Daily Water Intake Calculator

What this calculator estimates

Your body loses water all day through urine, breath, sweat and stool, and it has to be replaced. This calculator estimates total water intake, meaning every millilitre that should reach you from drinks and from food, and then splits out the part you are expected to drink.

A flat rule such as eight glasses ignores the fact that a 50 kg office worker and a 100 kg builder lose very different amounts. Here the target follows your energy expenditure: the more energy your body burns each day, the more water it turns over, so size, age, sex and activity all feed into the answer.

This is a population-based estimate for a mild climate and ordinary health, not medical advice. Kidney, heart or liver conditions, some medicines and fluid-restricted diets all change what is safe. See the disclaimer and ask your doctor before changing how much you drink.

How to use the calculator

1. Pick US units (feet, inches, pounds) or Metric units (centimetres or metres, kilograms), and enter your age and sex.
2. Fill in your height and weight. Check that the unit beside the weight box matches the number you typed.
3. Choose the activity level that describes a normal week, from sedentary to extreme (very heavy exercise or a physical job).
4. If you selected female, a pregnancy row appears: leave it on Not pregnant, or choose Pregnant or Lactating.
5. Press Calculate. You get two lines: the recommended total intake, and the portion to take as fluids.

Each line is shown four ways, for example 2,000 mL / day (~2 L, 68 oz, or 8 cups). Ounces are US fluid ounces and a cup is 8 fl oz, which is about 237 mL.

How the estimate is built

The calculation has three steps. First it estimates resting energy use with the Mifflin-St Jeor equation, then scales it by your activity level to get daily energy expenditure in kilocalories. Second, it assumes your body needs about 1 millilitre of water per kilocalorie burned. Third, it adjusts for pregnancy or lactation and splits the total into water from drinks and from food.

resting energy (kcal) = 10 × kg + 6.25 × cm − 5 × age + 5 (men) | − 161 (women)

total water (mL) = resting energy × activity factor × pregnancy factor

For a woman who is pregnant the total is multiplied by 1.25, and for a woman who is lactating by 1.38, reflecting the extra water that pregnancy and milk production demand. The pregnancy setting has no effect when male is selected.

Drinks supply roughly four fifths of total water for adults and teenagers from age 14, so the fluids line is 80% of the total. For children under 14 it is 75%, because food makes up a slightly larger share of what they take in.

Worked examples

An active man. He is 30, 5 ft 10 in (177.8 cm) tall, weighs 160 lb (72.57 kg) and exercises one to three times a week. Resting energy is 10 × 72.57 + 6.25 × 177.8 − 5 × 30 + 5 ≈ 1,692 kcal. Multiplying by 1.375 gives about 2,326 kcal, so the target is 2,326 mL a day, about 79 oz. Fluids are 80% of that, 1,861 mL or roughly 8 cups, and the remaining 465 mL arrives in food.

A woman across three stages. She is 25, 165 cm, 60 kg and sedentary. Resting energy is 600 + 1,031.25 − 125 − 161 = 1,345.25 kcal, and × 1.2 gives 1,614 kcal. Her target is then:

Quick reference: energy to water

Because the rule is one millilitre per kilocalorie, you can estimate a target from any energy figure, for example from the result of the TDEE calculator or the BMR calculator. The fluids column uses the adult 80% share.

What the estimate leaves out

The result assumes a mild climate and no unusual losses. You will need more than the figure shows if you:

• work or train in heat or at altitude, where sweat losses can reach a litre or more per hour during hard effort;
• have a fever, vomiting or diarrhoea, which drain fluid quickly;
• breathe dry air for long periods, for instance on flights or in heated buildings;
• eat a diet that is high in protein or salt, which pushes the kidneys to excrete more.

For official benchmarks, the European Food Safety Authority sets adequate total water intakes of about 2.5 L a day for adult men and 2.0 L for adult women, and the US Institute of Medicine uses higher figures of about 3.7 L and 2.7 L. Both count water from all sources, so your personal number from this tool may sit above or below them.

Where the water comes from

Plain water is only one source. Tea, coffee, milk, juice and soup all count towards the fluids target, and many fruits and vegetables are more than 80% water. Cucumber, lettuce, watermelon and oranges are among the wettest foods, while bread, cheese and meat contribute less. Metabolism also releases a small amount of water as food is burned.

Thirst is a reliable guide for most healthy people. Pale yellow urine usually means you are well hydrated, while dark amber urine suggests you should drink more. Drinking far more than you need, especially a large volume in a short time, can dilute blood sodium (hyponatremia), which is rare but dangerous.

Common mistakes

• Treating the total as the amount to drink. Roughly a fifth of your water arrives with food, so drink the fluids line, not the headline total.
• Mixing units. Entering 160 as kilograms instead of pounds adds more than a litre to the result. Check kg versus lb before you press Calculate.
• Overstating activity. Pick the level that matches an average week, not your best one. Each step up adds 17.5% of your resting energy, typically 200 to 330 mL.
• Forgetting extra needs. Hot weather, long workouts and illness are not modelled, so add to the figure on those days.

To work out how much of your body is water, try the total body water calculator; for a deficit caused by a high sodium level, see the free water deficit calculator, a clinical estimate for people with a high blood sodium level.