Sweat Rate Calculator — Your Hourly Hydration Target
Weigh yourself before and after a training session, log the fluid you drank during it, and get your sweat rate in liters per hour — the most useful single number for building a race-day hydration strategy.
Enter your pre and post weight to see your sweat rate.
How the sweat rate calculator works
Sweat rate is the simplest way to know how much fluid you actually lose per hour during exercise. Every other hydration metric — thirst, urine color, body water percentage — is less reliable. A 60-minute test in conditions close to race day gives you a number you can actually use.
The protocol: weigh yourself naked (or in dry shorts) just before the session. Train for a measured duration (60 to 90 minutes works best). During the session, track every ounce or milliliter of fluid you consume. Immediately after, towel off, strip down, and weigh yourself again.
The math: sweat loss in liters = (pre-weight − post-weight) + fluid consumed. Assume 1 kg = 1 L of water. Divide by hours to get sweat rate per hour.
Most endurance athletes sweat 0.5 to 1.2 L/hr in cool conditions and 1.0 to 2.5 L/hr in the heat. Your target during a race is usually 70 to 100 percent of your sweat rate — you rarely need to replace every drop, but you want to avoid losing more than 2 percent of body weight in the process.
Sweat loss (L) = (pre-weight − post-weight) + fluid consumed · Sweat rate (L/hr) = sweat loss ÷ duration
Frequently asked questions
Should I fully replace my sweat rate during a race?
Not usually. Full replacement — drinking exactly what you sweat — is hard on the stomach, especially at intensities close to race pace. A realistic target is 70 to 85 percent of sweat rate for most long races, and closer to 100 percent when racing in hot weather where dehydration risk is higher.
How often should I retest my sweat rate?
At least twice per year: once early in a cool-weather training block and once during heat acclimation. If you move climate or jump a season, retest. Sweat rate changes with heat acclimation (it usually goes up, then plateaus), with altitude, and with fitness.
Do I need electrolyte drink or plain water?
If the session is under 60 minutes, plain water is fine. Over 60 minutes at moderate or higher intensity, you need sodium to match what you lose in sweat — typically 300 to 800 mg of sodium per hour, higher in salty sweaters. This calculator gives you fluid volume; sodium is a separate calculation based on sweat sodium concentration.
Why did I gain weight during a workout?
You drank more than you lost — an easy trap in cool weather or on a trainer indoors with a fan. Net weight gain during exercise is a red flag for over-hydration and can lead to hyponatremia. If you gain weight, drink less next time.
Cora builds your hydration plan
Enter your sweat rate once and CoreRise applies it to every session and every race brief: how much to drink per hour, how much sodium to carry, and how to adjust for heat. Cora also checks your post-workout weight trend and flags you if you are drifting toward chronic dehydration.