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Vitalis
Instrument 04 · litres

Hydration planner

Works out a realistic daily fluid figure from your size, your climate and how hard you sweat — food and coffee counted in.

Status
Calibrated
Runs
In your browser
Account
Not required
Desk
Nutrition
Intake sheet
Awaiting press
Units
BodyweightSets the whole baseline
AgeNeeds step down after 55
SexA small lean-mass correction
Climate

The reference condition. Roughly 18–24 °C, nothing extreme in either direction, and the baseline needs no correction.

How hard you sweatOn a day you train

Most gym sessions and steady running. A damp shirt at the end of the hour, and about three glasses to replace it.

ExerciseMinutes on this kind of day
How you eatFood is banked, not poured

The usual assumption, and the one behind every public-health figure: about a fifth of your water comes in with your food.

Coffee and teaCounted in full
AlcoholCharged, never credited
Anything elseTick what applies today

The sheet is filled in. Take the reading when you are ready.

What this instrument is arguing with

Four claims about water that almost everyone repeats, and what the evidence actually says. The calculator above is built to disagree with all of them.

  • “Everyone needs eight glasses of water a day.”

    Two litres of plain water suits a specific body in a specific climate and nobody else. The figure traces back to a 1945 recommendation whose very next sentence — that most of it comes from food — was dropped in the retelling.

  • “Coffee and tea dehydrate you.”

    They do not, at habitual intakes. The diuretic studies behind this used large single doses in people who never drink caffeine. Your morning coffee is counted in full on this instrument.

  • “By the time you feel thirsty it is already too late.”

    Thirst is a good regulator for healthy adults doing ordinary things. It becomes unreliable in three specific cases: older age, long exercise in heat, and illness — which is exactly what this instrument adjusts for.

  • “You cannot drink too much water.”

    You can. Hyponatraemia is rare but real, and it happens when people drink to a plan rather than to thirst during endurance events. Kidneys clear roughly a litre an hour; sustained drinking past that is the risk.

How it is calibrated

The baseline is the clinical 30–35 ml per kilogram per day, with the standard step-down after fifty-five as kidney concentrating capacity and lean mass both fall. Climate multipliers and sweat rates come from sports-medicine replacement figures; food is assumed to supply a fifth of the total, which is the assumption behind the European and American adequate-intake numbers.

Coffee and tea count in full. Alcohol is the only line charged rather than credited. None of this replaces medical advice — if you have heart failure, kidney disease, or you are on diuretics, your fluid figure is set by your doctor and not by a calculator. Our hydration guide walks through the whole argument.

Elsewhere on the bench

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