Daily water intake calculator

Water intake — bodyweight + activity + climate, not "8 glasses a day"

The "8 glasses a day" rule is a 1945 USDA recommendation that included water from food. Most people misremember it as 8 glasses of plain water on top of food. Here's the real math.

The components

1. Baseline: ½ your bodyweight in ounces. A 175 lb person needs ~88 oz baseline. This is calibrated to roughly 30 ml/kg.
2. Exercise: 12 oz per 30 minutes of training. Heavy sweaters need more.
3. Climate: cool +0, moderate +8 oz, hot +24 oz, extreme +40 oz. Hot weather increases insensible losses (breath, skin) by 2-4×.
4. Sweat rate: ±15-20% on the exercise + climate buffer.

Why food matters

Fruits, vegetables, soup, even meat contain water. A typical mixed diet provides 20-30% of daily water — about 20-30 oz of "free" water. The intake numbers above are fluid only because most people undercount and over-correct.

Sodium is the missing input

Drinking lots of water without replacing electrolytes during long exercise sessions causes hyponatremia (dilutional low sodium) — headaches, nausea, in extreme cases seizures. Heavy sweaters lose 1,000-1,500 mg sodium per hour. Plain water replaces volume but not electrolytes.

For sessions over 60 minutes (especially in heat), use:

• Sports drink: 14-20 mEq/L sodium (Gatorade Endurance, Skratch)
• Salt tablets: 200-400 mg per tablet, 1-2 per hour
• Homemade: ¼ tsp salt + 8 oz juice + 16 oz water = ~600 mg sodium