A drinking-water starting point
This formula starts with 30 millilitres per kilogram of body weight, adds 0.4 litres for each hour of entered exercise, then adds the stated hot-weather amount. It estimates drinking water for planning; it does not measure sweat loss, total body water, or hydration status.
The default day in litres
At 72 kg, the baseline is 72 × 0.03 = 2.16 litres. Forty-five minutes of exercise adds 0.30 litres, and the entered 0.25 litre heat allowance brings the result to 2.71 litres. Changing those allowances changes the result directly, but the form does not observe how much fluid a particular workout actually loses.
Sweat is not measured by minutes
A short hard run in humid heat can lose more fluid than a longer easy indoor session. Clothing, acclimatization, pace, altitude, and access to drinks also matter. Before-and-after body mass under comparable conditions can describe a specific workout more directly, but it still needs sensible replacement in context.
Drinks are only part of water intake
Food, soup, milk, and other drinks contribute water. During prolonged sweating, replacing only plain water may be a poor plan when electrolyte loss matters. This page does not prescribe sports-drink concentration, sodium intake, or a drinking schedule; it only makes a simple volume assumption visible.
Fluid plans can need clinical limits
Heart failure, kidney disease, some medicines, endurance events, vomiting, diarrhoea, and heat illness need reasoning beyond body weight. A prescribed fluid restriction overrides this estimate. Confusion, fainting, persistent vomiting, or severe symptoms call for medical assessment rather than another calculation.
Check the arithmetic in context
The default 2.16 litre baseline supplies about four fifths of the 2.71 litre result; exercise and heat account for the remaining 0.55 litres. Keeping those pieces separate makes it possible to remove an allowance on a cool rest day instead of carrying it into every situation. It is an estimate for one described day, not a bottle-by-bottle rule.
A day log can distinguish a baseline from a particular workout or heat adjustment. That is more useful than treating every drink as proof that one total was correct, especially when appetite, weather, travel, illness, and medical advice change from day to day.
It is informational only and is not medical or professional advice.