Hydration Calculator
Hydration is not just about drinking water — it is about matching what you take in to what you lose, through sweat, breathing, urine and digestion. When the two do not match, you feel it: headaches, tiredness, dry lips, dark urine. The hydration calculator above works out how much water you need a day from your weight, activity, climate and exercise, so you can stay ahead of dehydration instead of chasing it.
This page explains how hydration works, how to tell if you are hydrated, what counts toward your intake, and how exercise, heat and illness change what you need.
What Hydration Means
Your body is about 60% water. It carries nutrients, regulates temperature, cushions joints and flushes waste. You lose roughly 2 to 3 litres a day through urine, sweat, breathing and digestion, and you replace it through drinks and food. Hydration is simply keeping that balance: take in what you lose, and your body runs well. Fall behind by even 2% of body weight — about 1.4 litres for a 70 kg person — and performance, mood and concentration start to suffer. That is why a target matters: thirst alone is a late signal, and by the time you feel it, you are already behind.
Your Daily Hydration Target
The calculator works out your target from your weight: 35 ml per kilogram, adjusted for activity and climate, plus 12 ml per minute of exercise. For a 70 kg moderately active person with 30 minutes of exercise, the total is 3,423 ml — 3.42 litres. About 20% of that comes from food, so the drinking target is roughly 2.7 litres. The table below shows the target for a moderately active person with 30 minutes of exercise at different weights.
| Body weight | Total water needed | From drinks (80%) | From food (20%) |
|---|---|---|---|
| 50 kg | 2,548 ml | 2.0 litres | 0.5 litres |
| 60 kg | 2,985 ml | 2.4 litres | 0.6 litres |
| 70 kg | 3,423 ml | 2.7 litres | 0.7 litres |
| 80 kg | 3,860 ml | 3.1 litres | 0.8 litres |
| 90 kg | 4,298 ml | 3.4 litres | 0.9 litres |
| 100 kg | 4,735 ml | 3.8 litres | 0.9 litres |
Check the 80 kg row: 80% of 3,860 ml is 3,088 ml, or 3.1 litres from drinks, and 20% is 772 ml, or 0.8 litres from food. The food share is an average — a diet rich in fruit, vegetables and soup can push it higher, while a diet of dry, salty food pushes it lower.
Are You Hydrated? The Checks That Work
There are three reliable ways to check your hydration, and none of them needs equipment.
| Check | Hydrated | Dehydrated |
|---|---|---|
| Urine colour | Pale yellow, like straw | Dark yellow or amber |
| How often you urinate | Every 3 to 4 hours | Rarely, or very little at a time |
| How you feel | Alert, normal energy | Tired, headache, dry mouth, dizzy |
Urine colour is the most useful single check. Pale yellow means you are drinking enough. Dark yellow means drink more. Completely clear usually means you are drinking more than you need — which is not dangerous for most people, but it is wasted effort. First thing in the morning your urine will be darker than later in the day, so judge by the afternoon colour, not the morning one. Vitamins, especially B vitamins, can turn urine bright yellow and confuse the check — if you take them, rely on how you feel instead.
What Counts Toward Hydration
Almost everything you drink counts, and so does much of what you eat. The table below shows the water content of common foods — the reason food supplies about a fifth of your daily water.
| Food or drink | Water content | Notes |
|---|---|---|
| Cucumber | 96% | One of the most hydrating foods |
| Lettuce | 96% | Mostly water by weight |
| Watermelon | 92% | A 200 g slice gives 184 ml |
| Soup | About 92% | Counts almost fully as water |
| Oranges | 87% | Fruit is a real source of water |
| Milk | 87% | Counts toward hydration |
| Yoghurt | About 85% | A 150 g pot gives about 128 ml |
| Tea and coffee | About 99% | Count toward intake at normal amounts |
| Soft drinks | About 90% | Count as water, but add sugar and calories |
Tea and coffee count. The idea that they dehydrate you is a myth at normal intakes — a cup of tea is 99% water, and the mild diuretic effect of caffeine does not cancel that out. Soft drinks count too, but they add sugar and calories, so water is the better choice for most of your intake. Alcohol is the exception: it increases urine output beyond what you drink, so it dehydrates rather than hydrating.
Hydration and Exercise
Exercise is when hydration matters most, because sweat losses can be large: 0.5 to 2 litres an hour depending on intensity and heat. The calculator adds 12 ml per minute of exercise, which covers a moderate session. For longer or hotter sessions, drink during exercise, not just after: about 150 to 250 ml every 15 to 20 minutes is a good rule. Weigh yourself before and after a long session if you want to know your sweat rate — every kilogram lost is roughly a litre of sweat to replace. For sessions over 90 minutes, or in extreme heat, add electrolytes: water alone can dilute blood sodium if you drink large volumes without salt.
