What this tool does
Enter weight, daily exercise minutes, and climate. The calculator returns a daily fluid target in liters and milliliters using 35 ml per kilogram of body weight, plus 350 ml for each 30 minutes of exercise, plus 500 ml when you select hot. Sex is collected for consistency with other tools; the math here uses weight, sweat time, and heat only. The number is total fluid — drinks plus the water in food.
How it works
Published adequate intakes (IOM: about 2.7 L/day women and 3.7 L/day men from all beverages and food; EFSA: about 2.0 L and 2.5 L in temperate climates) describe population averages, not your personal sweat rate. This tool uses a 30–35 ml/kg clinical rule of thumb instead — common in dietetics for healthy adults — then layers exercise and heat add-ons. It does not model pregnancy, altitude, fever, or medical fluid limits.
Formula and methodology
ml/day ≈ 35 × weight(kg) + (exerciseMinutes ÷ 30) × 350 + (hot climate ? 500 : 0). At 70 kg with no exercise in temperate weather: 2,450 ml (2.5 L), roughly 10 US cups at 240 ml. Add 60 min training: +700 ml. Hot climate: +500 ml more. Food typically supplies about 20–25% of total water (IOM/EFSA); you may drink less from a bottle if soups, fruit, and vegetables are already wet.
How to interpret the result
Use the liter output as a planning anchor, not a chug-by-noon quota. Pale-straw urine and normal thirst usually beat forcing an extra bottle. Dark urine, dizziness, headache, or very little output during a hot day are different signals — not solved by doubling a website number. Coffee and tea count toward fluid for most habitual drinkers. Spread intake through the day; drinking the full target in one sitting is unnecessary and can feel awful.
Example
70 kg, 45 minutes of training, temperate: 2,450 ml baseline + 525 ml exercise ≈ 3.0 L (about 12 cups at 240 ml). Same person in heat: 3.5 L. A 55 kg desk worker with no exercise: 1,925 ml (~1.9 L) — below the eight-glasses meme but reasonable if meals include yogurt, fruit, and vegetables. A 90 kg lifter with 90 minutes in heat: 3,150 + 1,050 + 500 ≈ 4.7 L.
Common mistakes
- Treating eight 8-oz glasses as a universal rule — cup size and body weight both move the target.
- Counting only plain water and ignoring soup, fruit, milk, and other wet foods already in the total.
- Adding a full exercise bonus on rest days when you did not sweat.
- Chugging the daily target in a few hours instead of spreading it.
- Skipping sodium on multi-hour hot sessions while only increasing water.
When this estimate may be off
- Kidney, heart, liver, or SIADH conditions with prescribed fluid limits.
- Pregnancy or breastfeeding — needs are higher and not modeled here.
- Ultra-endurance events where sweat-rate testing beats a 350 ml/30 min guess.
- High altitude, fever, or diuretic medications.
- Very low or very high body weight where a flat ml/kg rule mis-scales.