Overview
A quick estimate of how much water to drink in a day, based on body weight, how much exercise you plan, and whether the weather is hot. The result appears in litres and in 250 ml glasses so you can translate it into a fridge full of bottles. The calculation is a common heuristic, runs entirely in your browser, and the page says plainly that thirst remains the best everyday guide.
How It Works
Enter your weight in kilograms (the field accepts 20 to 250), the minutes of exercise you expect to do today (0 to 300), and choose Yes or No for a hot climate. The two result cards show the suggested daily intake in litres and the equivalent number of 250 ml glasses. Change any input and the figures update immediately. There is no unit toggle: weights are kilograms, so imperial users should convert pounds to kilograms first (pounds divided by 2.2).
Step-by-Step Usage Guide
- Enter your weight in kilograms.
- Enter how many minutes of exercise you plan for the day.
- Indicate whether today is hot where you are.
- Read the litres figure and the 250 ml glass equivalent.
Technical Specifications & Standards
The base figure is 33 millilitres per kilogram of body weight per day - a 70 kg adult gets about 2.3 litres before adjustments. That sits inside the widely cited 30 to 35 ml/kg band used in general guidance, though it is a heuristic, not a measured physiological need; official reference intakes are set by population surveys, not formulas, and food contributes maybe a fifth of total fluid. Exercise adds 500 ml for every 30 minutes entered, pro-rated linearly, reflecting the sweat replacement range commonly suggested for moderate activity. A hot climate multiplies the total by 1.12, a conservative bump compared with the much larger losses possible in extreme heat. All foods and other drinks count toward real hydration - the output is a drinking-water target, not a total-fluid requirement. People with kidney, heart or liver conditions, and pregnant or breastfeeding women, have individual requirements that no weight-based formula captures.
Targeted Use Cases
- Setting a daily bottles-to-drink target for an office job.
- Planning hydration for a summer training day with both exercise and heat toggled.
- Checking whether current water habits are far below a weight-based baseline.
- Travel packing: estimating how much water to carry for a hot-climate hike.
Notes & Gotchas
- Spread intake across the day rather than drinking the total at once.
- Count coffee, tea and food moisture toward real hydration; plain water need not be the only source.
- Let thirst and pale-yellow urine guide adjustments on top of the estimate.
- Increase the exercise figure for intense sessions; sweat rates vary widely by person.
Frequently Asked Questions
How accurate is the 33 ml per kg rule?
It is a commonly used heuristic within the 30-35 ml/kg band of general guidance, not a clinical measurement. Individual needs vary with diet, climate, health and activity, so use it as a baseline.
Does this account for water in food?
No. The output is a drinking target. Food typically supplies around 20 percent of total fluid intake, so real total hydration is higher than the number shown.
Is this medical advice?
No. If you have kidney, heart or liver disease, take diuretics, or are pregnant or breastfeeding, your fluid needs are individual - ask your clinician.
Why does heat add only 12 percent?
The multiplier is deliberately conservative. In serious heat with heavy sweating, losses can far exceed 12 percent, which is why the exercise field and your own thirst should also shape the day's intake.