When You Need More Water
- Hot weather. Sweat losses rise sharply. A hot, dry climate adds 15% to your base intake before exercise is counted.
- Illness. Fever, vomiting and diarrhoea all increase water loss. Sip small amounts often, and use oral rehydration salts if losses are large.
- High altitude. You breathe faster and lose more water through the lungs, and the air is usually dry.
- Pregnancy and breastfeeding. Pregnancy adds 300 ml a day and breastfeeding adds 700 ml. Check your due date to track your stage.
- Air travel. Cabin air is very dry. Drink an extra glass per hour of flight.
- Saunas and hot baths. You can lose half a litre in a 20-minute sauna. Drink before and after.
Can You Drink Too Much?
Yes, though it is rare outside endurance sport. Drinking far more than you need can dilute the sodium in your blood — hyponatremia — which causes confusion, nausea and, in severe cases, seizures. It mostly affects marathon runners and triathletes who drink large volumes of plain water over many hours. For everyday life, drinking to thirst plus a sensible target is safe. If your urine is completely clear all day and you are urinating very often, you are probably drinking more than you need, and you can ease back.
Related tools: your energy plan from the calorie calculator and your TDEE calculator, and your BMI for the weight behind the number. The Mayo Clinic explains hydration, dehydration and how much water you need.
Quick Reference
Use the water intake calculator above with your own numbers, and keep the reference figures below handy while you plan.
| Person | Daily total | Fluid ounces | 8-oz glasses |
|---|---|---|---|
| 50 kg, sedentary | 1,750 ml | 59 fl oz | 7.4 glasses |
| 70 kg, moderately active | 3,423 ml | 116 fl oz | 14.5 glasses |
| 90 kg, moderately active | 4,298 ml | 145 fl oz | 18.2 glasses |
| 70 kg, pregnant | 3,723 ml | 126 fl oz | 15.7 glasses |
| 70 kg, breastfeeding | 4,123 ml | 139 fl oz | 17.4 glasses |
What to remember
- Your base is 35 ml per kilogram of body weight a day, before activity and climate.
- A 70 kg moderately active person needs about 3.42 litres a day; a 50 kg sedentary person about 1.75 litres.
- About 20 percent of daily water comes from food — the rest must be drunk.
- Exercise adds 12 ml per minute on top of your base intake.
- Pale yellow urine in the afternoon is the simplest sign you are drinking enough.
Your checklist
- Enter your weight, exercise, activity level and climate in the calculator above.
- Note your target in litres, ounces, glasses and bottles.
- Fill your bottles in the morning so you can see the day target.
- Drink a glass with every meal and one between meals.
- Recalculate whenever your weight, activity or the season changes.
Mistakes to avoid
- Trusting the eight-glass rule — it is a minimum for a small sedentary person, not a target.
- Waiting for thirst, which is a late signal, not an early one.
- Forgetting that food counts — fruit, soup and yoghurt all contribute.
- Counting alcohol as hydration — it dehydrates rather than hydrating.
- Drinking far more than you need, which can dilute blood sodium.
Frequently Asked Questions
How do I know if I am hydrated?
Check your urine colour: pale yellow like straw means you are hydrated, dark yellow means drink more. You should urinate every 3 to 4 hours, and feel alert with normal energy. Thirst is a late signal — do not wait for it.
How much water do I need a day?
A 70 kg moderately active person with 30 minutes of exercise needs about 3.42 litres a day in total, of which about 2.7 litres comes from drinks and 0.7 litres from food. Use the calculator above for your own target by weight, activity and climate.
Does coffee dehydrate you?
No, not at normal intakes. Coffee and tea are about 99% water, and the mild diuretic effect of caffeine does not cancel that out. They count toward your daily intake, though water is still the best choice for most of it.
How much water should I drink when exercising?
Add about 12 ml per minute of exercise. Drink 150 to 250 ml every 15 to 20 minutes during the session, not just after it. For sessions over 90 minutes or in extreme heat, add electrolytes as well as water.
Can food hydrate you?
Yes. About 20% of daily water comes from food, especially fruit, vegetables, soup and yoghurt. Cucumber and lettuce are 96% water, watermelon 92%, and oranges 87%. A diet rich in these foods lowers how much you need to drink.
What are the signs of dehydration?
Dark yellow urine, urinating rarely, dry mouth and lips, tiredness, headaches and dizziness. Severe dehydration — confusion, very low urine output, rapid heartbeat — needs medical attention urgently.
Disclaimer: This calculator provides estimates for general information only and is not medical advice. Water needs vary with health conditions, medicines, pregnancy and climate. Consult a doctor if you have kidney disease, heart failure, or any condition that affects fluid balance.