How Much Water Should You Drink Per Day? (Calculator + Science)

“Drink 8 glasses a day” is the most common hydration advice — and it’s overly simplistic. Your actual water needs depend on your body weight, how active you are, and the climate you live in.

The formula

The evidence-based starting point:

Base water need = Body weight (kg) × 0.033 litres

A 70 kg person needs approximately 2.31 litres as a baseline. From there, adjustments are made.

Activity level multipliers

Physical activity increases water loss through sweat. The multipliers:

Activity Level	Multiplier	Example
Sedentary	×1.0	Desk job, no exercise
Light	×1.2	Walking, light yoga 1–3×/week
Moderate	×1.4	Gym/running 3–5×/week
Active	×1.6	Intense training 6–7×/week
Very Active	×1.8	Athlete-level daily training

Climate adjustment

• Hot climate: +0.5 litres (more sweating)
• Temperate: No adjustment
• Cold climate: -0.2 litres (less sweating, but don’t under-drink)

Worked example

An 80 kg person, moderately active, in a hot climate:

Base = 80 × 0.033 = 2.64 L
After activity = 2.64 × 1.4 = 3.70 L
After climate = 3.70 + 0.5 = 4.20 L
That's about 17 glasses or 8.4 bottles (500ml)

Signs you’re not drinking enough

• Dark yellow urine (should be pale straw color)
• Persistent headaches, especially in the afternoon
• Fatigue and difficulty concentrating
• Dry lips and skin
• Feeling hungry when you just ate (thirst mimics hunger)

Tips for staying hydrated

1. Keep a water bottle at your desk — visibility is a reminder
2. Drink a glass immediately after waking up
3. Set hourly reminders if you forget
4. Eat water-rich foods (cucumbers, watermelon, oranges)
5. Tea and coffee count — despite mild diuresis, net hydration is positive

Calculate your personal target with the OurDailyCalc water intake calculator.

TL;DR

• Base formula: weight(kg) × 0.033 L
• Adjust for activity (×1.0 to ×1.8) and climate (+0.5L hot, -0.2L cold)
• A 70 kg moderately active person needs ~3.2 L/day
• Track urine color — pale yellow means you’re hydrated