Water Intake Calculator by Weight

How this water intake calculator works

This tool calculates 35 millilitres per kilogram of body weight, plus 350 millilitres for each 30 minutes of exercise. It is a rule of thumb. What comes out is a starting estimate, and nobody has measured your actual fluid requirement. Results update as you change the inputs.

Read the result as a rough total-water estimate. Water in food and all beverages contributes to hydration. The cups and ounces are that same total expressed in other units, so they are a unit conversion and not a second helping on top. A cup here is 240 ml; fluid ounces are US units.

The National Academies’ water reference report gives adequate intakes of 3.7 litres for adult men and 2.7 litres for adult women, including food and beverages. Those are population reference values. The same report never tested the weight-based rule this calculator uses, and never tested a fixed allowance per half hour of exercise either.

Daily water estimate by body weight

Each row uses the same arithmetic as the calculator. Exercise columns add a fixed allowance; actual sweat loss can be very different.

Worked example: 70 kg and 30 minutes of exercise

70 × 35 = 2,450 ml. The exercise allowance adds 350 ml, giving 2,800 ml, or 2.80 litres. That is about 12 cups of 240 ml, or 95 US fluid ounces. Select pounds to enter the same body weight as about 154.3 lb; switching units converts your entry.

How much of that needs to be a drink?

Food supplies part of your water. The National Academies report describes about one fifth coming from food in the population data, though the share swings with what you eat. Soup, fruit and other water-rich foods count, as do tea, coffee and milk, and whether coffee counts toward water intake goes through the caffeine research. Drinking the full calculator result in plain water, on top of your meals and your coffee, overshoots the estimate by several hundred millilitres.

Where the estimate stops being useful

Heat, exercise intensity, clothing and your own sweat rate all move fluid losses around. Minutes of exercise is a crude stand-in for all four, and the fixed allowance goes furthest wrong on long sessions and on training in heat. Two people can finish the same hour of work having lost volumes that differ by a litre.

This tool is written for general adult education. It cannot set fluid intake for children, pregnancy, breastfeeding, illness or anyone with a prescribed fluid restriction. Ask your clinician for guidance if you have kidney or heart disease, or take medicines that affect fluid balance. Do not force fluids down to reach a calculator number.

For the background, read the water-intake guide. To estimate energy needs separately, use the calorie and TDEE calculator.

How many water bottles should I drink a day?

Bottle size, food and your other drinks all change the answer, so no single bottle count works for everyone. The conversion below puts the calculator’s rough estimate into your own container size. Read it as a volume, not as a quota to finish.

The bottle conversion first separates an assumed food share from the total. Choose 0%, 10%, 20%, 30% or 40% to see the effect. The default 20% matches the approximate food contribution in the population data above, and your own meals may sit well either side of it.

Water from drinks = total estimate × (1 − food share ÷ 100). Divide that volume by your selected bottle size. With the 2.80 L example and a 20% food assumption, food contributes 0.56 L and drinks contribute 2.24 L. That equals 4.48 bottles of 500 mL, or about 2.99 bottles of 750 mL.

The tool also supports 1-litre, 20-US-fluid-ounce and 32-US-fluid-ounce bottles. Select My bottle size for another container, enter its capacity, and choose millilitres or US fluid ounces. Changing the bottle unit converts your entry. These figures cover all your drinks together, so your morning coffee is already inside the count.

A 24-ounce bottle example

A 24-US-fluid-ounce bottle holds about 709.76 mL. For the illustrative 2,240 mL drinks portion above, 2,240 ÷ 709.76 = about 3.16 bottles. The conversion uses 1 US fluid ounce = 29.5735295625 mL. A fluid ounce measures volume, so it has no fixed relationship to an ounce on the kitchen scales.

Copy or print your calculation

Open How your result is calculated and you get the weight conversion, the 35 ml multiplication, the exercise allowance and the cup and ounce equivalents, all worked through with your own numbers. The steps run the same arithmetic as the headline figure, with intermediate values rounded so they read cleanly.

Copy results puts your inputs, the labelled results, the calculation steps and the notes on your clipboard. Print / save PDF opens a report preview and then your browser’s print dialogue, where Save as PDF is usually one of the options. Read the report before you send it to anyone, because it carries the measurements you typed in. Reset example puts the sample values back and leaves your unit choices alone. All of this happens in your browser, and the tool keeps no history of your calculations.

Common questions

Is this a water intake calculator by weight?

Yes. Enter kilograms or pounds, then minutes of exercise. The formula converts weight to kilograms before calculating the result.

Does the result include food and other drinks?

Yes. Read it as a rough total-water estimate. The equivalent cups and ounces are that same total in other units, so your meals and your coffee already count toward them.

Is half my body weight in ounces accurate?

It is another rough rule, pitched at the same level of precision as this one, and it ignores your food, your climate, your activity and your health in exactly the same way. Start from either figure, then let thirst and your usual urine pattern correct it over a week.

Why are cups rounded?

The underlying result is calculated in millilitres. Cups are rounded to a whole 240 ml cup and ounces to a whole US fluid ounce, so displayed equivalents are approximate.

Are my weight and exercise entries saved?

The calculation runs in your browser. The tool does not send these input values to the site or save them for your next visit.

Written and maintained by Rowan Hale, the site author’s pen name. Updated September 13, 2026. No medical or dietetic qualification is claimed. Read our editorial policy or report a calculation error.