Why this Sweat Rate Calculator and Two Numbers on a Bathroom Scale Will Tell You More Than Any Hydration App will. Most triathletes hydrate by feel, or by whatever a bottle-per-hour rule of thumb they picked up years ago. Feel is a bad instrument for this. Thirst lags actual fluid deficit, and by the time it shows up, the workout is already compromised. The fix isn't a smarter app or a fancier bottle — it's two weigh-ins and a little arithmetic.