How do you measure your sweat rate?
With a bathroom scale and a kitchen scale, nothing more. Weigh yourself before the ride and after it, add whatever you drank in between, subtract anything you passed, and divide by the hours. One kilogram of weight change is one liter of water, which makes the arithmetic honest: weigh in at 75.0 kg, out at 74.4, drink 750 ml over 90 minutes and you lost 1.35 L — a sweat rate of 0.9 L/h, about 30 oz an hour. That's the number every hydration plan on the internet is guessing at, and you just measured it.
Empty your bladder first. Weigh in dry, in minimal clothing, on the scale you'll use afterward — scales disagree with each other by more than a ride's worth of sweat. Ride 60–90 minutes at the effort and in the weather you want the number for. Afterward, strip the wet kit, towel off completely and weigh again the same way. Weigh your bottles before and after rather than trusting the markings; a kitchen scale reads them in grams, which are milliliters. Don't eat during the test, and log the temperature — you'll want it when you compare tests.
What is a normal sweat rate for cycling?
Anything from under half a liter an hour to nearly two. The ACSM's published range for exercising adults runs about 0.4 to 1.8 L/h, and the same rider covers a good chunk of it depending on the day, which is why one test is a data point and three are a profile:
| The ride | Before → after | Drank | Sweat rate |
|---|---|---|---|
| Cool morning, easy | 75.0 → 74.7 kg | 300 ml | 0.40 L/h (14 oz/h) |
| Warm afternoon, tempo | 75.0 → 74.4 kg | 750 ml | 0.90 L/h (30 oz/h) |
| Hot day, hard group ride | 75.0 → 73.9 kg | 1200 ml | 1.53 L/h (52 oz/h) |
Same 75 kg rider, 90-minute rides, computed by the calculator above. Heat, humidity, intensity, airflow and acclimatization all move the number — measure in the conditions you're planning for, and again a few weeks into summer.
What do you do with the number?
Turn it into bottles. The hydration calculator takes your measured rate — automatically, once you've run this test — and builds the plan: aim for about 80% of losses, never more than the gut empties, never more than you sweat. For the 0.9 L/h rider on a three-hour ride that comes to 24 oz an hour, roughly 1.2 standard bottles, finishing about 0.54 L down — well inside the 2%-of-body-mass line that the research says is fine. If your test shows you finished heavier, you drank more than you sweated; on a long ride that's the direction that leads to hyponatremia, and the answer is less fluid, not more salt.
A kilogram lost between weigh-ins is a liter of water, gone by tomorrow morning. A kilogram of body fat is about 7,700 kcal, and at a steady 150 W the pedals only burn about 538 kcal an hour — so fat-loss of one kilo would take roughly 14 hours of riding, not 90 minutes. The post-ride scale is a hydration instrument. The calories-burned calculator is the honest one for the other question.
Why one test isn't enough
Sweat rate tracks heat production and heat loss, and both change: you sweat more as you acclimatize (a good thing — earlier, more dilute sweat cools you better), more indoors without airflow, more at race pace than at conversation pace, and less on a cold morning than the summer number would predict. The useful habit is a test at the start of the season, one in mid-summer heat, and one on the trainer — three numbers that bracket every ride you'll do. The Garage keeps the latest; re-run the test and it updates the hydration plan the same moment.