One workout, two calculators
A 70kg person running for 60 minutes burns a real 720 calories and should drink a real 3.15 liters of water that day — two different numbers, from two different formulas, both computed from the exact same 70kg-and-60-minutes workout.
The same run, a heavier runner
A 100kg person running for the identical 60 minutes burns 1,029 calories — noticeably more than the 70kg runner — but needs only 4.2 liters of water, not proportionally as much more as the calorie figure jumped. Both numbers rise with weight, but at different rates, because they come from unrelated formulas measuring unrelated things.
Why weight affects the two numbers so differently
Calories burned scales with weight because moving more mass takes proportionally more energy — a heavier runner works harder for the same distance and time. Water intake's weight component is a hydration baseline (35 mL per kg) unrelated to how much energy was expended — it reflects the larger body's baseline fluid needs, not the workout's energy cost.
Why running these two calculators together makes sense
Neither number substitutes for the other — knowing calories burned doesn't tell you how much to drink, and knowing recommended water intake doesn't tell you how much energy was used. Someone tracking both fitness and hydration after a workout genuinely needs both calculators, not just one used as a proxy for the other.