Why measure instead of guess
Generic advice like “drink 500 ml an hour” is wrong for most people, because sweat rates range from under half a liter to over three liters per hour. Measuring yours takes one session and a bathroom scale, and it turns every future hydration plan from a guess into a number. Endurance coaches have used this method for decades; it's the standard in sports science labs.
How to run the test properly
- Use the bathroom, then weigh yourself in minimal clothing before you start.
- Note exactly how much you drink during the session — a marked bottle helps.
- Train as normal for at least 45–60 minutes in the conditions you care about.
- Towel off, then weigh again in the same clothing. Record any urine passed.
The calculator treats each kilogram lost as a liter of sweat, adds what you drank, and subtracts urine. Fuel from gels or food is small enough to ignore.
Reading the result
The headline is liters per hour. The percentage of body weight lost tells you whether you drank enough: staying under 2% is the usual target, above which endurance, coordination and mood measurably fall. The suggested drinking rate replaces about three-quarters of your sweat, capped at roughly a liter an hour. If your sweat rate is much higher than that, you'll finish sessions with a deficit — that's normal, and the fix is rehydrating afterwards, not chugging mid-run. Our guides to hydration for runners and gym sessions cover the details.
What about salt?
Sweat carries sodium as well as water. The calculator shows a rough hourly sodium loss assuming average sweat; if you see white marks on dark kit or your sweat stings your eyes, you likely lose more. For sessions over 90 minutes, run the numbers through the electrolyte replacement calculator.
