Water Intake Calculator
Estimate daily hydration needs from body weight, activity and climate.
About Water Intake Calculator
This uses the common guideline of about 35 ml of water per kilogram of body weight, plus extra to replace fluids lost during exercise. Hot climates and illness increase needs further.
Formula
Daily water intake
Water (ml) = 35 × weight (kg) + 12 × weight (kg) × exercise (hours)
- 12
- Exercise top-up: 12 ml per kg per hour of activity
- 35
- Base requirement: 35 ml per kg of body weight
- W
- Total daily water in millilitres
- w_kg
- Body weight in kilograms
- t_h
- Daily exercise duration in hours
Worked example
With the default values below, Water Intake Calculator returns Daily Water (liters) of 2.87. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Weight Unitsone of Kilograms, Pounds
- Kilograms
- Body Weightaccepts 1–400 kg / lb
- 70 kg / lb
- Daily Exerciseaccepts 0–300 min
- 30 min
Results
- Daily Water (liters)
- 2.87
- Cups (8 oz)
- 12.0
- Fluid Ounces
- 97
Step by step
- 1Base water from body weight: 35 ml/kg= 2,450 ml
- 2Convert exercise duration to hours= 0.500 h
- 3Exercise adjustment: 12 ml/kg per hour= 420 ml
- 4Total daily water= 2.87 L (12.0 cups / 97 fl oz)
Assumptions and limits
- The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
- This is an estimate and not medical advice. Individual hydration needs vary with health, climate, and activity level. Consult a healthcare professional if you have concerns about fluid intake.
- This is an estimate for general information and not professional medical advice. See the full disclaimer for the limits that apply to this kind of calculator.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Water Intake Calculator — full guide
Where the guideline comes from
Fluid needs scale with body size and with how much you sweat, so a single "eight glasses" rule fits almost nobody. This page uses a weight-based guideline of about 35 ml per kilogram per day, then adds a top-up for time spent exercising.
The formula
Water (ml) = 35 × weight (kg) + 12 × weight (kg) × exercise (hours)
The first term is the baseline requirement. The second adds 12 ml per kilogram for each hour of activity, so the replacement scales with body mass as well — a heavier person sweating for an hour loses more than a lighter one.
Worked example
Default inputs of 70 kg and 30 minutes of exercise:
- Baseline:
35 × 70 = 2,450ml - Exercise in hours:
30 ÷ 60 = 0.5 - Top-up:
12 × 70 × 0.5 = 420ml - Total:
2,450 + 420 = 2,870ml, shown as 2.87 L - Cups:
2,870 ÷ 240 = 12.0; fluid ounces:2,870 ÷ 29.5735 = 97
The half-hour of exercise contributes 420 ml, about 15% of the day's total. Scale it up and the effect compounds: a 90 kg person training for a full hour needs 35 × 90 + 12 × 90 × 1 = 4,230 ml, or 4.23 L.
Pounds instead of kilograms
Choosing imperial converts first, at 1 lb = 0.453592 kg. A 154 lb body becomes 69.85 kg, and with 45 minutes of exercise the result is 3,074 ml — 3.07 L.
Interpreting the target
- It is total fluid, not plain water only. Tea, coffee, milk, soup and water-rich food all count towards it.
- Spread it across the day. Large volumes drunk quickly are handled worse than the same amount paced out.
- Urine colour is a faster feedback loop than arithmetic: pale straw usually means you are on track, dark yellow means catch up.
What the formula leaves out
- Climate and humidity are not inputs, yet heat can raise sweat losses substantially. Add to the figure on hot days.
- Illness with fever, vomiting or diarrhoea increases needs sharply and may call for oral rehydration rather than water.
- Long or intense sessions lose electrolytes as well as water, so sodium replacement matters beyond a couple of hours.
- Some conditions and medications require fluid restriction instead. This is an estimate, not medical advice — if you have heart or kidney disease, follow the limit your clinician sets rather than this figure.
Round out the picture with calorie needs, split those calories using the macro calculator, or check your weight-to-height ratio on the BMI calculator.
This is an estimate and not medical advice. Individual hydration needs vary with health, climate, and activity level. Consult a healthcare professional if you have concerns about fluid intake.
Frequently Asked Questions
Does other fluid count?
Yes — water in tea, coffee, milk, and water-rich foods all contribute to hydration, though plain water is the best primary source.
Can I drink too much water?
Rarely, but drinking extreme amounts in a short time can dangerously dilute blood sodium. Spread intake through the day and listen to your thirst.
How much is the 30-minute exercise default actually adding?
420 ml for a 70 kg person, from 12 × 70 × 0.5. Set exercise to zero and the target drops from 2,870 ml to 2,450 ml — the baseline alone. So the default half-hour accounts for roughly 15% of the day's figure, and an hour instead of 30 minutes would add another 420 ml on top.
How does 2.87 L translate into bottles I can count?
A 750 ml sports bottle takes 3.8 refills; a 500 ml bottle takes 5.7. Picking a vessel and counting it is easier than tracking millilitres — four 750 ml bottles slightly overshoots the 70 kg target, which is close enough for a daily habit.
The cups row says 8 oz but the maths uses 240 ml — which is it?
The page divides millilitres by 240, the rounded US customary cup. A US fluid cup is actually 236.588 ml, so an exact 8-fl-oz count would be 12.13 cups rather than 11.96 for the 2,870 ml example — about 1.4% higher. Immaterial for hydration, but worth knowing if you are reconciling figures.
Should I raise the number for a heatwave or a hot yoga class?
Yes, manually — temperature and humidity are not inputs, so the 35 ml/kg baseline assumes comfortable conditions. A practical approach is weighing yourself before and after a hot session: each kilogram lost is roughly a litre of sweat to replace, on top of the calculated target.