Water Intake Calculator

See our free water intake tracker or water challenge

Use the free water intake calculator above to find out exactly how much water you should drink a day — personalized to you in seconds. Enter your weight, age, sex, activity level and climate, choose metric or imperial units, and you’ll instantly see your daily target in cups, fluid ounces, liters, milliliters and water bottles, along with a personalized drinking schedule that spaces your water sensibly from the moment you wake up until a couple of hours before bed. You can also add pregnancy or breastfeeding and a weight-loss goal for an even more tailored number.

Most advice online gives everyone the same flat “eight glasses a day.” Your body isn’t average, though — a 60 kg office worker in a mild climate and a 90 kg athlete training in the heat have very different needs. This calculator works out your number and shows you how it was built, so you can trust it and actually hit it.

How much water should you drink a day?

The most quoted guideline comes from the U.S. National Academies of Sciences, Engineering, and Medicine (formerly the Institute of Medicine). It suggests a total daily water intake — from all drinks and food combined — of about:

• 3.7 liters (125 fl oz, ~15.5 cups) a day for men
• 2.7 liters (91 fl oz, ~11.5 cups) a day for women

Because roughly 20% of the water you take in comes from food (fruit, vegetables, soup, yogurt and so on), the amount you actually need to drink is a little lower than those totals — which is why the headline figure in the calculator is your drinking target, with the food-inclusive total shown alongside it.

The famous “8×8” rule — eight 8-ounce glasses, about 1.9 liters — is easy to remember and a reasonable floor for many adults, but it is not based on your individual size or lifestyle. The calculator on this page gives you something better: a number matched to your body weight, how much you move and how hot it is where you live.

How to calculate your water intake (the formula)

You don’t have to do any maths — the tool does it for you — but here is exactly how it reaches your number, so you can see it’s grounded in real guidance rather than a guess.

Step 1 — Start with your body weight. A widely used clinical rule of thumb is 30–35 milliliters of water per kilogram of body weight, which is almost identical to the popular American version, “drink half your body weight in ounces.”

Rule of thumb: body weight (lb) ÷ 2 = ounces of water per day Example: 150 lb ÷ 2 = 75 oz ≈ 2.2 liters.

Metric version: body weight (kg) × 35 mL ≈ daily water in mL Example: 70 kg × 35 = 2,450 mL ≈ 2.45 liters.

Step 2 — Add water for exercise. Physical activity increases fluid loss through sweat. Following American College of Sports Medicine guidance, the calculator adds roughly 300–1,000 mL (about 12–34 oz) depending on how active you are — from light exercise a few times a week up to daily intense training.

Step 3 — Adjust for climate. Hot, humid or high-altitude conditions raise how much you sweat, so the tool adds around 10% for warm climates and 20% for very hot and humid ones.

Step 4 — Add for pregnancy or breastfeeding. Following Institute of Medicine and European Food Safety Authority figures, the calculator adds about 300 mL during pregnancy and 700 mL while breastfeeding.

Add those together and you have your personal daily target. The calculator’s “What makes up your target” bar shows each of these pieces so nothing is hidden.

Water intake calculator by weight

Body weight is the single biggest factor in how much water you need, which is why water intake by weight is the backbone of this tool. Bigger bodies have more cells, more blood volume and lose more fluid, so they need more water. Here is roughly what the weight-based portion of your target looks like before activity and climate are added:

These are starting points. Add a workout, a heatwave, or pregnancy and your real target climbs — exactly what the calculator does for you automatically.

How many ounces (and cups) of water should I drink a day?

If you think in ounces and cups rather than liters, the calculator has you covered — it shows your result in both at once. As a quick reference, a common healthy range for adults is 64 to 100 fluid ounces (8 to 12.5 cups) a day, with active people and those in hot climates at the higher end. One “cup” here means a standard 8 fl oz (about 240 mL) glass, and a typical single-use water bottle holds about 16.9 oz (500 mL) — so 100 oz is roughly six of those bottles. The tool converts your personal number into liters, milliliters, ounces, 8-oz cups and 500 mL bottles so you can track it however you like.

How much water should I drink to lose weight?

Water is one of the simplest tools for weight management, and searches for how much water to drink to lose weight are hugely popular for good reason. Staying well hydrated helps in three practical ways: it can reduce appetite when you drink a glass 20–30 minutes before meals; it helps you stop mistaking thirst for hunger; and, most importantly, replacing sugary drinks, juice and alcohol with water removes liquid calories from your day.

Research on “pre-loading” water before meals has found that people who drank about 500 mL (17 oz) before each meal tended to eat less and lost modestly more weight than those who didn’t. For weight loss, aim for the target the calculator gives you (select the “Support weight loss” goal for a tailored tip), front-load a glass before each meal, and make water your default drink. Water has zero calories, so you cannot “drink too many calories” of it — the only cautions are the extreme cases covered below.

Water intake and exercise

When you exercise you lose water and electrolytes through sweat, and even mild dehydration of around 2% of body weight measurably reduces strength, endurance and concentration. The American College of Sports Medicine suggests drinking before, during and after activity: roughly 500 mL (17 oz) in the two hours before exercise, 200–300 mL (7–10 oz) every 10–20 minutes during it, and enough afterwards to replace what you lost. Choose your activity level in the calculator — from sedentary up to athlete — and it builds this extra fluid into your daily number. For long or intense sessions over an hour, or heavy sweating, add an electrolyte drink so you replace sodium as well as water.

