💧 Water Intake Calculator

Personalised daily hydration goal based on your weight, activity & climate

Please enter a valid weight
Activity level
Climate
2.45 litres / day
≈ 83 oz  ·  ≈ 10 cups (250 ml each)

💧 Hydration Reminder Schedule

Enter your wake and sleep times to get an hourly drinking schedule.

How the formula works

This calculator uses a three-layer approach based on published guidelines from the WHO, EFSA, and the National Academies of Sciences:

Step 1 — Baseline (Holliday-Segar derived)
35 ml × body weight in kg = baseline daily fluid need

Step 2 — Activity adjustment
Sedentary: +0% · Lightly active: +15% · Moderately active: +30% · Very active: +50% · Athlete: +70%

Step 3 — Climate adjustment
Cool: +0 ml · Hot/humid: +500 ml · Very hot/desert: +1,000 ml

Step 4 — Goal adjustment
General health: +0 ml · Weight loss: +500 ml · Performance: +750 ml

Step 5 — Pregnancy/breastfeeding
Pregnant: +300 ml (WHO AIF) · Breastfeeding: +700 ml (WHO AIF)

A worked example

A 75 kg moderately active person in a hot climate with a general health goal:

  • Baseline: 75 × 35 ml = 2,625 ml
  • Activity (moderate, ×1.30): +788 ml → 3,413 ml
  • Climate (hot): +500 ml → 3,913 ml
  • Goal (general health): +0 ml
  • Total: ~3.9 litres/day

Important note on food water

Roughly 20% of daily water intake comes from food (especially fruits and vegetables). The figures above represent total fluid intake including food-derived water. If you eat a typical Western diet, subtract ~20% for your pure-drink target. If you eat a diet rich in raw produce, subtract up to 30%.

Signs you're not drinking enough

Dehydration can set in before you feel thirsty. Watch for these early and late warning signals:

Mild dehydration (1–2% fluid loss)

  • Dark yellow or amber urine — pale straw yellow is your target
  • Urinating fewer than 4 times per day
  • Mild headache or difficulty concentrating
  • Dry mouth, lips, or skin
  • Fatigue or feeling sluggish, especially in the afternoon

Moderate dehydration (3–5% fluid loss)

  • Decreased urine output or very dark urine
  • Dizziness when standing up
  • Reduced exercise performance (up to 30% reduction in endurance)
  • Muscle cramps, especially during or after exercise

The simplest daily check: look at your urine colour first thing in the morning. Pale yellow = well hydrated. Bright or dark yellow = drink more before you eat breakfast.

Best times to drink water

  • On waking: 400–500 ml to rehydrate after overnight fasting and support morning metabolism
  • Before meals: 400–500 ml 20–30 minutes before eating — shown to reduce calorie intake by ~75–90 kcal
  • During exercise: 150–250 ml every 15–20 minutes
  • In the heat: drink before you feel thirsty — thirst lags behind actual need by up to 20–30 minutes in heat
  • Before bed: small sip (100–200 ml) — enough to prevent overnight dehydration without disrupting sleep

Water vs. sports drinks for exercise

For most workouts under 60 minutes at moderate intensity, water is the optimal (and cheapest) hydration choice. Sports drinks become genuinely beneficial when:

  • Exercise lasts more than 60–90 minutes
  • You're sweating heavily in heat (losing significant sodium)
  • You're competing in endurance events where fuelling matters as well as hydration

A practical mid-point: plain water plus a small amount of electrolyte powder or a pinch of salt in a 500 ml bottle — you get the sodium and potassium replacement without the sugar load of commercial sports drinks.

If your goal is weight loss, note that a standard 500 ml sports drink contains 120–160 kcal. For a 45-minute gym session that burns 300 kcal, that's a meaningful fraction of the caloric deficit you're trying to create.

Frequently asked questions

Most adults need 2–3.7 litres (68–125 oz) per day depending on body weight, activity level, and climate. Use the calculator above for a personalised recommendation based on your specific factors.
The "8×8" rule (eight 8-oz glasses ≈ 1.9 L) is a rough heuristic, not a science-backed guideline. The actual amount varies considerably with body size, activity level, climate, and diet. A 50 kg sedentary adult in a cool climate needs far less than a 90 kg athlete in summer heat.
Research suggests drinking 500 ml of water 30 minutes before meals can reduce calorie intake. For weight loss, select the "Weight loss" goal in the calculator, which adds ~500 ml above the general health baseline to support metabolism and appetite control.
The American Council on Exercise recommends drinking 240–360 ml (8–12 oz) of water every 15–20 minutes during exercise. For activities lasting over 60 minutes, consider electrolyte drinks to replace sodium and potassium lost through sweat.
Yes — moderate consumption of caffeinated beverages (up to 400 mg caffeine/day) counts toward fluid intake. The diuretic effect of caffeine is mild and doesn't negate the hydrating benefit. However, alcohol does not count and actually increases water requirements.
Key signs of mild dehydration include dark yellow urine, infrequent urination (less than 4 times/day), headache, dry mouth, fatigue, and difficulty concentrating. Pale yellow urine is the best indicator of good hydration.