How Much Water Should You Drink a Day?
By Jesse · Last updated August 20, 2026
Every widely quoted answer to this question is either a fixed number that ignores your body, or an official reference value that is not a drinking target at all. This guide takes the three answers in circulation, works out what each one actually implies for a real adult, and shows where they disagree. If you want a number now, the water intake calculator applies the weight-based approach described below.
Where "8 glasses a day" came from
The rule has no scientific basis. Its likely origin is a 1945 US Food and Nutrition Board recommendation of about 2.5 litres of water a day — a document that also noted most of that amount is already contained in food. The qualification was dropped, the number survived, and the "eight 8-ounce glasses" version spread from there. A 2002 review in the American Journal of Physiology went looking for evidence supporting it as a universal recommendation and found none.
Its deeper problem is structural rather than historical: it is a fixed volume applied to bodies of every size. Eight 8-ounce glasses is 1.9 L, or 64 fl oz. Compared against the weight-based baseline of 32.5 ml per kilogram, it lands correctly at exactly one body weight:
| Body weight | Weight-based (32.5 ml/kg) | 8x8 rule | Difference |
|---|---|---|---|
| 50 kg (110 lb) | 1.6 L (6.9 cups) | 1.9 L | −0.3 L |
| 60 kg (132 lb) | 1.9 L (8.2 cups) | 1.9 L | +0.1 L |
| 70 kg (154 lb) | 2.3 L (9.6 cups) | 1.9 L | +0.4 L |
| 80 kg (176 lb) | 2.6 L (11.0 cups) | 1.9 L | +0.7 L |
| 90 kg (198 lb) | 2.9 L (12.4 cups) | 1.9 L | +1.0 L |
| 100 kg (220 lb) | 3.3 L (13.7 cups) | 1.9 L | +1.4 L |
| 110 kg (243 lb) | 3.6 L (15.1 cups) | 1.9 L | +1.7 L |
Baseline before any exercise adjustment. The difference is the weight-based figure minus the fixed rule.
The two agree at around 58 kg and diverge in both directions. For a 110 kg adult the rule is short by roughly 1.7 L, which is the sort of error a rule of thumb should not be making for people it is handed to indiscriminately.
What the official reference values actually say
The European Food Safety Authority set Adequate Intake values for water from large population surveys. The critical detail, and the one most often dropped when these figures are quoted, is that they are total water from all sources — food included. Roughly 20 to 30% of daily water intake comes from food for most people, particularly fruit, vegetables, and anything cooked in water.
| Group | Total water (all sources) | From drinks, if food supplies 20 to 30% |
|---|---|---|
| Adult men | 2.5 L | 1.8 L to 2.0 L |
| Adult women | 2.0 L | 1.4 L to 1.6 L |
| Pregnant women | 2.3 L | 1.6 L to 1.8 L |
| Breastfeeding women | 2.7 L | 1.9 L to 2.2 L |
Total water values from EFSA (2010). The drinks-only column is our arithmetic, not an EFSA figure.
The US National Academies set higher totals — 3.7 L a day for men and 2.7 L for women — which also include food. Neither body publishes a single number to drink, and that is deliberate: requirements vary too much with body size, climate, and activity for one figure to be meaningful.
An honest note about the number this site uses
Our calculator uses 32.5 ml per kilogram, the midpoint of the 30 to 35 ml/kg range widely used in clinical nutrition. Two caveats belong with it, and they are easier to see once the arithmetic is on the page.
First, sources quoting 30 to 35 ml/kg are not consistent about whether it means total water or fluid from drinks. We treat it as a drinking target, which is the more generous reading: at 80 kg it gives 2.6 L, against roughly 2.0 Lof drinks implied by EFSA's total for men. Second, a per-kilogram rule scales with all body mass, including fat, so it over-estimates for people carrying a lot of it. Neither caveat makes the estimate useless — it is a starting point that adapts to your size, which is more than the fixed rule manages — but it is not a physiological requirement, and drinking somewhat less than it while showing no signs of dehydration is not a problem to fix.
Exercise is the adjustment that matters
Sweat loss dwarfs every other variable you control. A general estimate is 0.5 to 1 litre of extra fluid per hour of moderate exercise, and a warm weather run can exceed that comfortably. Because sessions are spread across a week, the calculator prorates the addition rather than front-loading a single day:
| Training volume | Added per day | Daily total |
|---|---|---|
| Sedentary (little or no exercise) | — | 2.6 L (11.0 cups) |
| Lightly active (1-3 days/week) | +250 ml | 2.9 L (12.0 cups) |
| Moderately active (3-5 days/week) | +500 ml | 3.1 L (13.1 cups) |
| Very active (6-7 days/week) | +750 ml | 3.4 L (14.2 cups) |
| Athlete (twice-daily training) | +1000 ml | 3.6 L (15.2 cups) |
Weight-based baseline of 2.6 L at 80 kg plus the activity adjustment.
