The formula: half an ounce per pound
The most widely used personal water formula is simple: multiply your body weight in pounds by 0.5. The result is your baseline daily target in fluid ounces. A 160-pound person gets 80 ounces. A 200-pound person gets 100 ounces. That is the starting line, before activity, climate, and everything else.
Why weight? Because water needs scale with body size. Blood volume, muscle mass, and metabolic rate all rise with weight, and each of them moves water. A fixed rule like eight glasses treats a 120-pound woman and a 250-pound man as the same machine. They are not, which is why the weight formula beats fixed rules for almost everyone.
The chart: daily water by body weight
Here is the baseline for common weights, before activity and climate adjustments. All figures are fluid ounces, with cups in parentheses:
120 lb: 60 oz (7.5 cups). 130 lb: 65 oz (8 cups). 140 lb: 70 oz (8.75 cups). 150 lb: 75 oz (9.4 cups). 160 lb: 80 oz (10 cups). 170 lb: 85 oz (10.6 cups). 180 lb: 90 oz (11.25 cups). 190 lb: 95 oz (11.9 cups). 200 lb: 100 oz (12.5 cups). 210 lb: 105 oz (13 cups). 220 lb: 110 oz (13.75 cups). 230 lb: 115 oz (14.4 cups). 240 lb: 120 oz (15 cups). 250 lb: 125 oz (15.6 cups).
Notice that eight 8-ounce glasses is 64 ounces, which only covers the 120 to 130-pound range. Anyone heavier is under-hydrated on the old rule. That single comparison is the best argument for the weight formula.
Worked example: a 200-pound man
Take a 200-pound man with a desk job in a temperate climate. Baseline: 200 times 0.5 equals 100 ounces. That is 12.5 cups, about 3 liters, or 6 of the standard 16.9-ounce bottles (100 divided by 16.9 is 5.9, so 6 bottles).
Now put him on a very active day with an hour of intense training. Very active adds about 40 percent: 100 times 1.4 equals 140 ounces. Add a hot climate day on top and the calculator tacks on 16 more ounces, landing at 156 ounces, nearly 5 liters. That sounds like a lot until you remember he sweated out a large share of it. The water intake calculator runs this exact adjustment chain automatically.
If you weigh yourself in kilograms
The formula works in metric with one extra step. Convert kilograms to pounds first by multiplying by 2.2, then apply the half-ounce rule. A 70 kg person: 70 times 2.2 equals 154 pounds, times 0.5 equals 77 ounces, about 2.3 liters.
The direct metric shortcut: multiply your weight in kg by 0.033 to get liters. For 70 kg: 70 times 0.033 equals 2.31 liters, which matches the 77-ounce figure (77 times 0.0296 is 2.28 liters). Either path lands in the same place. Use whichever units you think in.
Adjusting for activity, heat, and life
The weight baseline assumes a normal day. Real days vary, so adjust. Exercise is the biggest lever: add roughly 12 ounces for every 30 minutes of moderate activity, more for intense or long sessions. A 90-minute workout adds about 36 ounces on top of baseline. Hot weather adds about 16 ounces for the day, since heat and humidity raise sweat and respiratory water loss even at rest.
Illness raises needs too: fever, vomiting, and diarrhea all increase fluid loss, and steady rehydration matters more than hitting a number. Pregnancy and breastfeeding raise fluid needs as well, but the amounts are personal enough that a doctor or midwife should set the target rather than a general formula.
Weight and age: what changes
For most adults between 18 and 65, weight plus activity plus climate is the whole story. At the edges of age, two tweaks apply. Children need less water in absolute terms but more per pound of body weight, and they are unreliable reporters of thirst, so scheduled drinking beats formulas for kids. Adults over 65 often have a blunted thirst response and reduced kidney concentrating ability, meaning they can slide into dehydration without feeling thirsty. For older adults, the habit matters more than the number: steady intake through the day, every day.
What counts toward your number
Your weight-based target is total water, not glasses of plain water. About 20 to 30 percent typically comes from food, especially fruits, vegetables, and soups. Coffee, tea, milk, and sparkling water all contribute their water content. Only alcohol works against you. So a 100-ounce target might mean 70 to 75 ounces from beverages and the rest from meals, which is far less daunting than it sounds.
Note: this guide is general hydration education, not medical advice. If you are pregnant, nursing, managing a condition that affects fluid balance, or taking medications that affect fluids, talk to your doctor about your personal target.