Water needs vary with body size, sweat rate, diet, and health conditions. Treat this as a general planning estimate, not medical advice.
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General estimate for planning only — not medical advice. Consult a doctor about your specific situation.
Calculation details
Planning estimate only, not financial, tax, or legal advice. Verify assumptions and current rules before making decisions.
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What does this calculator estimate?
Daily hydration calculator: find how much water you should drink a day based on your body weight and activity level. Free, runs in your browser, no signup.
How this calculator works
Formula
Base = 33 ml per kg of body weight, plus about 12 ml per minute of daily activity, scaled by a climate factor.
Worked example
A 170 lb (77.1 kg) person with 30 minutes of daily activity in a temperate climate: about 98 fl oz (2.9 L), 2.54 L base plus 0.35 L for activity.
Assumptions to verify
- Weight converts to kilograms at 2.20462 lb/kg.
- Baseline of ~33 ml per kg reflects common hydration guidance for healthy adults.
- Activity adds about 0.35 L per 30 minutes of exercise.
- Climate factor scales total need up in heat and down in cool conditions.
- Hydration from food and other beverages counts toward total intake.
- This tool provides general estimates for planning and education only; it is not medical advice and not a substitute for a qualified healthcare professional.
- Always consult your doctor or another healthcare provider about your specific situation.
Frequently asked questions
Is the 8x8 '8 cups a day' rule accurate?
It is a rough starting point. A weight- and activity-based estimate is more personalized.
Do I need more water when I exercise?
Yes. This calculator adds about 0.35 L per 30 minutes of activity and scales further in heat.
Does food count toward my water intake?
Yes. Roughly 20% of daily intake typically comes from food, so this estimate is a fluid target, not a strict total.
Is this medical advice?
No. It is a general planning estimate. Speak with a clinician for condition-specific guidance.
Cite this tool
BoringToolsKit. “Daily Hydration Calculator.” boringtoolskit.com/daily-hydration-calculator/. Free to reference in articles, syllabi, and answer posts with a link.
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