The U.S. National Academies of Sciences, Engineering, and Medicine recommends roughly 15.5 cups (124 oz) of water per day for men and 11.5 cups (92 oz) for women — but this includes water from food (about 20% of intake). The "eight glasses" rule was a misreading of a 1945 recommendation that was never intended as a target.
The formula we use
Base water = body weight (lbs) × 0.5 oz — a widely-used clinical heuristic. We then adjust for: activity (+12 oz per 30 min of exercise), climate (+16 oz for hot/very hot), altitude (+24 oz above 8,000 ft), and special conditions (+24 oz pregnant, +32 oz nursing, +16 oz fever). We add a small caffeine offset (+8 oz per caffeinated drink, since caffeine is mildly diuretic).
You do not need to drink all of this
About 20% of your daily water comes from food — especially fruits, vegetables, soups, and cooked grains. Coffee, tea, milk, and juice also count (the diuretic effect of caffeine is small for regular drinkers). The number above is total water; subtract roughly 20% for the food contribution to get your actual drinking target.
The color test
The simplest hydration check is urine color. Pale straw or light yellow = well hydrated. Dark yellow = drink more. Completely clear = you may be over-hydrated (rare, but possible — hyponatremia can occur in endurance athletes who drink too much water without electrolytes). Track the number for a week, then let thirst and color be your guide.