Estimate daily water needs from body weight and exercise, with climate, pregnancy, and breastfeeding adjustments and a glasses-per-day conversion.
Content last reviewed
Climate
Special condition
How it works
Target = 33 ml × body weight (kg) + 0.5 L per hour of exercise, plus adjustments for hot climates and pregnancy or breastfeeding. All fluids count — tea, coffee, and the water in food included — so the target is easier to reach than it looks. The simplest real-world check is urine colour: pale-yellow means you're hydrated.
Your body weight plus typical daily exercise hours — the two main drivers of fluid needs.
Choose temperate, warm, or hot climate, and mark pregnancy or breastfeeding if they apply.
See the daily total in litres, fluid ounces, and glasses of your chosen size, with a breakdown of where it comes from.
Baseline 33 ml/kg plus half a litre for 30 minutes of exercise.
Input
70 kg · 0.5 h exercise · temperate climate · no special condition · 250 ml glassOutput
≈ 2.6 L per day (≈ 87 oz), about 10 glasses of 250 ml — 2.3 L from body weight + 0.3 L from exerciseExercise and climate additions stack on the weight baseline.
Input
80 kg · 1 h exercise · hot/humid climateOutput
≈ 4.1 L per day — 2.6 L baseline + 0.5 L exercise + 1.0 L climate"Drink eight glasses a day" is the most repeated hydration advice around — and it is a one-size answer to a question that depends on your body, your training, and your climate. A 50 kg office worker in autumn and a 90 kg builder in July do not need the same water. This calculator scales the target to you instead.
The baseline follows the weight-based rule used in clinical nutrition: about 33 ml of fluid per kilogram of body weight per day. On top of that it adds half a litre per hour of daily exercise to replace sweat, a climate adjustment (an extra 0.5 L for warm conditions, 1 L for hot or humid ones), and an addition for pregnancy (+0.3 L) or breastfeeding (+0.7 L). The sidebar shows the total in litres and fluid ounces, converts it into glasses of whatever size you actually drink from, and itemizes exactly where each part of the target comes from — no black box.
Two framing notes the tool itself makes: all fluids count (tea, coffee, and the water in food included, so the target is easier to reach than it looks), and more is not automatically better — drinking very large amounts in a short window can dangerously dilute blood sodium. People with kidney, heart, or liver conditions may need to restrict fluids on medical advice, which overrides any calculator. The simplest everyday check remains urine colour: pale yellow means you are hydrated.
Choose Metric or Imperial and enter your body weight.
Set Exercise per day in hours (0–12, with a slider) — the calculator adds roughly half a litre per hour to replace sweat losses.
Pick your Climate: temperate, warm (+0.5 L), or hot/humid (+1 L).
If applicable, select Pregnant (+0.3 L) or Breastfeeding (+0.7 L) under special conditions.
Optionally set Your glass size (100–1,000 ml) so the target is also shown as a count of glasses.
Read the daily target in litres, ounces, and glasses, and check the Where it comes from breakdown. Use Copy for a text summary or Reset to start over.
33 ml per kilogram of body weight — the clinical-nutrition rule of thumb, instead of a flat eight glasses.
Half a litre per exercise hour, plus additions for warm or hot climates and for pregnancy or breastfeeding.
The target appears in litres, US fluid ounces, and glasses of your configured size.
A panel itemizes how much of the target comes from body weight, exercise, climate, and special conditions.
One click copies the target and its full breakdown as plain text.
Replace the generic eight-glasses rule with a figure scaled to your weight and routine.
See how much extra an hour or two of exercise adds, and plan bottles accordingly.
Quantify the extra fluid a heatwave or humid climate calls for.
Apply the standard additions for increased fluid needs during pregnancy and lactation.
Convert the target into glasses of your actual cup size to make tracking concrete.
The target is assembled from four transparent parts:
Target = 33 ml × weight (kg) + 0.5 L × exercise hours + climate adjustment + condition adjustment
The weight term reflects the 30–35 ml/kg range used in clinical nutrition, taking the middle-upper value. The exercise term approximates typical sweat replacement. Climate adds 0.5 L for warm and 1 L for hot or humid conditions, and the special-condition term adds 0.3 L for pregnancy or 0.7 L for breastfeeding, in line with common guidance. The glasses figure divides the total by your configured glass size, and the ounce figure converts litres to US fluid ounces. Every component is listed separately in the breakdown panel so you can see — and sanity-check — where the total comes from.
