How Much Water Should You Actually Drink?

"Drink eight glasses of water a day" is probably the most repeated piece of health advice there is — and it's not really based on anything specific to you. Your actual water needs depend on your body size, how active you are, and the climate you're in, which is why a fixed number was always going to be wrong for most people some of the time.

Where "8 glasses a day" actually came from

The rule traces back to a 1945 U.S. Food and Nutrition Board recommendation of roughly 2.5 litres of water a day — but the same report noted that most of that amount is normally already contained in the food you eat. That caveat got dropped somewhere along the way, and "drink 8 glasses" spread as a standalone rule disconnected from body size, diet, or activity. It's not dangerous advice, it's just imprecise for anyone who isn't an average-sized, sedentary adult eating an average diet.

A number that actually accounts for your body

A more useful baseline scales with bodyweight: roughly 35ml of water per kilogram of bodyweight per day. For a 75kg adult, that's about 2.6 litres before accounting for exercise or climate — larger bodies have more metabolic tissue turning over fluid, so they need proportionally more. This is the baseline FitCalc's hydration calculator starts from, before layering on the adjustments below.

Exercise and heat change the number

Sweat is mostly water, so anything that makes you sweat pushes your requirement up. A moderate session (30-60 minutes) typically calls for an extra 300-400ml on top of your baseline; a longer or harder session can add 700ml or more. Hot or humid climates increase baseline sweat losses even without exercise — your body works harder to regulate temperature, and that fluid has to be replaced regardless of what you did that day.

Signs you're already getting enough

You don't need to hit an exact number to be well hydrated. Pale straw-coloured urine is a reliable everyday signal that your intake is roughly on track; dark yellow is a sign to drink more. Thirst itself is a reasonably good guide for most healthy adults doing normal daily activity — the main time it under-reports your needs is during and immediately after intense or prolonged exercise, which is exactly when the extra top-up in the calculator above matters most.

Can you drink too much?

Yes, though it's uncommon outside of specific situations. Drinking very large volumes of plain water in a short window — most often seen in endurance athletes during long events — can dilute sodium levels in the blood and cause a condition called hyponatraemia, which is dangerous. For anyone exercising for more than a couple of hours, replacing some fluid with an electrolyte drink rather than plain water alone helps prevent this. For everyday hydration, this isn't something the average person needs to worry about.

Does coffee actually dehydrate you?

Not meaningfully, despite the popular myth. Caffeine has a mild diuretic effect, but for people who drink it regularly, the body adapts and the net fluid loss from a normal cup of coffee or tea is small — the water in the drink itself more than offsets it. Coffee and tea count towards your daily fluid intake just like plain water does. Alcohol is a different story: it's a genuine diuretic with no adaptation effect, which is a large part of why a night of drinking leaves you dehydrated and is why alternating alcoholic drinks with water is worth doing beyond just pacing yourself.

Electrolytes, not just volume

Hydration isn't only about how much fluid you take in — it's about fluid and electrolytes together. Sweat contains sodium, and in meaningful quantities: typically 400 to 1,500 mg of sodium per litre, with substantial variation between people and some adaptation to heat over time.

This is why replacing heavy sweat losses with plain water alone can leave you feeling worse rather than better. Without accompanying sodium, added water dilutes what's left and is more readily excreted, so you urinate frequently while still feeling flat and headachy. For ordinary daily drinking, a normal diet supplies plenty of sodium and no thought is needed. For sessions beyond about 90 minutes, in real heat, or for people who finish training visibly crusted with salt, including sodium alongside fluid makes a genuine difference — whether from an electrolyte product or simply from salted food afterwards.

Hydration around training

A practical approach, without turning it into a project:

• Beforehand — arrive already hydrated rather than trying to catch up. Roughly 400–600 ml in the two to three hours before a session gives time to absorb what's needed and pass the rest.
• During — for sessions under an hour in comfortable conditions, drinking to thirst is entirely adequate. For longer or hotter work, periodic sips of 150–250 ml every 15–20 minutes tend to sit better than large volumes at once.
• Afterwards — weighing yourself before and after a hard session gives a direct read on fluid lost, since almost all of that immediate change is water. Replacing roughly 1.2–1.5 litres per kilogram lost accounts for continued urine output during rehydration.

That last point deserves emphasis because it's the only genuinely personalised hydration measurement available to most people, and it costs nothing. Two people doing the same session in the same room can differ in sweat rate by more than a litre an hour.

What else counts toward the total

Total water intake includes everything, not just what you drink from a glass. Food typically supplies 20–30% of it — fruit, vegetables, soups, yoghurt, and rice or pasta cooked in water all contribute meaningfully. Tea, coffee, milk, and soft drinks all count as fluid too. Even in the case of caffeinated drinks, the mild diuretic effect is comfortably outweighed by the water they contain, and regular consumers develop partial tolerance to it in any case.

Alcohol is the genuine exception. It suppresses vasopressin, the hormone that signals the kidneys to conserve water, which is why an evening of drinking produces more urine than the volume consumed and why dehydration contributes substantially to how a hangover feels.

When the formula doesn't apply

A weight-based baseline suits healthy adults under ordinary conditions. Several situations change the picture enough that a general formula shouldn't be the guide: kidney disease, heart failure, and some liver conditions can require deliberately restricted fluid intake; several medications, diuretics in particular, alter fluid balance directly; and pregnancy and breastfeeding raise requirements. In any of those cases, follow the guidance you've been given rather than a calculator.

This guide is for general education, not medical advice. Fluid needs vary with individual health conditions — anyone with kidney, heart, or other conditions affecting fluid balance should follow guidance from their doctor rather than a general calculator.