Before: start hydrated, not full
The goal before exercise is to begin in fluid balance, not to bank extra water — the body can't store it. Sports medicine guidelines suggest around 5–7 ml per kilogram in the two to four hours beforehand, which for most adults is a large glass or two. Pale urine before you start is the easiest check.
During: match your sweat, within reason
The calculator estimates a sweat rate from intensity, conditions and body size, then suggests replacing about three-quarters of it, capped at roughly a liter an hour because that's about what the gut can absorb. For short sessions it simply says drink to thirst — a 30-minute lifting session doesn't need a schedule. If you want your real number instead of an estimate, measure it with the sweat rate calculator.
After: replace what you didn't drink, plus a bit
Whatever deficit you finish with, replace about 1.25–1.5 times that volume over the following few hours. The extra covers ongoing sweat and urine losses. Pairing it with food or something salty helps your body hold onto it; plain water alone tends to pass through quickly. See our guides to hydration for gym sessions and hydration for runners.
When electrolytes actually matter
For most workouts, water is all you need. Sodium becomes worth adding when sessions pass about 90 minutes, when it's hot, or when you know you're a salty sweater. The calculator flags this for you. And if you get muscle cramps, note that the evidence linking them to dehydration is weaker than most people think — fatigue and pacing are usually the bigger culprits.
