Estimate the water to drink each day for your weight, how much you move and the climate you are in. Requirement varies more between people than any single quoted figure suggests, so treat this as a target to aim at.
A general guideline, not a prescription. Requirements change with illness, medication, pregnancy and heart or kidney conditions. Drink to thirst as well as to a number, and spread intake across the day.
There is no single correct amount of water for a person, which is why every figure you have seen quoted is different. Requirement moves with body size, how much you sweat, the air around you, what you eat and what you drink alongside it. What a calculator can give you is a sensible target to aim at rather than a number to obey.
The base figure is half your body weight in fluid ounces — the rule of thumb most commonly used in practice. It is then adjusted upward for activity, since sweat losses rise steeply with training volume, and for climate: heat and humidity increase losses, altitude increases them through faster breathing and greater urine output, and cold reduces them slightly. The figure is trimmed for women, who on average carry a lower proportion of body water at the same weight.
The result is total water from all sources. For reference, the Institute of Medicine puts adequate total water intake at about 125 fluid ounces a day for men and 91 for women, of which roughly one fifth comes from food — so the amount to actually drink is around four fifths of the figure above.
More is not automatically better. Drinking far beyond thirst over a short period can dilute blood sodium, which is dangerous. Spread intake across the day rather than front-loading it.
Water contains no calories, and drinking it before a meal modestly reduces how much people eat at that meal in several trials. It also matters for a reason people notice more: mild dehydration produces fatigue and blunted appetite signalling that are easy to misread as hunger, and the scale can move a couple of pounds in either direction on fluid alone — which is one more thing a daily weigh-in cannot separate from actual fat loss.
Every calculator on this page works the same way: it takes your height, your weight, your age and a guess about how much you move, and returns the average answer for people who look like that on paper. It is a reasonable place to start. It is not a reading of you, and it cannot change when you do.
Rysflo measures instead. One sixty-second exhale each morning reads three molecules in your breath, and the day’s protein, carbohydrate, fat and fibre come out in grams — against the goal you set, from what your metabolism is actually doing rather than from a formula on your dimensions.