How Much Water Do I Need? case study

The problem

“Drink eight glasses a day” survives because it is simple, not because it is true. Hydration needs scale with body weight and shift with age, activity, diet, and health — the same target cannot be right for a six-month-old, a marathon runner, and someone working through a summer heat wave. Most calculators answer anyway, emitting one number with no method, so there is no way to tell personalization from rounding. How Much Water Do I Need? takes the opposite position: an estimate is only useful if you can see what it was built from.

Who it’s for

Anyone who wants a defensible daily target: athletes and outdoor workers replacing what they sweat out, parents dosing a child’s intake by age band, older adults whose thirst signals have dulled, and people whose medications or conditions change the math. The calculator states its limits plainly — results carry a medical disclaimer, because hydration in a clinical situation is a clinician’s conversation.

The method

The calculation starts from four baselines — body weight, sex, age group, and activity level — then layers on the factors that move real-world needs: coffee and tea consumption, high-fiber and high-protein diets, fever or illness, pregnancy, breastfeeding, kidney disease, menstruation, binge drinking, nicotine use, and stress. Between baselines and adjustments the calculator draws on up to 18 parameters, each mapped to published research the page links directly: EFSA intake guidelines by age and sex, National Library of Medicine research on body-water homeostasis, WHO growth standards behind the youngest age bands, and sports-medicine work on activity-related losses. In the product today:

• Body weight, sex, age group, and activity level baselines
• Diet, health, and lifestyle adjustments, from caffeine and protein to pregnancy and stress
• Daily fluid intake with the share expected to come from food
• Cited sources and a medical disclaimer shown with every result

Results come back as a daily fluid-intake figure, the share expected to come from food, and a glasses-per-day translation.

How it’s built

The build keeps friction low: one page, no account, weight entered in kilograms or pounds. Every result screen repeats its sources and the disclaimer, so the receipts travel with the number instead of living in a footnote. Around the calculator, a library of plain-language guides — from caffeine and electrolytes to hot-weather hydration — follows the questions the parameter list provokes.

Where it stands

Live and free at howmuchwaterdoineed.com, with the guide library growing alongside the calculator. Factor suggestions and source tips are welcome — the contact page below is the fastest route.

Try it

Calculate your target at howmuchwaterdoineed.com — free, no signup, sources included. Commissioning or building a research-backed calculator? Webuildstuff designs, builds, and operates products like this one — start a conversation on the contact page.

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