Water Intake Calculator

Toolsda's Water Intake Calculator works out a personalized daily drinking-water target from body weight, activity level, climate, and pregnancy — plus an hourly schedule to actually hit it. Free, no login, and every calculation runs in your browser; nothing you enter is sent anywhere.

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For informational purposes only — not medical advice. Consult a healthcare professional.

Weight

kg

Age: 30

Gender

Activity level

Climate today

Wake up

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Your daily target

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Adults need roughly 30-35ml of drinking water per kg of body weight daily, per National Academies (formerly IOM) 2004 guidance. For a 70kg moderately active adult in a temperate climate, that's about 2.5-2.95L of drinking water (3.1-3.7L total including food) — more with heat, exercise, pregnancy, or breastfeeding. Enter your own details above for a personalized number, cups/bottles conversion, and an hourly drinking schedule.

What is a water intake calculator, and why go by weight?

A water intake calculator estimates how much you should drink daily from inputs like body weight, age, activity, and climate, instead of a flat "8 glasses a day" rule that ignores how different a 50kg and a 100kg adult's fluid needs actually are. The National Academies of Sciences, Engineering, and Medicine (formerly the Institute of Medicine) set the Adequate Intake figures — 2.7L/day total water for women, 3.7L/day for men — that a weight-based, per-kg formula personalizes.

The formula, with citations

Component	Adjustment	Source
Base (weight)	35ml/kg (30ml/kg at 65+)	National Academies (IOM) 2004 [1]
Activity	+300 to +1000ml/day	Sawka et al. 2005 [2]
Climate	+500 (hot) to +700ml (humid)	Sawka et al. 2005 [2]
Pregnancy / lactation	+300ml / +700ml	Consistent with National Academies 2004 AI figures [1]

Children under 18 use a different, well-established formula instead — Holliday-Segar maintenance fluid: 100ml/kg for the first 10kg of body weight, 50ml/kg for the next 10kg, and 20ml/kg for the remainder. It doesn't take activity or climate inputs.

How to use it

Enter your weight, age, and (for adults) activity level.
Select your climate today, and pregnancy/breastfeeding status if relevant.
Get your target in litres, cups, and bottles — plus an hourly drinking schedule from your wake to sleep time.

The hourly schedule matters because sipping steadily is more effective than front-loading or back-loading your intake — and spreading it out, especially away from the hour before bed, is a simple way to actually hit your number without disrupting sleep.

Why calculate on your device?

Every number here is computed in your browser — nothing about your weight, age, pregnancy status, or daily drink log is ever uploaded. Health-adjacent inputs like these deserve that by default, not as an opt-in. Tools that process this kind of data server-side create a record that didn't need to exist.

Frequently Asked Questions

How much water should I drink per kg of body weight?

About 30-35ml per kg for adults (30ml/kg if you're 65+). A 70kg adult's baseline is roughly 2.1-2.45L/day, before activity, climate, or pregnancy adjustments.

How much water should I drink based on my weight?

Multiply your body weight in kg by 30-35ml. This calculator does that automatically and adds activity, climate, and pregnancy/lactation adjustments on top.

Does activity increase water needs?

Yes — roughly +300 to +1000ml/day depending on how active a typical day is, per Sawka et al. 2005. This calculator applies that as a flat addition for your selected activity tier.

How much water do I need during pregnancy?

About +300ml/day pregnant, +700ml/day breastfeeding, on top of your normal baseline — broadly consistent with ACOG's general "8-12 glasses a day" pregnancy guidance.

Is 3L of water too much? How much is too much?

3L is a normal, even typical, target for an active adult — not too much. Risk rises when intake sustains well above 5-6L/day, which can dilute blood sodium (hyponatremia). This calculator warns once your computed target passes 5L/day.

Medical disclaimer

This tool is for informational purposes only and is not medical advice. It is based on National Academies of Sciences, Engineering, and Medicine (formerly IOM) Dietary Reference Intakes for water and related published guidance. Consult a doctor for medical conditions, or before making significant changes to your fluid intake.

References: [1] National Academies (formerly IOM) 2004, Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. [2] Sawka, Cheuvront & Carter, "Human Water Needs," Nutrition Reviews, 2005. [3] Mayo Clinic, "Water: How much should you drink every day?" [4] StatPearls (NCBI Bookshelf), "Exercise-Associated Hyponatremia."