How much water should you drink a day?

Most water calculators multiply your body weight by a fixed number. afiet starts somewhere else, because water leaves your body mainly through the work it does: breathing, sweating, digesting, keeping your temperature steady. All of that is energy.

So the calculation follows your energy rather than your weight alone. It estimates your basal metabolic rate from your sex, age, height and weight, widens it into a maintenance range using how much you move, and then applies the guideline dietitians use for adults: about 1 ml of fluid per kilocalorie of daily energy.

The result is converted into glasses, counting one glass as 200 ml (about 6.8 fl oz), and smoothed into a band between 6 and 15 glasses. The smoothing is deliberate. The underlying estimate carries a wide margin, and printing "11.3 glasses" would be false precision dressed up as science.

This is the same calculation the afiet app runs, so the number here matches the number on your phone. Nothing you type leaves your browser.

Treat the number as a starting point for an ordinary day, not a quota to hit. Several ordinary things move it:

Heat and sweat. A hot day, a warm office or a workout can add several glasses. If you exercise hard for an hour, drinking to thirst around the session usually covers it.

Illness. Fever, vomiting and diarrhoea raise the need sharply, and that is a moment to ask a doctor rather than a calculator.

Pregnancy and breastfeeding. Both raise fluid needs meaningfully. This tool does not model them, and a midwife or doctor should.

What else you drink. Tea, coffee, milk and the water inside soup, fruit and vegetables all count toward fluid. The old idea that coffee dehydrates you does not hold at normal intakes; it just does not replace water either.

Your body also gives you two cues that no calculator can beat. Thirst is the first, and it is reliable for most healthy adults. Urine colour is the second: pale straw usually means you are fine, dark amber usually means you are behind. If you have a kidney or heart condition, or you take medication that affects fluid balance, follow your doctor rather than either cue.

One myth is worth retiring: the famous "eight glasses a day" has no strong origin in research. It is usually traced to a 1945 recommendation that also said most of that water comes from food, a sentence that got dropped somewhere along the way.

afiet does not ask you to log millilitres. On your phone, water is a row of glasses you tap as you go, and the day fills up next to the five food groups rather than in a separate spreadsheet.

That is the whole idea behind this tool: give you the number once, then let a habit carry it. A glass with each meal and one between meals is already most of the way there for many people, and it takes no counting at all.