Water Intake Calculator
How much water to drink per day
Enter your weight and daily exercise for a practical drinking target in ounces, cups and litres. This water intake calculator uses the common body-weight rule, and shows it next to the official reference intake so you can see how a popular rule of thumb compares with the published guidance — they are closer than you would expect, but they measure different things.
The rule this calculator uses
The estimate is the widely used heuristic: half your body weight in pounds, taken as ounces, plus roughly 12 oz for every 30 minutes of exercise, plus 16 oz in a hot or humid climate. For a 170 lb adult doing 30 minutes a day that is 97 oz — about 12.1 cups, 2.9 litres, or roughly six standard bottles.
It is a rule of thumb, not a clinical guideline, and it is worth saying so plainly. Its virtue is that it scales sensibly with body size and activity and lands close to published reference figures for an average adult. Its limitation is that it takes no account of diet, health conditions, medication, pregnancy or altitude.
The official figures measure something different
The US National Academies' adequate intake for adults is 3.7 litres a day for men and 2.7 litres for women. Those numbers are widely quoted as drinking targets and that is a misreading: they are total water, and roughly 20% of it comes from food.
Converted to what you actually have to drink, 3.7 L of total water is about 3.0 litres — 12.5 cups. For women, 2.7 L total is about 2.2 litres, or 9.1 cups. Set the optional reference field and the calculator shows both figures side by side rather than making you do that conversion.
Notice how close the two methods land for an average adult male: 2.9 litres from the body-weight rule against 3.0 litres from the reference figure. That agreement is the best argument for the heuristic — and the divergence at the extremes of body weight is the best argument for not treating it as precise.
How much water should I drink by weight
| Body weight | Temperate climate | In cups | In litres | Hot or humid |
|---|---|---|---|---|
| 120 lb | 72 oz | 9 | 2.1 L | 88 oz |
| 150 lb | 87 oz | 10.9 | 2.6 L | 103 oz |
| 170 lb | 97 oz | 12.1 | 2.9 L | 113 oz |
| 200 lb | 112 oz | 14 | 3.3 L | 128 oz |
| 250 lb | 137 oz | 17.1 | 4.1 L | 153 oz |
Including 30 minutes of daily exercise. The hot-climate column adds a flat 16 oz.
What exercise adds
| Daily exercise | Daily water | In litres |
|---|---|---|
| None | 85 oz | 2.5 L |
| 30 minutes | 97 oz | 2.9 L |
| 60 minutes | 109 oz | 3.2 L |
| 90 minutes | 121 oz | 3.6 L |
| 120 minutes | 133 oz | 3.9 L |
170 lb adult, temperate climate.
Twelve ounces per half hour is an average across moderate activity. Real sweat rates vary several-fold between individuals and rise sharply with heat and intensity, which is why endurance athletes weigh themselves before and after training rather than using a formula: each pound lost is roughly 16 oz of fluid to replace.
Food and other drinks count
Around a fifth of total water intake arrives as food, and it is not evenly spread — fruit, vegetables, soup, yoghurt and rice are mostly water, while bread, cheese and nuts are barely any. A diet heavy in fresh produce genuinely reduces how much you need to drink.
Tea, coffee and most soft drinks also count toward intake. The idea that caffeine is so dehydrating that a coffee is a net loss does not survive contact with the evidence at ordinary consumption levels; the fluid outweighs the mild diuretic effect. Alcohol is the real exception, and drinks with a lot of sugar bring other issues with them.
Thirst, urine colour, and when a number stops helping
For a healthy adult with normal kidneys, thirst is a reliable guide, and pale straw-coloured urine is the simplest daily check there is. A calculated target is most useful as a rough anchor for people who reliably forget to drink at all, and least useful as something to force yourself to hit.
More is not better. Drinking far beyond need dilutes blood sodium, and in extreme cases — most often endurance events and drinking contests — causes hyponatraemia, which is dangerous. The sensible reading of any figure on this page is "roughly this much, spread across the day, adjusted for how you feel".
