Water Intake Calculator
How much water you need each day depends on your size, activity, climate, and health. This water intake calculator uses a common rule of thumb — about 30 to 35 millilitres per kilogram of body weight — to give a baseline. It is a general guide, not a clinical prescription: needs rise with exercise, heat, illness, pregnancy, and breastfeeding, and fall in cooler, less active conditions. Food and other drinks also contribute to your fluid intake. Use the figure as a starting point and let thirst and urine colour fine-tune it.
Calculate
Default result: 2.44
Water Intake Calculator · Result
calculators.dev
Daily water
154 lb × 35
- In millilitres
- 2,445
This calculator provides general estimates for educational purposes only and is not medical advice. Results are based on population formulas and may not reflect your individual circumstances. Consult a qualified healthcare professional before making changes to your diet, exercise, or health routine.
Reviewed by the calculators.dev team · Last updated 2026-06-23
Formula reviewed against 30–35 mL/kg/day adult fluid rule of thumb (general clinical convention; total fluid, not plain water only)
How to calculate
Enter your body weight and a rate between 30 and 35 mL/kg (use the higher end when you are active or in the heat). The calculator multiplies the two and shows the total in litres and millilitres. A 70 kg person at 35 mL/kg gets about 2.45 L/day. Remember this is total fluid from all sources, including food, so you do not need to drink the entire amount as plain water. Adjust upward on hot days or hard training sessions.
water_mL = body_weight_kg × rate_mL_per_kg, then litres = mL ÷ 1000. Variables: body weight in kilograms and a rate of roughly 30–35 mL/kg. The rate is a general convention, not a measured requirement, so the output is a baseline estimate. Activity, climate, and health can move actual needs well above or below it.
Example calculation
A 70 kg (about 154 lb) person at 35 mL/kg needs 70 × 35 = 2,450 mL, or about 2.45 litres of water a day. Using the lower 30 mL/kg end of the rule gives 2,100 mL — the right amount sits within that range and rises with heat and exercise.
- liters
- 2.45 L/day
- milliliters
- 2,450 mL
Assumptions
- The 30–35 mL/kg rate is a general rule of thumb, not a clinical prescription, so the result is a baseline rather than an exact requirement.
- The figure is total daily fluid from all sources — food and other drinks count, not just plain water.
- Needs increase with exercise, heat, fever, pregnancy, and breastfeeding, and decrease in cool, sedentary conditions; the rule does not adjust for these automatically.
Common mistakes
- Treating the number as plain-water-only. The estimate is total fluid intake, and food plus other drinks contribute a meaningful share.
- Ignoring conditions. The same person needs much more on a hot day or during hard exercise than the baseline rule suggests.
- Over-drinking to hit a target. Forcing far more than thirst dictates can, in rare cases, dilute blood sodium — let thirst and urine colour guide you.
Frequently asked questions
How much water should I drink a day?
A common guide is about 30–35 mL per kg of body weight, so roughly 2.1–2.5 litres for a 70 kg person. That is total fluid from all sources, and your real need rises with activity and heat.
Does the “8 glasses a day” rule apply to everyone?
It is a rough average, not a rule. Needs scale with body size, activity, and climate. A weight-based estimate like this one is a better starting point, then adjust to how you feel.
Does food count toward water intake?
Yes. Foods — especially fruits, vegetables, and soups — and other drinks all contribute. The estimate here is total fluid, so you do not need to drink it all as plain water.
Can I drink too much water?
Rarely, but yes — drinking far more than your body needs can dilute blood sodium (hyponatraemia). Use thirst and pale-yellow urine as your guide rather than forcing a fixed number.