Water Intake Calculator

Estimate daily water needs from body weight, activity, climate, and lifestyle factors.

Daily water target

3.8 L

3,790 ml

Base need

2,450 ml

35 ml per kg

Exercise add-on

540 ml

12 ml per exercise minute

Climate add-on

500 ml

Editable assumption

Hydration inputs

Estimate daily water needs from weight, activity, climate, and caffeine.

Body weight

Exercise

min/day

Hot climate add-on

Caffeinated drinks

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What is the water intake calculator?

This calculator estimates personalized hydration need from weight, age, exercise, and environment. It follows the 2026 Korean Nutrition Society, WHO, and EFSA framing in the source page rather than using a single universal eight-cups rule.

Weight-based formula

A practical baseline is 30 to 35 mL/kg per day. Exercise adds about 350 mL for every 30 minutes, and hot or dry environments can raise the need further.

Why hydration changes performance

Body water and dehydration signals

The body is about 60% water, blood is more than 90% water, and the brain is about 75% water.
A 1% body-water loss can cause thirst, dry mouth, and mood changes.
A 2% loss can reduce concentration and short-term memory.
A 3% loss can cause severe fatigue, headache, and impaired judgment.

Daily recommendation ranges

Infant 0 to 6 months: about 680 mL from breast milk or formula.
Age 1 to 3: 1,000 to 1,300 mL; age 4 to 8: 1,200 to 1,600 mL; age 9 to 18: 1,900 to 2,500 mL.
Adult men 2,500 to 3,700 mL, adult women 2,000 to 2,700 mL.
Pregnancy adds about 300 mL for 2,300 mL or more; lactation adds about 700 mL for 2,700 mL or more.
Adults 65 and older often use 1,500 to 2,000 mL as a practical range, with thirst sensitivity monitored carefully.

Beverage contribution and exercise hydration

How drinks count

Water, mineral water, barley tea, and corn tea count as 100% water contribution.
Green tea and black tea count around 99%, ion drinks about 94%, americano about 95%, cold brew about 90%, and energy drinks about 85%.
Milk and soy milk count about 87%, fruit juice 85% to 90%, carbonated beverages 89% to 90%, and smoothies about 85%.
Alcohol causes net water loss in the source table: beer -20%, wine -30%, and soju -50%.

Practical FAQ guidance

The eight-cups or 2 L rule is not universal; body size, activity, and environment change the target.
Coffee and tea count, but caffeine lowers effective contribution compared with plain water.
For exercise, drink 500 mL or more about 2 hours before, then 150 to 200 mL every 15 to 20 minutes during activity.
For high-intensity exercise longer than 1 hour, a sports drink can help replace electrolytes.
Up to about 120% to 150% of a general daily target is usually safe, but rapid excessive intake should be avoided.