Frequently asked questions

Most healthy adults need roughly 2–3.7 litres of total water a day (including food), depending on weight, sex, age, activity and climate. Use the calculator for a figure adjusted to you rather than a flat "8 glasses" rule, which was never based on strong evidence in the first place.

Not really — it is a rough rule of thumb that ignores body size, activity and climate entirely. It happens to land close to the average adult need, but it can be well off for a large athletic man in summer or a small sedentary person in a cool climate.

Yes. The headline number is what you should drink; we separately show how much you'll typically get from food (about 20% of baseline needs) so the total adds up honestly.

Roughly 0.4–1.0 litres per hour depending on intensity, based on typical sweat rates. Heavy sweaters or long endurance sessions can need more, plus electrolytes — plain water alone is not always enough for sessions over 60–90 minutes.

Yes, meaningfully. Heat and humidity both raise sweat losses; dry heat also increases losses through faster breathing and evaporation from skin, which is why the calculator asks about humidity separately from temperature.

Above roughly 2,500 m, yes. Thinner, drier air increases water loss through breathing, and altitude can cause increased urination. The calculator adds an altitude term above that threshold.

Yes. EFSA guidance recommends roughly 300 ml/day extra during pregnancy and 700 ml/day extra while breastfeeding, which the calculator adds automatically.

Total need per kg of body weight decreases somewhat with age, but the bigger risk is that the sensation of thirst weakens with age — meaning older adults are more prone to under-drinking without noticing. Scheduled drinking (see the hourly plan) helps.

Children generally need more water per kilogram of body weight than adults because of a higher metabolic rate and body-surface-area-to-mass ratio. The calculator uses higher ml/kg figures for under-18s.

Not meaningfully at typical intake. Caffeine has a mild diuretic effect, but the fluid you drink still counts toward hydration — coffee and tea are roughly 85–90% as hydrating as plain water, not a net loss.

It works against you. Alcohol suppresses the hormone that helps your kidneys retain water, producing a net fluid loss — which is why it is weighted as reducing, not adding to, your hydration in the beverage log.

Yes — very rarely from normal daily drinking, but it is a real risk during endurance exercise if you drink large volumes of plain water without replacing electrolytes, diluting blood sodium (hyponatraemia). Spreading intake across the day, as our schedule does, is safer than large boluses at once.

Dark yellow urine, headache, dry mouth, fatigue, dizziness and reduced concentration are common early signs. Thirst is a lagging indicator, especially in older adults — don't wait for it.

Commonly-cited guidance is an extra 250–500 ml (about 8–16 fl oz) a day on top of your normal target, since creatine draws additional water into muscle cells and adds a small extra load for your kidneys to process. Use the calculator's "Taking Creatine" mode to get this added to your personal baseline automatically rather than guessing a flat number.

Generally yes. A loading phase (~20 g/day, usually split into several doses across the day) involves a much larger daily amount than maintenance (~3–5 g/day), so the recommended extra fluid sits toward the top of the typical range during loading and the lower end during maintenance. The calculator scales the adjustment by both your dose and your selected phase.

Two main reasons: creatine increases the water content inside muscle cells (part of how it supports strength and size gains), and processing it adds a small extra solute load for your kidneys. Neither is dangerous at typical doses for a healthy adult, but staying a little ahead on fluids is standard, sensible advice — not medical guidance, so check with a doctor if you have any kidney condition or other concern.