Hydration advice tends to swing between rigid rules and vague hand-waving. The truth sits in between: your water needs are personal, but they are easy to estimate and even easier to monitor.
TL;DR — Enter your weight and exercise in the water intake calculator for a daily target in liters, ounces or cups.
What drives your needs
Bigger bodies need more water, and so do hot climates, illness, pregnancy and exercise. The calculator starts from your body weight and adds for activity, which captures the two factors most people can control day to day. It is an estimate of total fluid, not a hard quota.
Does “8 glasses” hold up?
The famous eight-glasses rule is a reasonable ballpark for an average adult, but it was never precise. Some people need more, some less, and crucially, the fluid in food, coffee, tea and other drinks all counts. Fixating on water alone misses where a lot of your hydration actually comes from.
Let your body guide you
The simplest hydration check needs no calculator: pale yellow urine and rarely feeling thirsty usually mean you are doing fine. Dark urine, headaches or sluggishness can mean you need more. Use the water intake calculator for a target, then let thirst and these signals fine-tune it — and avoid the rare opposite problem of drinking far more than you need.