Guide September 2026 · 6 min read

Where "8 glasses" came from

The usual origin cited is a 1945 US Food and Nutrition Board note recommending about 2.5 L of water a day — followed by a sentence most people never quote: "most of this quantity is contained in prepared foods." The second half got lost, the first half became a rule, and the rule became eight 8-ounce glasses (about 1.9 L) because it is easy to remember.

Reviews of the evidence since then have found no study showing that healthy adults need to drink 8 glasses of water on top of a normal diet. Thirst, in healthy adults, is a good regulator.

The actual guidelines

These are population adequate-intake estimates, not minimums. They assume moderate climate and activity. They also count every liquid: tea, coffee (yes — caffeine's diuretic effect is small and disappears with habitual use), milk, juice, soup. Water is the best choice for other reasons (no sugar, no calories), but it is not the only thing that counts.

How to estimate your own number

A common working formula: 30–35 ml per kg of body weight as total daily water, of which roughly 70–80% should come from drinks.

Then adjust:

• +500 ml to 1 L per hour of sweaty exercise.
• +500 ml or more in hot weather or a heated office.
• More on a high-protein diet — protein metabolism produces urea, which needs water to excrete. If you are eating 1.6–2 g/kg protein, err toward the top of the range.
• More after a high-salt day. Sodium pulls water; you will be thirsty for a reason.
• More when pregnant or breastfeeding (about +300 ml and +700 ml respectively).
• Less if your diet is fruit- and vegetable-heavy — a cucumber is 96% water; a bowl of soup is mostly water.

Signs you are not drinking enough

• Urine darker than pale straw (the single best check; some vitamins and foods like beets change the colour, but the trend is reliable).
• Headache, especially in the afternoon.
• Fatigue and poor concentration — mild dehydration of 1–2% of body weight measurably reduces cognitive performance.
• "Hunger" that goes away after a glass of water. Thirst and hunger signals overlap; people on a calorie deficit often eat when they are thirsty.
• Dry mouth, dry lips, less frequent bathroom trips.
• Constipation — particularly if you have recently increased fiber without increasing water.

Can you drink too much?

Yes, but it is hard. Hyponatremia (dangerously low blood sodium) happens when large volumes of plain water are drunk quickly — several litres in a few hours — mostly in endurance events or with certain medications. For a healthy adult drinking to thirst plus a reasonable margin, it is not a concern. Drinking 5 L a day "for skin" or "for weight loss" has no evidence behind it and adds nothing except bathroom trips.

Does water help with weight loss?

Modestly, in three real ways:

1. Replacing sweet drinks. Swapping one sugary drink a day for water removes 100–200 kcal and 25–40 g of sugar. This is the biggest effect by far.
2. Pre-meal water. Drinking about 500 ml before a meal leads to eating slightly less at that meal in several trials. Effect is small but consistent.
3. Thirst-as-hunger. Correctly identifying thirst avoids snacks that were never about food.

The "drinking water burns calories" effect exists but is tiny — a few kcal per glass. Water is not a fat-loss tool; it is a way to avoid mistaking thirst for hunger and to stop drinking sugar.

Tracking water without it being a chore

Water tracking apps fail because logging every glass is tedious. Two approaches that stick:

• Fixed vessels. Know that your bottle is 750 ml and your mug is 300 ml, and count vessels, not sips.
• Anchors. A glass on waking, a glass with each meal, the bottle at the desk. Four anchors cover most of the target without thinking.

CalMePlease includes water as one of its ten daily metrics, alongside food — so a high-salt lunch or a high-protein day automatically shows up next to a low water count, and the AI coach connects the two ("salt is high today; drink more") rather than leaving water in a separate app.

FAQ

How much water should I drink a day? Roughly 1.5–2.5 L from drinks for most adults, more with exercise, heat, high protein or high salt. Total water including food: about 2–2.7 L for women, 2.5–3.7 L for men.

Is 8 glasses of water a day a myth? The number has no scientific origin. It is close to a reasonable intake for many people, but it is not a rule, and food and other drinks count toward it.

Does coffee count toward water intake? Yes. The diuretic effect of caffeine is small and largely disappears in regular drinkers. Coffee and tea contribute to daily fluid.

How much water per kg of body weight? About 30–35 ml per kg as total water, with 70–80% from drinks.

How do I know if I'm drinking enough water? Pale-yellow urine, no afternoon headaches, and not feeling "hungry" shortly after a full meal are the practical signs.

Should I drink more water on a high-protein diet? Yes. Higher protein means more urea to excrete, which needs more water. Aim for the top of your range.

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Related: How much salt per day · How much protein per day · Why you crash after lunch

CalMePlease tracks water next to your food, not in a separate app, and tells you when salt or protein means you need more. Free on the App Store.