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Water Intake Calculator

Find out how much water you should drink each day based on your weight, sex, activity level, climate and whether you're pregnant or breastfeeding.

Calculator

Used as the main basis for your hydration estimate.

kg
20 kg200 kg

Affects baseline hydration needs.

Older adults often need a reminder to drink more.

yrs
1100

Exercise increases sweat loss and fluid needs.

Heat and humidity increase sweat and fluid loss.

Your recommended daily water intake

calculating…
Daily target
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250 ml glasses
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500 ml bottles
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750 ml bottles
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1 litre bottles
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Breakdown of your daily target
Base requirement (weight-based) —
Activity adjustment —
Climate adjustment —

How the calculation works

The calculator uses a weight-based formula consistent with guidance from the European Food Safety Authority (EFSA) and the NHS:

Base intake (ml) = weight (kg) × 35 ml  [male]  or  × 31 ml  [female]

Additional amounts are then added based on activity level (sweat loss), climate (elevated heat and humidity), and life stage (pregnancy or breastfeeding). The EFSA recommends an additional 300 ml/day during pregnancy and 700 ml/day during breastfeeding.

Age adjustments apply a small upward nudge for adults over 55, as the thirst sensation weakens with age and older adults are at higher risk of dehydration.

Note: This estimate covers total fluid intake, around 20% of which typically comes from food. If you want a drinks-only target, reduce the figure by approximately 20%. This calculator is for general guidance only and is not a substitute for medical advice.

Signs you may not be drinking enough

  • •Dark yellow urine — pale straw is the target colour
  • •Headaches — often the first sign of mild dehydration
  • •Fatigue and low energy — even 1–2% fluid loss affects performance
  • •Dry mouth and lips
  • •Difficulty concentrating — dehydration impairs short-term memory
  • •Infrequent urination — fewer than 4 times a day suggests low intake

Practical tips to hit your target

  • •Start the day with a glass of water. It rehydrates after sleep and sets a positive habit for the rest of the day.
  • •Use a marked water bottle. A 1-litre bottle with time markers makes it easy to pace your intake throughout the day.
  • •Set reminders. A simple phone alarm every 90 minutes is enough to prevent long dry spells, especially during desk work.
  • •Eat water-rich foods. Cucumbers, tomatoes, oranges, and soups all count towards your daily fluid balance.
  • •Drink more around exercise. Aim for an extra 500 ml for every hour of moderate exercise — more in hot conditions.
  • •Herbal teas and diluted juices count. They aren't as hydrating as plain water but still contribute meaningfully to your daily total.

Common questions

Does coffee and tea count towards my water intake? ⌄
Yes, mostly. Despite the mild diuretic effect of caffeine, research shows that moderate coffee and tea consumption (up to 3–4 cups a day) contributes positively to fluid balance. Heavily caffeinated drinks or energy drinks in excess can have a net diuretic effect, but standard cups of coffee and tea absolutely count.
Is it possible to drink too much water? ⌄
Yes — overhydration (hyponatraemia) occurs when sodium in the blood becomes dangerously diluted. It is rare in everyday life but a risk for endurance athletes who drink large volumes without replacing electrolytes. For most healthy adults, the kidneys can handle up to 0.8–1 litre per hour. Stick to consistent sipping throughout the day rather than drinking large amounts quickly.
Does the "8 glasses a day" rule hold up? ⌄
It's a reasonable rule of thumb for average adults, but it's not rooted in precise science. The actual amount needed varies with body size, activity, climate, and diet. A 50 kg sedentary woman and a 100 kg athlete have very different needs — a weight-based calculation like this one gives a more personalised estimate.
Why does the calculator include food as part of intake? ⌄
Water balance is about total fluid in vs. total fluid out. Food contributes roughly 20% of daily fluid intake — fruits, vegetables, soups and even bread contain significant amounts of water. Most hydration guidelines, including those from the NHS and EFSA, quote total fluid intake including food, not drinks only. If you want a drinks-only target, subtract around 20% from the result.
Do children need less water per kg than adults? ⌄
Children actually need proportionally more water per kilogram of body weight than adults, because they have a higher metabolic rate and surface-area-to-volume ratio. The EFSA recommends around 1,300 ml/day for 4–8 year olds rising to 1,900 ml for teenage girls and 2,100 ml for teenage boys. This calculator uses adult EFSA values and is most accurate for adults aged 18+.

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