How Much You Actually Need

The old rule of eight glasses a day was never based on solid science — and no single formula fits everyone. A few things matter more:

• Body weight gives a rough starting point — a common rule of thumb is about half your weight in ounces
• Activity level can change needs sharply — intense exercise may require an extra liter per hour
• Climate and altitude both raise water loss through sweat and breathing
• Food counts — roughly 20% of your daily water comes from fruits, vegetables, and other foods

The simplest check is urine color. Pale straw yellow means you are well hydrated. Dark yellow means drink more. Clear means you might be overdoing it.