Water Intake Calculator

What sets your fluid requirement

Water requirement is not a fixed number. It scales with body size, with sweat losses during training, with the climate you live in and with how much water your food already contains. The widely quoted eight glasses a day is a rough population figure with no individual basis, and it happens to be about right for a small sedentary adult and much too low for a 95 kg runner in summer.

The body is also good at regulating this when it has the chance: thirst plus access to water handles most situations. The exceptions are heavy training, hot weather, illness with vomiting or diarrhoea, and older age, where the thirst signal becomes less reliable.

How the calculator estimates your target

The baseline is 35 ml per kilogram of body weight per day, a common clinical rule of thumb that sits in the middle of the 30 to 40 ml range. On top of that, 350 ml is added for every 30 minutes of training, which approximates typical sweat rates for moderate exercise in a temperate climate.

The result is a total fluid target, not a 'plain water only' target. Tea, coffee, milk, soup and fruit all contribute, and food typically supplies 20% to 30% of daily fluid for people eating normally. If you drink a litre of tea a day, count most of it.

Reading your result

The first three rows show the same target in millilitres, ounces and litres, and the next two translate it into 8 oz glasses and 500 ml bottles so you can plan without converting anything. The last row isolates the training addition, which is the part that changes most from day to day.

Spread the total across the day rather than drinking it in two large sittings: the kidneys can only excrete about 0.8 to 1 litre per hour, and large boluses are uncomfortable. A glass on waking, one with each meal, and one or two around training covers most of the target without thinking about it.

Is eight glasses a day a myth?

It is a rough average, not a rule. For a 60 kg sedentary adult, 35 ml per kilogram gives about 2.1 litres, which is close to eight 8 oz glasses. For a 95 kg person training daily the same formula gives 3.3 litres plus training losses, which is well over eight glasses.

Caffeinated drinks count toward the total. The idea that coffee and tea dehydrate you has been repeated so often that it is treated as fact, but controlled studies show that moderate caffeine intakes do not produce a net fluid deficit in people who consume them regularly.

Signs of dehydration and overhydration

Mild dehydration shows up as dark urine, headache, tiredness and a dry mouth, and it is enough to reduce training performance and concentration. Urine colour is the simplest practical check: pale straw is the target, dark amber means drink more, and completely clear urine for hours means you are drinking more than you need.

• Dark yellow urine, headaches and fatigue suggest dehydration.
• Very pale urine all day and frequent urination suggest over-drinking.
• Thirst is unreliable during heavy training and in older age.
• Weighing yourself before and after a long session shows your real sweat loss.

Hydration around training

For sessions under an hour in normal conditions, drinking to thirst is sufficient. For longer or hotter sessions, start well hydrated, drink 400 to 800 ml per hour, and include sodium if the session runs beyond about 90 minutes or if you are a salty sweater - visible white streaks on clothing are the giveaway.

Weighing before and after a hard session tells you what you actually lost: every kilogram of body weight lost during training is roughly one litre of fluid, and replacing 125% to 150% of that over the following hours restores the balance properly.

Frequently asked questions