Why body weight is the best single predictor
Of all the things that influence how much water you need, body size matters most. More tissue means more metabolic activity, more surface area to lose water through, and a larger blood volume to maintain. That's why most hydration formulas, including the one Realwater uses to suggest a goal, begin with weight and then adjust from there.
Which ml-per-kg rate to use
Thirty ml per kg suits people who spend most of the day sitting in a temperate climate. Thirty-five is a better fit if you move a lot or exercise most days. Forty is for people training hard every day or working physically in warm conditions. If you're unsure, pick the middle option — it's close to the population reference values used by health agencies. In imperial terms, the three rates are roughly 0.46, 0.54 and 0.61 fluid ounces per pound.
What this calculator leaves out
A weight-only estimate can't see the heatwave, the long run, or the fact that you're breastfeeding. For those, the full daily water intake calculator adds the right adjustments. And if you're working out a target for a child, use the kids calculator, which is age-based rather than weight-based. For serving sizes, see how big a glass of water really is.