Metric or imperial weight entry; the result is shown in litres and fluid ounces regardless.
0–12 hours, in half-hour steps with a slider.
Temperate (+0), warm (+0.5 L), or hot/humid (+1 L).
None, pregnant (+0.3 L), or breastfeeding (+0.7 L).
100–1,000 ml, used only to express the target as a number of glasses.
The weight field is empty or zero — it is the one required input.
Check the exercise hours — an accidental 5 instead of 0.5 adds 2.25 extra litres. Also remember the total counts all fluids and food water, not just plain water.
It is simply the total divided by your configured glass size; a 250 ml default gives roughly four glasses per litre. Adjust the glass size to match your actual cup.
The formula is a healthy-adult baseline built from population rules of thumb — 33 ml/kg is a midpoint of the commonly cited 30–35 ml/kg range, and the exercise, climate, and condition additions are typical values rather than personal measurements. It cannot account for illness, medications, altitude, very high sweat rates, or medical fluid restrictions. Individual needs genuinely vary; the practical calibration is urine colour (pale yellow is the goal) and thirst, with medical advice taking precedence for anyone with a relevant condition.
The calculation runs in your browser — your weight and settings are not uploaded, stored, or logged. Copying the summary is the only way a result leaves the page.
Spread intake across the day; steady sipping beats forcing litres in one sitting.
Count all fluids — tea, coffee, and food water contribute, so the target is easier than it looks.
Use urine colour as the real-time check: pale yellow means hydrated, dark means drink more.
Add fluid proactively in heat and around exercise rather than waiting for strong thirst.
Follow your doctor's fluid guidance over any calculator if you have a kidney, heart, or liver condition.
Make the number visible and physical. Once you know your daily litres, translate it into containers you actually use — the glass-size setting exists precisely for this. "Ten glasses" or "three refills of my 750 ml bottle" is a plan; "2.6 litres" is trivia.
Anchor drinking to existing routines: a glass on waking, one with each meal, a refill when you sit back down at your desk. Habit-stacked hydration outperforms willpower, and it naturally spreads intake across the day — which is also the safe pattern.
Recalculate when circumstances change: a new training block, a heatwave, pregnancy. The target is a baseline to adjust, not a fixed prescription, and thirst plus urine colour remain the live feedback that no formula replaces.
Fully client-side. The target is computed as weight(kg) × 0.033 + max(0, exercise hours) × 0.5 litres, plus a climate constant (0/0.5/1.0 L) and a condition constant (0/0.3/0.7 L). Ounce conversion uses 33.814 oz/L; the glasses figure divides millilitres by the configured glass size. Weight accepts metric or imperial entry with internal conversion; results display to one decimal litre.
Anyone who wants a hydration number scaled to their body instead of folklore: desk workers building a drinking habit, people training in the heat, and pregnant or breastfeeding parents applying the standard additions. It is a general-wellness reference — people under medical fluid guidance should follow their clinician's numbers instead.
Evidence and boundaries
The page identifies the reference population and units used. Clinical interpretation, screening thresholds, and professional guidance can differ by country and population.
Evidence standard reviewed .
It is a rough average rather than science. Eight 250 ml glasses is 2 litres — close for a 60 kg adult, short for a 90 kg one who trains. Weight-based targets (30–35 ml/kg) scale with body size, which is why clinical nutrition uses them.
Yes. Caffeine's mild diuretic effect does not cancel the fluid — caffeinated drinks hydrate nearly as well as water. Food contributes too, typically around a fifth of daily fluid intake.
Yes — drinking many litres in a short window can dilute blood sodium (hyponatremia), which is dangerous. It is rare outside endurance events, but the principle holds: spread intake through the day and drink to thirst during long exercise. The tool's disclaimer highlights this explicitly.
They add the commonly recommended increases — about 0.3 L per day during pregnancy and 0.7 L while breastfeeding — on top of your weight-and-activity baseline. Your midwife's or doctor's advice takes precedence.
No — it is total fluid. Water, tea, coffee, milk, soup, and water-rich food all count toward it. Sugary drinks hydrate too, just with calorie baggage.
Urine colour is the simplest reliable check: pale yellow indicates good hydration, dark yellow means drink more. Thirst, energy, and how you handle heat are the supporting signals.