How much water per day? Stop guessing.
You've heard 8 glasses a day? 2 liters? half an ounce per pound? Every website says something different, and you still don't know whether you're under-drinking, on track, or overdoing it.
Enter your weight, click how you move, tick boxes for heat, pregnancy, sauna: the calculator compares the four most-cited formulas and shows an honest average tuned to your day. You get the result in milliliters, liters, glasses (250 ml), US cups (240 ml) and fl oz: pick what you like.
Plus a schedule for the whole day: when to drink the first glass, when before meals, how much around training. No pills, no app, no explaining to anyone that yes, you really do count every sip.
How to use it
- Enter your weight in kg or pounds (toggle at the top).
- Pick an activity level: none, light (walking, light cardio), moderate (gym, jogging), heavy (intense training). The training hours slider scales the water bonus to session length.
- Pick a climate: temperate (home, office), hot (summer 25-30°C / 77-86°F), very hot (above 30°C, tropics).
- Tick special states: pregnancy, breastfeeding, high-protein diet. Each adds a fixed amount of ml.
- Use the sliders for sauna sessions (each = +500 ml) and coffee cups (each = +250 ml to compensate for the mild diuretic effect).
- Optionally toggle calorie-based formula (TDEE): that adds a fifth method, "1 ml per 1 kcal".
- Look at the big number: that's how much water to drink during the day. The smaller number underneath is total fluid need (food + drinks combined).
- The daily schedule section shows when to drink each glass, eight points from waking up to evening.
When this is useful
Six typical reasons people reach for a water calculator:
- You're starting to take hydration seriously: you know you under-drink, but "how much is too little"? The calculator gives you a number and a schedule, no more guessing.
- Vacation in a hot country: Spain in August, Thailand, the desert. In heat you lose a liter or two more per day than at home. The calculator catches that and bumps your target, so you don't end up at the medical post with heat exhaustion.
- Intense training: gym 5x a week, marathon, CrossFit. Each hour of heavy effort = up to a liter of sweat. The calculator adds that liter automatically.
- Pregnancy and breastfeeding: your body needs +300 ml during pregnancy and +700 ml while breastfeeding. Easy to forget, easy to under-drink, the calculator reminds you.
- Doctor asks "how much do you drink?": checkup with a nephrologist, cardiologist, dietitian. Instead of "uh… two liters maybe?" you have a real number tuned to your weight and lifestyle.
- Kids, seniors, anyone in a risk group: older people feel thirst less often, kids forget. The calculator gives you a baseline you can check on as a caretaker.
Questions and answers
Short answer: for most adults 2-3 liters of total fluids per day (about 80% from drinks, the rest from food). The "8 glasses of 8 oz" rule is ~1.9 liters, a rule of thumb, not a medical norm. Realistically you need 35 ml per kg of body weight + bonuses for sport, heat, coffee. A 70 kg person in mild climate = 2.4 L of drinks. A 90 kg person at the gym in July = 3.5-4 L.