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Weigh yourself dry, after emptying your bladder, in as little clothing as you can manage.
Towel off first. Sweat still in your kit has left your body but not the scales.
1.37 L/h
1.37 L an hour, in the weather and at the effort of this session and no other.
Sweat loss is your mass before, minus your mass after, plus everything you drank, minus anything you passed — divided by how long you were out. One kilogram lost is counted as one litre of sweat. That is the field method in the American College of Sports Medicine position stand “Exercise and Fluid Replacement” (Sawka and colleagues, Medicine & Science in Sports & Exercise 2007;39(2):377–390), which is also where the 2% figure above comes from: it advises keeping body-mass loss during exercise under roughly that, because larger deficits cost performance, more so in heat.
This number belongs to one session. Sweat rate moves with temperature, humidity, how hard you went, what you wore and how acclimatised you were that week, and it can double between a cool morning and a hot afternoon at the same pace. Measure again when the conditions change rather than carrying this figure into them. General information for healthy adults, not medical advice; anyone managing a heart, kidney or sodium condition should be taking fluid instructions from their clinician instead.
Sweat rates between two athletes doing the same session in the same weather can differ by a factor of three. No table, no body-size formula and no weather chart will tell you which end of that spread you sit at. What will tell you is a set of scales and two minutes: weigh yourself before, weigh yourself after, keep track of what you drank, and the difference across the hours you were out is the litres an hour your body actually produced.
Weigh dry, undressed or nearly so, and after a trip to the toilet. Afterwards, towel down properly before stepping on the scales again: sweat sitting in a soaked jersey has left your body but not the reading, and a heavy kit can swallow several hundred millilitres. Use the same scales both times and do not change them halfway through a season.
Then account for traffic in both directions. Every mouthful you drank has to be added back, because it replaced sweat that had already gone, and anything you passed has to come off, because it was never sweat in the first place. Bottles are easy to track if you start them full and count what is left.
A rider starts at 72.4 kg, finishes at 71.1 kg, and gets through three quarters of a litre of drink in an hour and a half. The mass lost is 1.3 kg, which is 1.3 litres of sweat; add the 0.75 litres that went in and 2.05 litres came out. Over 1.5 hours that is about 1.37 litres an hour. The net mass change, the figure the two per cent guideline is written against, is 1.8 per cent.
Knowing you lose 1.4 litres an hour is only useful next to what you can actually absorb. Most people tolerate somewhere under a litre an hour while working hard, and a few tolerate far less, so a high sweat rate often means planning to finish a long effort down on fluid rather than level with it.
The practical output is a drinking schedule and a bottle count: how much per hour, how many mouthfuls that is, and where on the route it has to be refilled. A rate that outruns your stomach is a reason to start well hydrated and to plan the deficit, not a reason to force down more than you can hold.
What you have measured is one athlete, one intensity, one set of conditions. Move the same session from a fifteen degree morning to a thirty degree afternoon and the rate can double. Change the humidity, the clothing, the wind over your skin or how acclimatised you are that fortnight, and it moves again.
That is not a flaw in the method, it is the reason the method exists. Repeat it when the weather turns, when you travel somewhere hotter, and once in the middle of a hard block, and you end up with a small personal table that is worth more than any published one.
For a session of a couple of hours the error from gels and bars is small next to the sweat, and this calculator ignores it. On an all-day ride with real food in it, weigh the food and treat it the same way as fluid.
Because the scales only show what is left. Fluid you drank stayed in your body and hid an equal amount of sweat that had already been lost, so adding it back reveals the total your body actually produced.
Not in itself. Well-trained and heat-acclimatised people often sweat earlier and harder, which is an efficient way to shed heat. It simply means your drinking has more to keep up with than the person riding next to you.
Any time the conditions or the effort change meaningfully, and at least once each season as the weather turns. Three or four measurements across a year cover most of what you will actually race and train in.