How the workout hydration calculator works
The three phases
Pre-hydration (~7 oz per 100 lb in the final 2 hours) tops off reserves without sloshing. During, 16–28 oz/hour covers most sweat rates in mild conditions — more in heat, less in cold. Post, rehydrate at ~150% of fluid lost (the excess covers urine losses) spread over hours with food for sodium. Each phase has a different job; lumping them is why gym-bottle-only strategies underperform.
The sodium question
Sweat loses 200–2,000 mg sodium per liter (heavily individual). Under 90 minutes with a normal diet, water suffices. Beyond that, or in heat, or for salty sweaters (white streaks on clothing), 300–600 mg sodium per hour prevents the hyponatremia-and-cramps corner of the endurance world. Salt preferences and sweater streaks are real data — use them.
The sweat-rate self test
The gold-standard field method is embarrassingly simple: void, weigh, run one hour at representative effort tracking exactly what you drink, towel off, weigh again. Weight loss × 16 oz + fluid consumed = hourly sweat rate. One test per season beats any formula on this page — the calculators here are the fallback, not the front line.