How much water should you drink today?

A common practical formula is half your body weight in ounces, which works out to about 0.5 to 1 oz of water per pound (ounces per pound). For a 200 pound person that is roughly 100 oz, or close to 3 liters (3,000 mL). The National Academies of Sciences (NAS) frames the same idea as adequate intake: about 125 oz (3.7 L, 15.5 cups) per day for an adult man and 91 oz (2.7 L, 11.5 cups) per day for a woman, including water from food. The calculator starts from your weight and then adjusts for activity level, climate, caffeine, alcohol, pregnancy, and your goal.

It is a reasonable starting point, not a hard rule. Half body weight in ounces (the 0.5 to 1 oz per lb range) lands close to the NAS adequate intake for most adults, and it is easier to remember than 8 cups (64 oz) or a gallon (128 oz). It can under-shoot for very active people in hot weather and over-shoot for small, sedentary people, which is why this calculator layers activity, climate, and goal adjustments on top of the baseline rather than stopping at body weight alone.

Yes, mostly. Killer et al. 2014 (PLOS ONE) found that moderate coffee intake (up to 4 cups per day) provides hydration similar to plain water in habitual coffee drinkers. The diuretic effect of caffeine is mild and offset by the fluid in the drink itself. The calculator adds a small offset (about +10 oz per 200 mg caffeine) to account for the diuretic effect at higher doses.

Yes. Hyponatremia (water intoxication) happens when blood sodium drops too low from drinking too much water too fast. It is rare but documented in endurance athletes and people on strict water diets. Noakes 2003 (BJSM) describes cases of exercise-associated hyponatremia at intakes above 6 L (200 oz) per day. The calculator flags any target above 200 oz.

The NAS adequate intake is about 91 oz (2.7 L, 11.5 cups) per day for an adult woman and 125 oz (3.7 L, 15.5 cups) per day for an adult man, both including water from food. Women sit lower mainly because they average lower body weight, lower lean body mass, and lower total body water (about 52% of body weight vs 60% for men). The calculator applies a small downward adjustment for female biology and an upward one for male biology when weight is provided.

Add roughly 12 to 16 oz (350 to 470 mL) of water for every 30 minutes of exercise on top of your baseline, and more in hot weather. ACSM recommends 17 to 20 oz (500 to 600 mL) about 2 to 3 hours before exercise, 8 oz (240 mL) during warm-up, and 7 to 10 oz (200 to 300 mL) every 10 to 20 minutes during the session, then replacing 16 to 24 oz per pound of body weight lost in sweat. A higher sweat rate or hot, humid climate raises the loss, so the calculator adds water for both your activity level and climate, and reminds you to replace electrolytes (sodium and potassium) for long sessions.

Urine color is the most practical at-home check: pale straw or light yellow means good hydration, dark yellow to amber means dehydration, and persistently clear urine can mean over-hydration. The chart on this page uses the same color codes the Cleveland Clinic and Mayo Clinic publish. If you hit your target and still feel thirsty all the time, the cause can be high salt or caffeine intake, hot weather, exercise, dry indoor air (winter heating, AC), or medications such as diuretics, lithium, or some blood-pressure drugs. Constant thirst can also signal diabetes or thyroid issues, so see your doctor if it persists.

Water from food and other drinks counts, not just plain water. About 20% of total fluid intake comes from food in a typical US diet: watermelon is 92% water, cucumber 95%, lettuce 96%, strawberries 91%, oranges 87%, and soup is 85 to 95%. Plain unsweetened sparkling water hydrates the same as still water because carbonation does not affect fluid absorption, though you should watch for added sugar in flavored versions. The calculator already accounts for food in the NAS-based baseline, since the 91 oz / 125 oz numbers include it.

The Institute of Medicine recommends 101 oz (3.0 L) per day during pregnancy and 128 oz (3.8 L) per day while breastfeeding. The calculator adds 10 oz for pregnancy and 32 oz for breastfeeding to your personalized base.

Drink 16 to 20 oz (500 to 600 mL) about 30 minutes before each major meal. Dennis et al. 2010 (Obesity) showed adults who drank 500 mL before meals lost 44% more weight over 12 weeks than those who did not. Water blunts appetite hormones and adds zero calories, and unsweetened sparkling water can feel more filling. The Weight loss goal in the calculator adds 16 oz to your baseline target.

Mild dehydration tightens skin and accentuates fine lines, so reaching your daily target supports skin elasticity. The 'drink water to fix acne' claim is overstated: there is no strong randomized evidence that hydration alone clears acne. Use the Skin health goal for an extra 8 oz baseline.

Alcohol is a strong diuretic: about 4 oz of fluid is lost per standard drink. For a hangover, replace 16 to 24 oz of water plus electrolytes (sodium and potassium) over the first 2 to 3 hours. The Alcoholic drinks field in the calculator adds 6 oz per drink to your daily target to compensate.

Eight 8 oz glasses (64 oz total) is a useful rule of thumb but is not a personalized target, and a full gallon (128 oz) is more than most people need. The NAS and IOM publish higher numbers than 8 cups: 91 oz (2.7 L) for women and 125 oz (3.7 L) for men including food. The calculator gives a personalized target in ounces, milliliters, liters, and cups that beats the generic 8-glass rule for almost everyone.