When the number does not apply to you
This calculator is for healthy adults. Fluid needs change materially with pregnancy and breastfeeding, kidney or heart conditions, diabetes, certain medications including diuretics, fever or illness, high altitude, and age — older adults often have a blunted thirst response. Several of those conditions require restricting fluid rather than increasing it, on medical advice.
If any of those apply, the figure here is not the one to follow. Ask the clinician who knows your case.
General information, not medical advice. Talk to a healthcare professional about your own fluid needs, particularly if you have a kidney, heart or metabolic condition.
How this water intake calculator works out its numbers
The drinking target is (weight in lb × 0.5) + (exercise minutes ÷ 30 × 12) + 16 if the climate is hot, in fluid ounces. Cups divide by 8, litres multiply by 0.0295735, and bottles use the standard 16.9 oz size.
- The body-weight rule is a heuristic, not a guideline. It is used because it scales sensibly with body size and activity and lands close to published reference intakes for an average adult — 2.9 L against 3.0 L for a 170 lb man. It has no clinical status.
- The reference figures are the National Academies' adequate intake for adults: 3.7 L a day for men and 2.7 L for women. These are total water, food included. The calculator applies the commonly cited 20% from food to convert them to a drinking figure, and labels both, because quoting the total as a drinking target is the standard error.
- 12 oz per 30 minutes of exercise is an average for moderate activity. Individual sweat rates vary several-fold; weighing before and after training (about 16 oz per pound lost) is far more accurate.
- The hot-climate allowance is a flat 16 oz, which is a simplification. Heat combined with exertion can raise needs well beyond any fixed figure.
- Healthy adults only. Pregnancy, breastfeeding, kidney and heart conditions, diabetes, diuretics, fever, altitude and older age all change fluid needs, and some require restriction rather than more. This model covers none of them.
Reviewed September 2026.
Figures reviewed . Every worked example on this page is checked against the calculator above.
Water Intake Calculator: frequently asked questions
How much water should I drink a day?
As a practical anchor, half your body weight in pounds taken as ounces, plus about 12 oz per 30 minutes of exercise. For a 170 lb adult exercising 30 minutes that is 97 oz — roughly 12 cups or 2.9 litres.
Is the 3.7 litres figure a drinking target?
No, and this is the most common misreading. The National Academies' 3.7 L for men and 2.7 L for women is total water including food, and about 20% comes from what you eat. As a drinking target that is closer to 3.0 L and 2.2 L.
How much is 8 glasses of water?
Eight 8 oz glasses is 64 oz, about 1.9 litres. It is a memorable phrase rather than a guideline, and it sits below what both the body-weight rule and the published reference figures suggest for most adults.
Does coffee or tea count toward my water intake?
Yes. At ordinary consumption levels the fluid outweighs caffeine's mild diuretic effect, so both count. Alcohol is the genuine exception, and sugary drinks bring other problems with them.
How much extra water do I need when exercising?
Roughly 12 oz per 30 minutes as an average, more in heat or at high intensity. Sweat rates vary several-fold between people, so athletes weigh themselves before and after — each pound lost is about 16 oz of fluid to replace.
How do I know if I am drinking enough?
For a healthy adult, thirst is a reliable guide and pale straw-coloured urine is the simplest daily check. A calculated figure is most useful for people who forget to drink at all.
Can you drink too much water?
Yes. Drinking far beyond need dilutes blood sodium and can cause hyponatraemia, which is dangerous and occurs most often in endurance events. More is not better.
Does food count toward daily water?
Around a fifth of total intake, and unevenly — fruit, vegetables, soup and yoghurt are mostly water while bread, cheese and nuts are barely any. A produce-heavy diet genuinely lowers how much you need to drink.
Do I need more water in hot weather?
Yes. The calculator adds a flat 16 oz for a hot or humid climate, but that is a starting point — heat combined with exertion can raise requirements far beyond any fixed allowance.