Measuring sweat loss
- Weigh yourself right before exercise, after using the bathroom, in light clothing.
- Note how much you drink during the session.
- Towel off and weigh again in the same clothing right after.
- Sweat loss ≈ (weight before − weight after) + fluid drunk. Treat 1 kg lost as about 1 L.
Do this once or twice per season for your usual sessions and you get a personal rule like "summer 1-hour run ≈ X L".
Before, during and after
| When | ACSM position stand, in short |
|---|---|
| A few hours before | About 5–7 ml per kg body weight, slowly (about 4 hours before) |
| During | Avoid losing more than 2% of body weight, but do not drink so much that you gain weight |
| After | About 1.25–1.5 L per kg lost, spread out |
The "a bit more than you lost" advice after exercise is because part of what you drink leaves as urine quickly.
A worked example
70 kg, one-hour summer evening run. Before: 70.0 kg. After: 69.2 kg. Drank 300 ml during.
- Sweat loss ≈ 0.8 L + 0.3 L = about 1.1 L
- Weight loss 0.8 ÷ 70 ≈ 1.1%, inside the 2% line
- Afterward: 0.8 kg × 1.25–1.5 L ≈ 1.0–1.2 L over the next few hours
Logging it in the app
Set Activity to High on training days
In Water Tracker settings, set Activity to High and tap Apply: the weight-based goal rises by about 15%. If your measured replacement needs are larger, raise the daily target by hand.
Log sports drinks by type
Choose Sports in "Add with drink type" and it counts at the same rate as water. Drinks spread over the hours after a session are logged with their times.
Compare training and rest days
Use the 14-day chart and monthly calendar on the History tab to check whether intake actually went up on training days.
FAQ
Do I need extra for a 30-minute gym session?
Short sessions without heavy sweating are usually covered by normal meals and thirst. If your weight barely changes, there is nothing to calculate.
I weighed more after my workout.
You drank more than you sweated. Repeated on long sessions this can be dangerous; read drinking too much water.
Sources: ACSM Position Stand, Exercise and Fluid Replacement (2007)
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