Do I need eight glasses a day?

That figure has no strong evidence behind it and appears to have been repeated into common knowledge. Requirements vary widely, and fluid from food, tea and coffee all counts towards the total.

Can I drink too much water?

Yes. Drinking far in excess of need can dilute blood sodium dangerously, which is rare but serious. Thirst is a reasonable guide for most healthy people; a calculated target is not something to force down.

Does the water intake estimate change with age?

No, age is not an input. The range starts at 30 to 35 ml per kilogram of body weight, then adds about 0.35 litres per 30 minutes of exercise, 0.5 litres for a hot or humid climate, and 0.3 or 0.7 litres for pregnancy or breastfeeding. Anyone with a medical condition affecting fluids should follow a clinician instead.

Water Intake Calculator

Estimate a daily fluid target from body weight and activity. A general guide only: climate, health and medication all change what is appropriate.

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2.45 L to 2.8 L per day

about 10–11 cups of 250 ml

Lower estimate2.45 L

Upper estimate2.8 L

Per hour awake0.14 Lspread over ~16 hours

From body weight: 2.1 L – 2.45 L
Exercise: + 0.35 L
Climate: + 0 L
Pregnancy / breastfeeding: + 0 L

How this is calculated

Base of 30–35 ml per kg of body weight (a common clinical rule of thumb; EFSA's adequate intakes are 2.0 L/day for women and 2.5 L/day for men including food), plus about 350 ml per 30 minutes of exercise, 0.5 L in hot weather, and EFSA's +0.3 L in pregnancy or +0.7 L when breastfeeding. Roughly a fifth of daily water comes from food, so treat the range as a guide, not a target to hit exactly. Kidney, heart or medication conditions change needs — ask a clinician.

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About Water Intake Calculator

A rough daily fluid target from body weight and activity. Worth being clear about the status of this number: hydration guidance is genuinely individual, and any figure a web page gives you is a starting estimate rather than a requirement. Free online, asking for no email. Your weight and activity feed the estimate and are then let go.

How to use Water Intake Calculator

Enter weight and activityExercise raises requirements substantially, and so does heat.
Read the estimateA daily total, which includes fluid from food and other drinks, not water alone.
Adjust for your dayA hot climate, illness, altitude or a long flight all change it more than the formula does.

Everything you drink counts, and so does what you eat

Fluid requirements are met by tea, coffee, juice, milk and soup as much as by water, and food contributes a surprising share — fruit and vegetables are largely water. The idea that caffeinated drinks do not count, or actively dehydrate, does not survive contact with the evidence at ordinary intakes. A target expressed as glasses of plain water overstates what you need to drink deliberately.

What it does not do

Individual requirements vary with climate, health, medication, pregnancy and kidney function, none of which are inputs here.
Anyone with a heart or kidney condition, or on medication affecting fluid balance, should follow clinical advice rather than a formula — over-drinking can be dangerous.
Not medical advice.