HydrationGuideEvery other hydration guideline is an average of other people. This one is yours: weigh in before and after a session, and the scale tells you exactly how much fluid you lost and how fast. It is the same protocol sports scientists use, and it needs nothing but a bathroom scale.
How this calculator works
Body mass lost during exercise is almost entirely fluid, so the scale is a sweat gauge. The standard field method is simple arithmetic: sweat loss = weight lost + fluid drunk, and dividing by the hours trained gives a rate per hour. If you drank half a litre and still finished a kilogram lighter, you sweated one and a half litres.
Sodium is the part the scale cannot see. Sweat carries roughly 400 to 1600 mg of sodium per litre, and where you sit in that spread is largely genetic. Only a laboratory sweat test pins it down, so this calculator gives a range and lets you nudge it with the one signal you can observe: whether your kit dries with a white crust on it.
Two thresholds do most of the useful work. Finishing more than 2% of body mass down is where endurance, heat tolerance, and concentration measurably drop. Finishing heavier than you started means you drank more than you sweated, which is the one direction genuinely worth avoiding.
These are estimates for healthy adults doing ordinary training, not medical advice, and not a protocol for athletes with a clinical reason to manage fluid closely.
How to measure your sweat rate
- Weigh in before. Empty your bladder, strip to minimal clothing, towel dry, note the number.
- Track what you drink. Start with a known volume in your bottle so you can measure what is missing at the end.
- Train as normal and note the time. An hour is plenty; anything under fifteen minutes is too short to be reliable.
- Weigh out. Same scale, same minimal clothing, towelled dry. Wet kit reads as body mass and will hide your losses.
- Repeat in different conditions. A cool session and a hot one give you a range, which is far more useful than one number treated as fixed.
Sweat rates from real sessions
Five ordinary sessions, run through the exact model behind the calculator above. The spread is the point: the same person in different conditions can double their hourly loss.
| Session | Time | Drank | Sweat rate | Sodium lost |
|---|---|---|---|---|
| Easy 5 km run, mild day | 30 min | 0 mL | 1.0 L/h | 300–550 mg |
| Gym session, air conditioned | 60 min | 400 mL | 0.8 L/h | 480–880 mg |
| Football match, warm evening | 90 min | 700 mL | 1.2 L/h | 1080–1980 mg |
| Long ride, hot afternoon | 120 min | 1500 mL | 1.5 L/h | 1740–3190 mg |
| Hot yoga class | 60 min | 250 mL | 1.0 L/h | 630–1155 mg |
Sodium figures assume average-salt sweat. A genuinely salty sweater can lose twice that at the same sweat rate, which is why the calculator asks.
What to do with your number
During exercise, drink at a rate that keeps you within about 2% of your starting weight, which for most people means covering most but not all of the sweat rate. Above roughly a litre an hour, matching losses exactly stops being realistic: the gut can only absorb so fast, and forcing plain water down brings its own risk.
After exercise, replace about 125 to 150% of the deficit you finished with, over the following few hours rather than in one go. The extra covers the urine you produce while rehydrating. Sodium alongside it, from food or a mix, is what actually keeps the fluid in you rather than passing it through.
If you finished heavier, drink less next time. Overdrinking during long events is how exercise-associated hyponatremia happens, and it sends more athletes to medical tents than dehydration does.
Frequently asked questions
How do I calculate my sweat rate?
Weigh yourself before and after exercise, note how long you trained and how much you drank, then use this: sweat loss equals the weight you lost plus the fluid you drank, and sweat rate is that total divided by the hours you trained. A kilogram of body mass lost counts as roughly a litre of sweat. The calculator above does the arithmetic and adds a sodium estimate.
What is a normal sweat rate?
Most people fall between roughly 0.5 and 2.0 litres per hour, with about 1.2 litres per hour being a common figure for a moderate session in warm conditions. Rates above 2.5 litres per hour are unusual and usually mean the measurement needs re-checking. The spread between people is wide enough that your own number is worth more than any average.
How much should I drink per hour when exercising?
Aim to cover most of your measured sweat rate, but drinking much more than a litre per hour is rarely practical or advisable in one sitting. The goal is to finish under about 2% of body mass down, not to match losses litre for litre. Anything left over is replaced afterwards, when there is no time pressure.
Should I weigh myself naked?
Minimally clothed and towel dried, both times, using the same scale. Sweat trapped in clothing and shoes still weighs the same as sweat in your body, so wet kit makes your loss look smaller than it was. Empty your bladder before the first weigh-in, and if you urinate during the session the number is less reliable.
Do I need electrolytes or is water enough?
Under about an hour in mild conditions, water is usually fine. Past that, and especially in heat, you are losing meaningful sodium: roughly 400 to 1600 mg per litre of sweat depending on the person. Replacing hours of heavy sweating with plain water alone can dilute blood sodium, which is more dangerous than mild dehydration because it looks similar and is treated oppositely.
Am I a salty sweater?
White crusting or gritty residue on skin, cap, or dark kit after a session, and stinging when sweat runs into your eyes, both point that way. It is a rough signal rather than a test: laboratory sweat sodium in athletes runs from about 18 to 71 mmol per litre, and only a sweat test pins down where you sit. The calculator uses that spread to give a range rather than a single figure.
Does my sweat rate change?
Yes, quite a lot. Heat, humidity, effort, clothing, and how heat acclimated you are all move it, and fitter athletes tend to sweat earlier and more. That is why one measurement is a starting point rather than a fixed personal constant. Measure a hard session and an easy one, in different weather, and you learn your range instead of a single point.
How much should I drink after exercise?
Roughly 125 to 150% of the fluid you are still down at the end, spread over the next few hours rather than drunk at once, and with sodium alongside it from food or a mix. The extra above 100% covers the urine you will produce while rehydrating. The calculator shows this figure once it knows your deficit.
Sources
- American College of Sports Medicine (2007). Position Stand: Exercise and Fluid Replacement.
- Baker, L.B. (2017). Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports Medicine 47(S1).
- Baker, L.B. et al. (2016). Normative data for regional sweat sodium concentration and whole-body sweating rate in athletes. Journal of Sports Sciences.
- CDC. Water and Healthier Drinks.