A worked example: 70kg, 30 minutes of exercise
A 70kg person exercising for 30 minutes should aim for about 2.8 liters of water a day — 2.45 liters from bodyweight alone, plus 0.35 liters for the exercise.
The formula: 35 mL per kg, plus 350 mL per 30 minutes of exercise
The baseline (weight × 35 mL) accounts for ordinary daily fluid needs regardless of activity. The exercise component adds roughly 350 mL for every 30 minutes of activity, to cover fluid lost through sweat — the same 70kg person with no exercise at all needs only 2.45 liters, exactly the baseline with nothing added.
Why the exercise component scales linearly
Doubling exercise time doubles the exercise-related water allowance — 60 minutes adds 0.7 liters instead of 30 minutes' 0.35 liters — reflecting that sweat losses accumulate roughly proportionally with how long someone is active, not just whether they exercised at all.
What this baseline doesn't adjust for
Climate, altitude, and individual sweat rate all meaningfully affect real fluid needs but aren't part of this formula — hot, humid, or high-altitude conditions increase fluid losses beyond what this baseline estimate accounts for.
What counts toward this total
This estimates total fluid intake from all sources combined, not just plain water — in practice, roughly 20% of typical fluid intake comes from food rather than drinks.