Water Intake Calculator
Estimate a daily fluid target from body weight, exercise and climate — with the caveats the round numbers usually leave out.
The eight-glasses rule has no evidence behind it
It appears to trace back to a 1945 US Food and Nutrition Board note which added that most of that water comes from food — a sentence that got dropped in the retelling. Roughly 20% of typical fluid intake arrives in food, and tea, coffee and juice all count towards the total.
Thirst works
For healthy adults, drinking to thirst and checking urine colour — pale straw, not clear — is more reliable than any formula. Deliberately drinking far beyond thirst can dilute blood sodium, which is dangerous. Pregnancy, breastfeeding, kidney conditions and some medications change the target, and those are conversations for a clinician rather than a calculator.
The formula
Baseline ≈ 35 mL per kg of body weight, plus ~350 mL per 30 minutes of exercise, adjusted for climate
Every result on this page comes from this formula. Nothing is rounded until the final display, so figures match a spreadsheet to the cent.
How to use the Water Intake Calculator
- Fill in each field with your own figures — the defaults are only there as an example.
- The result updates as you type, so there is no button to press.
- Check the breakdown under the main result for the supporting numbers.
- Use the copy button to take the result with you, or share the link to reload the same inputs.
Water Intake Calculator — frequently asked questions
Is this water intake calculator free to use?
Yes. Every calculator on the site is free, needs no sign-up, and runs entirely in your browser, so your figures never leave your device.
How accurate is the result?
The maths is exact for the formula shown on the page. Accuracy in the real world depends on your inputs and on assumptions the formula cannot see, so treat the output as a well-grounded estimate rather than a quote.
Can I use it on my phone?
Yes. The page is responsive, weighs a few kilobytes and works offline once loaded, so it behaves the same on a phone as on a desktop.
What people search for
This page answers each of these searches. They are the exact phrases visitors use to find the water intake calculator.
- water intake calculator
About this calculator
Written and reviewed by The Samsung Calculator editorial team. Every calculator is written against a published formula, reviewed against at least one independent reference implementation, and dated when it changes.
Last reviewed August 23, 2026 · Calculation runs client-side · No data collected · Report an error