The simplest rehydration math: weigh yourself before and after your workout, and for every pound of body weight you lost, drink back roughly 20–24 ounces of fluid over the next few hours. That's the guidance from the American College of Sports Medicine, whose position stand recommends drinking about 1.5 liters of fluid for every kilogram of body mass lost when you need to rehydrate fully between sessions (ACSM, Exercise and Fluid Replacement).
Why 150%, not 100%?
It seems logical that losing a pound of sweat means drinking a pound (16 oz) of water. But your kidneys keep producing urine while you rehydrate, so some of what you drink passes straight through before your fluid balance is restored. In a controlled study, researchers found that participants only returned to proper fluid balance when they drank around 150% of what they'd lost — drinking just 100% left them still net-dehydrated hours later (Shirreffs et al., Medicine & Science in Sports & Exercise, 1996). Overshooting the loss is the point, not a rounding error.
The weigh-in/weigh-out method
1. Weigh yourself right before exercise — minimal clothing, after a bathroom trip.
2. Weigh yourself again right after — towel off sweat first, same clothing situation.
3. Subtract. Each pound lost ≈ 16 oz of sweat. Multiply by 1.25–1.5 to get your drink-back target: that's the 20–24 oz per pound.
4. Spread it out. Sip over the following two to four hours rather than chugging it all at once — slower intake gives your body time to retain the fluid instead of flushing it.
Do this a few times and you'll learn your personal sweat rate, which the ACSM notes commonly ranges from about 0.5 to 2 liters per hour depending on the person, the workout, and the heat (ACSM position stand). After that, you can estimate your losses without a scale.
Worked example
Say you weigh 180 lb before a summer training session and 178 lb after. That's 2 lb lost — about 32 oz of sweat. Your drink-back target at 150% is roughly 48 oz over the next few hours. One full NuRich 32 oz Insulated Bottle ($29.99) plus a refill to the halfway mark gets you there — no measuring cups, no guesswork. That's the quiet advantage of a bottle with a known capacity: it doubles as your measuring tool.
Don't forget sodium (and food)
Sweat carries salt out with the water, and the same rehydration research found that fluid retention improves when sodium is replaced alongside water. You don't necessarily need a sports drink — a normal salted meal alongside your water does the job for most people after everyday workouts. Save electrolyte products for long, hot, or multiple-session days. And if you're working outside in the heat, the CDC's NIOSH heat stress guidance emphasizes drinking on a schedule rather than waiting for thirst.
The gulp-friendly setup
Rehydration favors bottles you can drink from fast. A straw lid is built for steady desk sipping; post-workout, you want volume. The NuRich Spout Chug Lid ($12.99) twists onto any wide-mouth bottle and pours fast without dumping water down your chin — exactly what you want in the gym parking lot. Pair it with the 32 oz bottle and your entire rehydration plan lives in one cup holder. Find both, plus every other lid style, in the full NuRich collection.
The habit in one line
Weigh in, weigh out, drink back 20–24 oz per pound over the next few hours with some salty food — and let a bottle with known capacity do the measuring for you.
Disclaimer: This article is general information, not medical advice. Fluid needs vary; athletes with medical conditions or on fluid-affecting medications should follow their clinician's guidance.
Sources: ACSM — Exercise and Fluid Replacement (2007); Shirreffs et al. 1996 (PubMed); CDC/NIOSH — Heat Stress