“8 glasses a day.” You’ve heard it a million times. It’s also wrong.

That advice is from the 1940s. One-size-fits-all for a 120lb woman and a 220lb man? Come on.

Here’s how to actually calculate your daily water intake.

The Simple Formula

Your weight (lbs) ÷ 2 = ounces of water per day

That’s it. A 160lb person needs roughly 80oz (about 2.4 liters).

In kilograms: Your weight (kg) × 30-35 = ml per day

But Wait — It’s Not That Simple

That’s your baseline. You need to adjust for:

Activity Level

  • Light exercise: Add 12oz (350ml)
  • Moderate exercise: Add 20oz (600ml)
  • Intense workout: Add 32oz+ (1L+)

Climate

  • Hot weather: Add 16-32oz (500ml-1L)
  • Dry/air-conditioned environments: Add 8-16oz (250-500ml)

Other Factors

  • Coffee/alcohol: Add 8oz for every caffeinated or alcoholic drink
  • Pregnancy: Add 10-12oz (300-350ml)
  • Breastfeeding: Add 32oz+ (1L+)

The Real-World Calculation

Let’s say you’re:

  • 170lbs (77kg)
  • Work out 3-4 times a week
  • Drink 2 coffees a day
  • Live in a moderate climate

Baseline: 170 ÷ 2 = 85oz Exercise adjustment: +16oz (average) Coffee adjustment: +16oz Total: ~117oz (3.5L)

That’s way more than “8 glasses.”

Signs You’re Not Drinking Enough

Your body tells you. Listen:

  • Dark yellow urine (should be light yellow)
  • Headaches (especially afternoon)
  • Fatigue that coffee doesn’t fix
  • Dry skin, chapped lips
  • Feeling hungry when you just ate (thirst mimics hunger)

Signs You’re Drinking Too Much

Yes, this is possible:

  • Clear, completely colorless urine
  • Bathroom trips every 30 minutes
  • Bloating
  • Nausea

The Best Way to Track

Calculating is one thing. Actually drinking enough? Different problem.

Most people forget. Get busy. Skip lunch water. Then chug a liter at 10pm (hello, midnight bathroom trips).

You need:

  1. A baseline calculation
  2. Reminders that adapt to your schedule
  3. Easy tracking (one tap, not 6 steps)

That’s why I built WaterDrop. It calculates your needs, reminds you at the right times, and tracks with one tap.

No math. No guessing. Just drink your water.

FAQ

How much water should I drink to lose weight? Same formula, but prioritize drinking water before meals. Studies show 16oz (500ml) before eating reduces calorie intake by 75-90 calories per meal.

Does coffee count toward water intake? Partially. Coffee is a mild diuretic, so you absorb about 60-70% of the water. Count it as half.

Can I drink too much water? Yes. Hyponatremia (water intoxication) is real but rare. Stick to your calculated amount spread throughout the day.

What about sparkling water? Counts the same as regular water. The carbonation doesn’t affect hydration.

Related reads:

— Dolce