Estimating baseline fluid needs from bodyweight
A common rough rule of thumb scales daily fluid to body mass, often cited around 30 to 40 mL per kilogram per day for a typical adult in temperate conditions. By that estimate, a 70 kg person lands somewhere near 2 to 2.8 liters from all sources combined. Importantly, food contributes a meaningful share of total water intake, so this figure is not all from drinks.
These are population estimates, not precise targets. Actual needs vary with climate, activity, body composition, pregnancy, and health status.
Adding fluid for exercise and heat
Physical activity, especially in heat, raises requirements substantially, with sweat losses that can reach roughly half a liter to more than a liter per hour. Replacing fluid lost to sweat, and including some sodium during long or heavy sessions, helps maintain both volume and blood sodium concentration. Drinking to thirst is a reasonable default for most people most of the time.
This is general educational information, not medical advice. People with heart, kidney, or hormonal conditions may need restricted or adjusted fluid intake and should follow guidance from a qualified professional.