Water Intake Estimator

A baseline, not a prescription

Hydration needs cannot be reduced to one number for everyone, but a body-weight baseline beats guessing. This page multiplies your weight in kilograms by a factor in millilitres per kilogram — the tool defaults to 30 ml/kg/day, a common and conservative rough rule depending on your situation — and converts the result into litres per day. It is a general estimate only; it does not include sweat income from heat or sport, medical conditions that change fluid balance, and it is not medical advice.

That last sentence is half the page's value: for kidney, heart or thyroid conditions, and during pregnancy or breastfeeding, the watering figure belongs to your clinician, not to a formula; some of those situations genuinely restrict fluids. For everyone else, the number is a floor-level marker, a target that includes drinks and food (about a fifth of intake typically arrives with meals, and more with fruit or soup), so drinking the whole figure in plain water is unnecessary for most people with normal kidneys.

A worked example, step by step

Problem: 58 kg adult, factor 33 ml/kg (a hot climate choice above the default).

1. Multiply: 58 × 33 = 1,914 ml of target intake.
2. Convert: 1,914 ÷ 1,000 = 1.91 litres per day.
3. Split the accounting sensibly: maybe 1.5 L drinks, and the remainder arrive with meals.
4. Activity adjustment: train an hour sweat-heavy and the day legitimately needs more; this page does not factor exercise — you do.
5. Sanity check: pale urine and thirst-free mornings are the everyday signals the number is roughly right; dark urine usually you drink more.

On colder sedentary days the same person would run the default 30 ml/kg and get 1.74 L/day — a smaller target that is still fully normal. Factor choice is the volume dial, in a range that reflects the day, not a fixed fact.

Baselines by weight at 30 ml/kg

Read the table as a felt sense of scale: moving 12 kg of body weight shifts the baseline by roughly 360 ml — noticeable, but far less than the choices (climate, sport, illness) that swing litres rather than glasses.

From the number to the decision

A litres-per-day figure only becomes a plan once it splits across the day: 1,914 ml is about 8 standard 240 ml glasses, which reads as one with breakfast, two through the morning, one at lunch, two through the afternoon and the rest at dinner — a rhythm rather than a quota. Paced sips keep up with the kidneys' actual processing; catch-up sessions in the evening mostly pass straight through. That pacing habit is the difference between a target the body can use and a number the schedule resents.

The factor dial stays where the day's shape lives: hotter weather, long flights, heavy training and illness all justify upward turns like the 33 ml/kg choice, while air-conditioned office weeks sit near the default. Thirst and urine colour are the daily feedback that beat any single-day arithmetic; and re-checking the factor on rest days prevents the slow creep of over-consumption that turns drinking into a chore. The standing sentence from the top applies unchanged: kidney, heart and thyroid conditions, pregnancy, breastfeeding and prescription diuretics each move fluid needs in their own direction, and those figures belong to your clinician, not to this estimator's factor lines.

Mistakes that misread hydration

• Forcing a number. Chugging to satisfy the calculator is the wrong target, and in specific conditions genuinely dangerous; follow your clinician's advice, not the litres.
• Counting only plain water. Food, coffee, tea and milk all contribute; chasing the whole figure in plain water over-restricts your day.
• Ever doubling for sport. Sweaty sessions add needs, but they vary; drink to sweat, thirst and your training history, not to a fixed rule.
• Using the baseline as medical truth. Kidney, heart or thyroid patients have individual instructions that override body-weight maths.
• Medication and conditions. Diuretics, blood-pressure and kidney medicines change fluid needs in their own direction; your clinician's plan overrides this estimate.

Keep exploring

• Daily Calorie Estimate — the connected daily target most people track.
• Weight Converter — convert kilograms cleanly if labels differ.
• BMI Calculator — a body-weight context check, not a prescription.
• Guide: BMI and Calorie Estimates Explained.