recommendation

In 2004 the Institute of Medicine (now the National Academy of Medicine) published its first comprehensive Dietary Reference Intakes for water. The numbers that emerged from that report are still the most widely cited benchmarks in the world:

A critical detail that is almost always lost when these figures are quoted in the press: total water includes all fluids and food moisture, not just plain drinking water. The IOM estimated that roughly 20 percent of daily water intake comes from solid food. Fruits, vegetables, soups, and dairy all contribute meaningfully. Subtract that 20 percent and the actual drinking-water targets become approximately 3.0 litres for men and 2.2 litres for women — much closer to everyday experience.

The ml per kg Body Weight Approach

Because body size varies enormously between individuals, many dietitians prefer a weight-based formula. The most widely used rule of thumb is 35 ml per kilogram of body weight per day for a healthy adult in a temperate climate at rest. A 70 kg person therefore needs around 2,450 ml (2.45 litres) of fluid from beverages alone.

Some clinical guidelines use a slightly different scale: 30 ml/kg for older adults (whose kidneys are less efficient at concentrating urine) and up to 40 ml/kg for people doing moderate physical work. These are starting points, not ceilings — the right amount for any individual depends on several additional factors described below.

Factors That Raise Your Needs

Body weight and composition

Larger bodies contain more metabolically active tissue and generate more heat. A person who weighs 100 kg needs substantially more water than someone who weighs 60 kg, even doing the same activities. Muscle tissue also holds more water than fat, so a muscular individual has higher baseline needs.

Physical activity

Sweat is the body's primary cooling mechanism. Depending on exercise intensity and environmental temperature, sweat losses range from 0.5 litres to over 2 litres per hour. Anyone who exercises regularly — even a 30-minute brisk walk — needs to add fluid to compensate. The American College of Sports Medicine recommends drinking 400–600 ml in the two hours before exercise and continuing to drink during and after.

Climate and heat

Living or working in a hot or humid environment increases sweat rate even at rest. Moving from a temperate climate to the tropics can double daily fluid losses. Air conditioning reduces but does not eliminate this effect. Acclimatisation takes one to two weeks and slightly reduces sweat sodium concentration, but total sweat volume often remains elevated long-term.

Health status

Fever, vomiting, diarrhoea, and urinary tract infections all increase fluid losses rapidly. Kidney stones are strongly associated with chronic mild dehydration; most urologists recommend urine output of at least 2 litres per day as a preventive measure, which typically requires drinking 2.5–3 litres. Certain medications (diuretics, some blood pressure drugs) also increase losses.

Pregnancy and breastfeeding

Plasma volume expands by roughly 50 percent during pregnancy, raising baseline needs. Breastfeeding transfers approximately 700 ml of fluid to the infant each day, so the IOM's recommendation for nursing mothers is the highest of any adult group.

The Practical Guide: Watch Your Urine

No formula can perfectly account for every individual variable. The most reliable real-time indicator of hydration status is urine colour:

Check first thing in the morning (before your first drink) for your baseline. Note that certain B vitamins, beets, and some medications can temporarily alter colour regardless of hydration status.

Does the Type of Beverage Matter?

Water is the ideal hydration source — zero calories, zero cost, immediately available to the body. However, research consistently shows that other beverages also count. Coffee and tea at moderate intakes are not dehydrating (see our article on coffee and hydration). Milk, juice, and even soft drinks contribute fluid, though they carry calories and other considerations. Alcohol is genuinely diuretic and does not count toward your daily target.

The Bottom Line

For most healthy adults, aiming for 2–2.5 litres of beverages per day will meet the IOM targets once food moisture is included, assuming a temperate climate and light activity. Add 500 ml for every hour of exercise. Adjust upward in heat, illness, or pregnancy. Use urine colour daily as your feedback loop — it is free, instant, and more accurate than any fixed number.