How much water should you drink a day?

How much water should you actually drink a day?

The honest answer is that there is no single number, and any calculator that gives you one without caveats is overselling its own precision. The most widely cited reference figures come from the US National Academies (NASEM, formerly the IOM): about 3.7 litres of total water a day for men and 2.7 litres for women. Total water includes what you get from food, which is roughly 20% of intake — so the amount you need to actually drink is closer to 3.0 and 2.2 litres.

The other common approach, and the one nearly every calculator on the first page of Google quietly uses, is two thirds of your bodyweight in ounces. It is easy to compute and it produces sensible-looking numbers for average adults. It also has no strong evidence base. This page shows you both, because the gap between them is more informative than either one alone.

How much water to drink by weight

The bodyweight rule worked out for common weights, for a day without exercise in mild weather. Add about 12 oz for every 30 minutes of sweaty exercise and 12–20 oz on a hot day, or put your own numbers into the calculator above.

For comparison, the NASEM adequate intake from drinks is about 73 oz for women and 100 oz for men at any weight. The two land close together for lighter and average-sized adults and drift further apart the heavier you are, because the reference figures stay put while the rule keeps climbing.

What actually changes your needs

• Exercise. Roughly 350–500 mL (12–16 oz) extra per 30 minutes of hard effort, and much more in heat. If you want a real number rather than an estimate, weigh yourself before and after a session — that is what the sweat rate calculator does.
• Heat and humidity. Humidity matters more than most people expect, because sweat that cannot evaporate does not cool you, so you keep producing more of it.
• Altitude. Above about 2,500 m you lose more water through breathing and increased urine output — roughly a litre a day.
• Health and medication. Kidney conditions, heart failure, diuretics and pregnancy all change fluid needs, sometimes in the direction of drinking less. This is the point where a calculator stops being useful and a clinician starts. Appetite-suppressing medicines are the exception that runs the other way: see how much water to drink on a GLP-1 for what changes when you eat less.

Does coffee count?

Mostly, yes. The diuretic effect of caffeine at normal doses is small enough that tea and coffee still contribute net fluid. They are not water, but they are not a hydration debt either.

A better signal than any calculator

Pale straw-coloured urine and an absence of thirst tell you more about your hydration than arithmetic based on your bodyweight ever will. Use the number here as a starting target, then let your body correct it.