Water Intake Calculator

About the Water Intake Calculator

This water intake calculator estimates your daily water requirement based on body weight and exercise duration, using the widely cited guideline of 35 ml of water per kilogram of body weight per day, plus an additional allowance for exercise. Enter your body weight and average daily exercise duration, and the tool returns your daily water target in milliliters, liters, cups, fluid ounces, and glasses.

The 35 ml/kg guideline comes from the European Food Safety Authority (EFSA) and the U.S. National Academies of Sciences, Engineering, and Medicine, which recommend approximately 2.5 L/day for adult women and 3.7 L/day for adult men (from all sources including food). The 35 ml/kg rule scales this for body weight: a 70 kg adult needs 2,450 ml (2.45 L) at baseline, while a 90 kg adult needs 3,150 ml (3.15 L).

Exercise increases water loss through sweat and respiration. The calculator adds 350 ml per 30 minutes of moderate-to-vigorous exercise (about 12 ml per minute, or roughly 350-700 ml per hour depending on intensity and climate). In hot or humid conditions, additional water is needed; the calculator does not adjust for climate automatically.

How It Works

The daily water requirement formula:

Total = (body_weight_kg * 35) + (exercise_minutes * 350 / 30)

The base requirement of 35 ml/kg comes from metabolic water needs (water produced by metabolizing food), water lost through urine, feces, respiration, and insensible perspiration. The 35 ml/kg figure already accounts for water obtained from food (about 20% of total intake) — the calculator returns total water need, which can come from beverages, food, or metabolic processes.

The exercise addition of 350 ml per 30 minutes reflects additional sweat and respiratory loss during moderate-to-vigorous activity. Intense exercise in hot conditions can lose 1-2 L/hour, so adjust upward in heat or for high-intensity endurance activity. Light walking may not require the full 350 ml addition.

Common conversions:

• 1 liter = 1,000 ml = 4.2 cups (240 ml) = 33.8 fl oz
• 1 cup (US) = 240 ml = 8 fl oz
• 1 glass (typical) = 250 ml

Worked Examples

For a 70 kg adult who exercises 30 minutes/day:

1. Base: 70 * 35 = 2,450 ml
2. Exercise addition: 30 * 350 / 30 = 350 ml
3. Total: 2,450 + 350 = 2,800 ml (2.8 L)
4. In cups: 2,800 / 240 = 11.7 cups
5. In fluid ounces: 2,800 / 29.57 = 94.7 fl oz

For a 90 kg adult who exercises 60 minutes/day:

1. Base: 90 * 35 = 3,150 ml
2. Exercise addition: 60 * 350 / 30 = 700 ml
3. Total: 3,150 + 700 = 3,850 ml (3.85 L)
4. In cups: 16.0 cups

For a 55 kg adult who does not exercise:

1. Base: 55 * 35 = 1,925 ml
2. Exercise addition: 0 ml
3. Total: 1,925 ml (1.93 L)
4. In cups: 8.0 cups

In hot or humid conditions, add 500-1,000 ml extra. At altitude (above 2,500 m), add 500 ml extra due to increased respiratory water loss.

When to Use This Tool

Use this water intake calculator when:

• Setting a daily hydration target for general health
• Adjusting water intake for exercise or sports training
• Planning hydration for hot weather or travel to high-altitude destinations
• Demonstrating the relationship between body size, activity, and water needs in health classes
• Comparing your current intake to evidence-based recommendations
• Setting a target for children, elderly, or pregnant women (with appropriate adjustments)
• Calculating fluid needs for backpacking, hiking, or other endurance activities

Thirst is a reliable guide for most healthy adults — drink when thirsty, and check that your urine is pale yellow (dark yellow indicates dehydration). The calculator provides a target; your actual need varies day to day based on climate, diet, and activity.

Limitations & Disclaimer

This calculator uses the 35 ml/kg/day guideline from EFSA and U.S. National Academies, plus 350 ml per 30 minutes of moderate exercise. It does not adjust for climate (hot/humid conditions may require 500-1,000 ml extra), altitude (above 2,500 m adds ~500 ml due to respiratory loss), pregnancy or lactation (which increase requirements by 300-700 ml), illness (fever, vomiting, diarrhea increase fluid needs), age (older adults have reduced thirst sensation and may need reminders to drink), dietary factors (high-sodium or high-protein diets require more water), or individual variation in sweat rate. The guideline covers total water intake from all sources (beverages, food, metabolic water), not just plain water. This is an educational estimate, not medical advice. Individuals with kidney disease, heart failure, or other conditions requiring fluid restriction should follow their physician's guidance. See our disclaimer for full terms.

Frequently Asked Questions

Last updated: July 21, 2026  ·  Author: HT99 Tools Editorial Team  ·  Reviewed by: HT99 Tools Editorial Team