Water intake in hot climates

Heat and humidity dramatically increase how much you sweat — you can lose 1 to 2 liters an hour exercising in the heat. If you live somewhere warm, work outdoors, or you’re traveling somewhere hot, set the climate option accordingly and the calculator raises your target by 10–20%. Signs you’re keeping up in the heat include pale-yellow urine and not feeling thirsty; dark urine, headache and fatigue mean it’s time to drink more.

Water intake during pregnancy and breastfeeding

Fluid needs rise in pregnancy to support the extra blood volume, amniotic fluid and the baby’s circulation, and they rise further during breastfeeding because breast milk is roughly 87% water. General guidance suggests around 300 mL extra per day during pregnancy and about 700 mL extra while breastfeeding — which is why the calculator has a dedicated setting for both. A reliable habit is to drink a glass of water every time you nurse. Always follow your own midwife’s or doctor’s advice, especially if you have any pregnancy complications.

Water intake for children and by age

Children need less total water than adults but more per kilogram of body weight, and their needs grow with age. As a rough guide, the European Food Safety Authority suggests total daily water (from drinks and food) of about 1.3 L for ages 2–3, 1.6 L for ages 4–8, 1.9 L for girls and 2.1 L for boys aged 9–13, rising to adult levels in the teens. The calculator gives an age-appropriate estimate when you enter a younger age, but a child’s appetite for water varies a lot day to day — offer water regularly, watch for pale urine, and check with a pediatrician if you’re unsure. Older adults should also pay attention: the sense of thirst weakens with age, so it’s easy to drink too little without noticing.

Signs you’re not drinking enough water

Your body is good at telling you when it needs more fluid. Common early signs of dehydration include dark yellow urine, thirst, dry mouth, tiredness, headache, dizziness and poor concentration. A simple daily check is the color of your urine: pale straw-yellow is well hydrated, while dark amber means drink more. Aim to sip steadily through the day rather than gulping a lot at once — which is exactly what the calculator’s drinking schedule helps you do.

Can you drink too much water?

Yes, although it’s rare. Drinking a very large amount of water in a short time can dilute the sodium in your blood, a dangerous condition called hyponatraemia (water intoxication). It mostly affects endurance athletes who over-drink plain water during long events, and people with certain heart, kidney, liver or hormonal conditions. For the vast majority of healthy people the risk is minimal — the target this calculator produces is a sensible daily amount spread across the whole day, not something to drink all at once. If you have a medical condition that requires fluid restriction, follow your doctor’s specific limit instead of a general calculator.

How to use this water intake calculator

1. Choose Male or Female.
2. Pick Metric (kg, mL) or US / Imperial (lb, oz) units.
3. Enter your age and weight (the tool accepts kilograms, pounds or stones).
4. Select your activity level and the climate where you live.
5. Optionally open more options to add pregnancy or breastfeeding, a weight-loss goal, and your wake and sleep times for a personalized schedule.
6. Press Calculate My Water Intake to see your daily target in cups, ounces, liters, milliliters and bottles, a breakdown of how it was calculated, and an hour-by-hour drinking plan — then copy or print it.

Frequently asked questions

How much water should I drink a day? A common guideline is about 3.7 liters (125 oz) a day for men and 2.7 liters (91 oz) for women from all drinks and food combined, but your ideal amount depends on your weight, activity and climate. A simple personal estimate is 30–35 mL per kg of body weight (or half your body weight in ounces), plus more for exercise and hot weather — which is exactly what the calculator above works out for you.

How do you calculate water intake? Multiply your body weight in kilograms by about 35 mL (or divide your weight in pounds by 2 to get ounces), then add roughly 300–1,000 mL for exercise, 10–20% for hot climates, and 300–700 mL for pregnancy or breastfeeding. For example, a 70 kg lightly active adult needs about 2.4–2.8 liters of water to drink per day.

How many ounces of water should I drink a day? For most adults, 64 to 100 fluid ounces (8 to 12.5 cups) a day is a healthy range, with active people and those in hot climates toward the top. The calculator gives your exact number in ounces, cups, liters and bottles.

How much water should I drink to lose weight? Aim for your calculated daily target and drink a glass (about 500 mL / 17 oz) 20–30 minutes before meals. Pre-meal water can reduce appetite, and swapping sugary drinks for water removes liquid calories — both of which support weight loss.

Is it better to measure water in cups, ounces or liters? Use whichever is easiest for you to track — they’re just different units for the same amount. This calculator shows all of them at once: 1 liter ≈ 34 fl oz ≈ 4.2 cups ≈ two 500 mL bottles.

Does coffee, tea or other drinks count toward my water intake? Yes. Water is best, but tea, coffee, milk and juice all count toward your fluid intake, and food provides around 20% of your daily water. Moderate caffeine has only a mild diuretic effect and still hydrates you overall. Alcohol is the exception — it dehydrates you.

How much water should a pregnant woman drink? Roughly 300 mL more than usual per day during pregnancy and about 700 mL more while breastfeeding. Select the pregnancy or breastfeeding option in the calculator for a tailored figure, and follow your own doctor’s advice.

Can you drink too much water? It’s uncommon, but drinking a very large volume in a short time can dilute blood sodium (hyponatraemia). Spreading your intake across the day, as the calculator’s schedule suggests, avoids this. People with kidney, heart or fluid-restriction conditions should follow their doctor’s limit.

How much water should I drink based on my weight? Around 30–35 mL per kilogram, or half your body weight in ounces. So a 60 kg (132 lb) person needs about 2 liters (66 oz) as a baseline, a 90 kg (198 lb) person about 3 liters (104 oz) — before adding exercise and climate.

This page is for general education and is not medical advice. If you have a heart, kidney or liver condition, are on a fluid-restricted diet, or have any concerns about your hydration, talk to a qualified healthcare provider about the right amount of water for you.