1. Enter your weight
Type your weight and pick kg or lb. Body size is the biggest factor in daily fluid needs.
Free online tool
Daily water intake calculator.
Enter your weight, pick your activity level, and get a daily water target in liters, glasses, and ounces — right in the browser. When you want reminders, widgets, and Apple Watch logging to actually hit that target, BigDrink keeps you on track.
How to use it
How to use the calculator.
Three inputs, instant answer. The formula uses about 35 ml per kilogram of body weight plus an activity bonus.
2. Pick your activity level
Choose how much you exercise on a typical day. Sweat loss adds roughly 350–700 ml to your baseline.
3. Spread it across the day
Drink steadily rather than all at once — a glass with each meal and around workouts covers most of the target.
Reference
The numbers behind the tool.
Formulas, reference tables, and practical guidance you can come back to.
How the estimate works: 35 ml per kilogram
The calculator multiplies your weight in kilograms by about 35 ml — a widely used rule of thumb for healthy adults. A 70 kg person lands near 2.45 liters per day as a baseline. On days you exercise, sweat loss justifies adding roughly 350 ml for a moderate session (30–60 minutes) and up to 700 ml for an hour or more of hard training.
General population references point the same way: the U.S. National Academies suggest about 3.7 liters of total water per day for men and 2.7 liters for women — that figure includes fluids from food, which typically covers around 20% of the total. Drinks alone therefore sit close to the 35 ml/kg estimate for most people.
Treat the result as a starting point, not a prescription. Individual needs vary, and medical conditions (kidney or heart disease) or certain medications can change them — follow your clinician's advice when it differs.
Daily water target by body weight
Baseline targets without exercise, rounded to the nearest 50 ml — the same rounding the calculator uses. Add 350–700 ml on training days.
| Weight | Liters per day | Glasses (250 ml) | Ounces |
|---|---|---|---|
| 50 kg | 1.75 L | 7 | ~59 fl oz |
| 60 kg | 2.1 L | 8–9 | ~71 fl oz |
| 70 kg | 2.45 L | 10 | ~83 fl oz |
| 80 kg | 2.8 L | 11 | ~95 fl oz |
| 90 kg | 3.15 L | 13 | ~107 fl oz |
What changes your needs — and how to self-check
Hot or humid weather, altitude, fever, and long flights all raise fluid loss. Pregnancy adds roughly 300 ml per day, and breastfeeding around 700–1,000 ml, according to common clinical guidance. Alcohol has a diuretic effect; pairing each drink with a glass of water is a practical counterweight.
Your body gives two reliable feedback signals: thirst and urine color. Pale-yellow urine roughly means you are on track; dark urine usually means drink more, and consistently clear urine with very frequent bathroom trips can mean you are overdoing it.
FAQ
Frequently asked questions.
Quick answers about this tool and how to train with it.
How much water should I drink a day?
A practical estimate is about 35 ml per kilogram of body weight — roughly 2.4 liters for a 70 kg adult — plus 350–700 ml on days you exercise. Common general references land in the same range: about 3.7 liters of total fluids per day for men and 2.7 liters for women, which includes water from food.
Does coffee or tea count toward my water intake?
Yes — despite the caffeine, coffee and tea contribute net fluid. Water is still the best default because it adds no sugar or calories, but your morning coffee counts toward the total.
Can I drink too much water?
It's rare but possible — drinking many liters in a short window can dilute blood sodium (hyponatremia). Spread intake across the day, and let thirst and pale-yellow urine be your feedback signals.
How do I actually remember to drink enough?
Tie drinking to existing habits (meals, breaks, workouts) and make the target visible. BigDrink does exactly this on Apple devices: smart reminders, home screen widgets, Apple Watch one-tap logging, and iCloud sync across iPhone, iPad, Mac, and Apple Vision.
Keep going
Related tools & further reading.
More free tools that pair well with this one, plus guides and articles to go deeper.
For site owners
Embed this tool on your website.
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Knowing your number is easy. Hitting it daily is the hard part.
BigDrink turns your water target into a habit with reminders, widgets, Apple Watch logging, and HealthKit sync — free on the App Store.