Sweat rates run from 0.5 to 2.5 L/h between people doing the same work. Weigh yourself before and after one session and you know yours; no published figure can tell you.
Over a training session essentially all the mass you lose is water, so the change on the scales plus whatever you drank is your sweat loss, and a litre of sweat weighs a kilogram. Losing about 2% of body mass is where endurance performance measurably declines — 1.4 kg for a 70 kg athlete — and at a middling 1.1 L/h you reach it in about an hour and a quarter with nothing to drink.
The measurement assumes mass lost is sweat, which drifts over very long events as fuel oxidation and breathing add up. And it says nothing about sodium, which varies tenfold between people and matters more the longer you go. Most importantly it does not mean drink more: taking on substantially more than you sweat causes hyponatraemia, which has killed runners while dehydration has not.
Estimate with the rule, then check it against the calculator that models it properly.
Open Sweat Rate & Hydration →