Water Intake Calculator
About 60% of the adult human body is water, and on a normal day you lose roughly 2 to 2.5 liters of it through urine, sweat and breathing.
- Based on standard published formulas
- Instant, easy-to-read estimates
- Private: nothing leaves your device
Water Intake Calculator
Enter your numbers and press Calculate
How to use the water intake calculator
You only need two numbers:
1. Weight (kg). Enter your weight in kilograms. If you think in pounds, divide by 2.2: a 165 lb adult is about 75 kg. 2. Exercise minutes per day. Count activity that raises your heart rate: a brisk walk, a gym session, a run along the river, pickup basketball. A desk day with no workout is 0 minutes.
The exact formula the tool runs, in plain text:
liters per day = weight in kg × 0.033 + (exercise minutes ÷ 30) × 0.35
That is 33 ml per kilogram of body weight, plus an extra 350 ml for each 30-minute block of exercise. The glasses figure comes from:
glasses per day = round(liters × 4)
since a 250 ml glass is a quarter of a liter.
Worked example: a 70 kg adult who works out 30 minutes needs 70 × 0.033 + (30 ÷ 30) × 0.35 = 2.31 + 0.35 = 2.66 liters, which rounds to 11 glasses a day. In US units that is about 90 fl oz — noticeably more than the old eight-by-eight rule (eight 8-oz glasses, 64 fl oz), and noticeably less than the gallon-a-day challenge (128 fl oz) that floods social media every January.
Where the numbers come from — and where they stop working
The U.S. National Academies of Sciences set adequate total water intake at about 3.7 liters per day for men and 2.7 liters for women — but that includes the water in food, which typically covers 20-30% of the total. Strip the food out and you land near the practical 30-35 ml per kilogram rule that clinicians and dietitians use as a quick screen; this tool uses 33 ml/kg as the midpoint.
The activity term covers sweat: moderate exercise drains roughly 300-500 ml per half hour, so the formula adds 350 ml per 30-minute block.
Keep the limits in mind:
- Heat and humidity. A summer run in Houston or a day at a Florida theme park can double your sweat losses compared with the same workout in a gym.
- Pregnancy and breastfeeding. Needs rise by roughly 300 ml and 700 ml per day, respectively.
- Medical conditions. With heart failure, kidney disease or diuretic medication, your fluid target should come from your doctor, not from any formula.
- Higher body weights. A linear rule overshoots at the extremes; read the output as an upper guideline rather than a quota.
For healthy younger adults, thirst is still a reliable signal. Past age 60 it dulls, which is why scheduled sips beat waiting until you feel dry.
Worked examples, start to finish
Jake, 82 kg (180 lb), runs 60 minutes most evenings in Austin. Base: 82 × 0.033 = 2.706 liters. Exercise: (60 ÷ 30) × 0.35 = 0.70 liters. Total: 2.706 + 0.70 = 3.41 liters, which the calculator shows as 14 glasses (3.406 × 4 = 13.6, rounded to 14). That is about 115 fl oz — close to a gallon only because Jake both weighs a lot and trains hard.
Dana, 66 kg (145 lb), desk job, no workout today. With 0 exercise minutes the answer is just the base: 66 × 0.033 = 2.18 liters, or 9 glasses (2.178 × 4 = 8.7, rounded to 9). A reusable 24-oz bottle filled three times gets her there.
Edge case: the formula floor. At the minimum weight of 20 kg with no activity, the output is 20 × 0.033 = 0.66 liters — a reminder that this tool is calibrated for adults; children's hydration follows pediatric tables, not this rule.
Notice the pattern: every 10 kg (22 lb) of body weight adds 330 ml per day, and every half hour of training adds 350 ml. Once you know your baseline, you can adjust on the fly for a double session or a rest day.
This calculator is for educational purposes only and is not medical advice; talk to a physician or registered dietitian about your individual fluid needs.
About this calculator
This calculator estimates how much you should drink back using just two inputs: your body weight and your daily exercise minutes. You get the answer in liters and in 250 ml glasses (about 8.5 fl oz, close to a classic American cup of water). The math starts from the common 33 ml-per-kilogram guideline and adds 350 ml for every half hour of exercise. Treat it as a baseline, not a prescription: a July afternoon in Phoenix, pregnancy, or certain medications can push your real needs higher.
Frequently asked questions
Do coffee, tea and other drinks count toward my daily water?
Yes. Fluid from any beverage counts: coffee, tea, milk, broth. Caffeine has a mild diuretic effect that does not cancel out the water it delivers. That said, sugary sodas and alcohol hydrate poorly and add calories, so most of your total should still come from plain water.
Where does the 33 ml per kilogram rule come from?
It is a practical rule used in clinical nutrition that approximates official recommendations (U.S. National Academies: about 2.7-3.7 liters of total water daily for adults) by scaling with body weight. The usual range is 30-35 ml/kg; this calculator uses 33 ml/kg as the midpoint and adds extra for exercise sweat.
Can you drink too much water? Is it dangerous?
Yes, though it is rare. Drinking several liters in a short window can dilute blood sodium (hyponatremia), a problem documented mostly in marathons and endurance events. The safe approach is to spread intake across the day and, during very long efforts, alternate water with electrolyte drinks. If you have kidney or heart disease, ask your doctor about your personal limit.
Should I drink more water in hot weather or with a fever?
Yes. Heat, high humidity and fever raise sweat losses beyond what this formula covers, since it assumes normal conditions. During heat waves, add fluid steadily through the day without waiting to feel thirsty, and watch for signals like dark urine, headache or fatigue — signs you are running behind your needs.