Losing about 2% of body weight as fluid is enough to measurably reduce endurance performance and concentration, which is the practical reason to replace what you sweat rather than a reason to over-drink in advance. For sessions under an hour in comfortable conditions, drinking to thirst before and after covers it.
Measure your own sweat rate instead of guessing
Sweat rates differ enough between people that any table is a rough guide. Measuring yours takes one session and no equipment beyond bathroom scales:
- Weigh yourself in minimal clothing immediately before training, after using the bathroom.
- Train as normal for a known length of time, noting how much fluid you drink during the session.
- Towel off and weigh yourself again in the same clothing immediately afterwards.
- Sweat loss in litres is roughly the weight lost in kilograms plus the fluid you drank. Divide by the session length to get your rate per hour.
One kilogram of body weight lost over a session is about one litre of sweat. Repeat it once in cool conditions and once in heat and you will have a range that is worth more than any published average, because it is yours.
Does coffee count?
Yes. The belief that caffeinated drinks dehydrate you does not survive contact with the evidence: caffeine's mild diuretic effect does not offset the fluid it arrives in. A controlled crossover study in habitual coffee drinkers found no difference in hydration status between four cups of coffee a day and the same volume of water. Water, milk, juice, tea, and coffee all count toward the total.
How to tell whether you are drinking enough
- Urine colour, through the day. Pale straw is fine. Dark amber suggests drinking more. Consistently colourless urine suggests you are drinking more than you need, which is not dangerous but is not doing anything either.
- Thirst, with one caveat. It is a real signal and adequate for most people in comfortable conditions, but it lags behind actual fluid deficit, and the lag is larger in older adults and during hard exercise.
- Weight change around training. The most direct measure available to you, per the protocol above.
- Headache, poor concentration, unusually hard sessions. Non-specific, but worth checking hydration against before looking for more exotic explanations.
What raises your needs
- Heat and humidity. Sweat losses rise substantially, and humidity makes sweat less effective at cooling, so more of it is produced.
- Altitude. Increased respiratory water loss plus a mild diuretic response in the first days at height.
- Fever. Roughly 150 ml extra per degree Celsius above normal body temperature.
- A high-fibre diet. Fibre needs water to move through the digestive tract comfortably.
- Pregnancy and breastfeeding. EFSA adds about 300 ml a day in pregnancy and 700 ml while breastfeeding.
Can you drink too much?
Yes, though it is rare for healthy adults eating normally. Drinking a very large volume of plain water in a short period can dilute blood sodium to dangerous levels — hyponatremia — and it is seen mostly in endurance events where athletes drink aggressively without replacing sodium. The failure mode is volume plus speed plus no electrolytes, not a high daily total spread across the day. At any sensible target, in a person with healthy kidneys, the risk is essentially zero.
Common questions
How much water should I drink a day?
There is no single correct number. A defensible starting point is 32.5 ml per kilogram of body weight, which is 2.6 L at 80 kg, plus roughly 0.5 to 1 litre for every hour of hard exercise. Authorities publish total water intakes rather than drinking targets because needs vary too much with body size, climate, and activity.
Is 8 glasses of water a day accurate?
Eight 8-ounce glasses is about 1.9 L. As a fixed number it happens to match the weight-based estimate at around 58 kg and drifts in both directions from there — too much for a small adult, too little for a large one. Its origin is a 1945 recommendation that already counted the water in food, and a 2002 review found no evidence for it as a universal rule.
Does coffee count toward water intake?
Yes. Caffeine has a mild diuretic effect that does not offset the fluid the drink delivers. A controlled crossover study of moderate coffee drinkers found no difference in hydration status between coffee and an equal volume of water. Tea, milk, and juice all count as well.
Do I need more water when I exercise?
Yes, and it is the largest adjustment most people make. Sweat losses of 0.5 to 1 litre an hour are typical in moderate conditions and can be considerably higher in heat. Losing about 2% of body weight as fluid is enough to measurably reduce endurance performance and concentration.
How can I tell if I am drinking enough?
Pale straw-coloured urine through the day is the simplest indicator; dark amber suggests drinking more, and consistently colourless urine suggests more than you need. For training, weighing yourself before and after a session gives a direct measure: each kilogram of body weight lost is roughly a litre of sweat.
Can you drink too much water?
Yes, though it is rare in healthy adults eating normally. Drinking a very large volume of plain water quickly can dilute blood sodium to dangerous levels, a condition called hyponatremia, and it is seen mainly in endurance events where athletes drink aggressively without replacing sodium. Drinking to a sensible target across the day carries essentially no risk.
These are general guidelines for healthy adults, not medical advice. If you have kidney disease, heart failure, or another condition that affects fluid balance, your doctor or dietitian should guide your fluid intake. See our methodology for the sources behind these recommendations.