Quick Tip: Weigh Yourself Before and After a Ride to Learn Your Sweat Rate
Ever wonder if you’re drinking enough on hot rides — or too much? Your bathroom scale can tell you, and it only takes one ride to find out.
Sports scientists call this a sweat rate test, and it’s the same method pro team staff use in the field. Every liter of sweat you lose weighs about one kilogram (2.2 pounds), so the weight you lose during a ride is mostly the fluid you didn’t replace.
Here’s how to do it:
Before the ride. Use the bathroom, then weigh yourself with minimal clothing on a digital scale. Write the number down. Fill your bottles and note how much fluid you’re starting with.
Ride for about an hour at your normal pace, and drink like you normally would. An hour to 90 minutes is the sweet spot — much longer and other factors start to muddy the numbers.
After the ride. Towel off the sweat, weigh yourself again in the same clothing (or lack of it) on the same scale, and note how much you drank.
Now the math. Take the weight you lost, add the weight of what you drank, and that’s your total sweat loss. A 16-ounce bottle of fluid weighs about a pound, so if you lost 1 pound on the scale and drank one bottle, you sweated out roughly 2 pounds — about a liter — in that hour.
That number is your sweat rate, and it’s surprisingly useful. Sweat rates vary enormously from rider to rider — anywhere from half a liter to more than two liters per hour — which is why the generic advice to “drink one bottle an hour” works fine for one cyclist and leaves another badly dehydrated on the same ride.
The goal isn’t to replace every drop while riding. Losing a little weight over a long, hot ride is normal and expected. What you’re trying to avoid is losing more than about 2-3 percent of your body weight, which is where performance measurably drops off — that’s about 3.5 pounds for a 150-pound rider. If your test shows you’re losing weight fast at your current drinking habits, you now know to drink more, and roughly how much more.
One test won’t be perfect, so repeat it a few times and use the average. And since sweat rate changes with the weather, it’s worth doing the test once in mild spring conditions and again when summer arrives. You’ll probably be surprised how different the numbers are.
Here’s USA Cycling’s page on the method if you want more detail: https://usacycling.org/article/why-and-how-to-calculate-your-athletes-sweat-rate
Recommendations for hydration per hour are all over the map, like a bottle an hour (with the size of the bottle varying widely). I’ve done this experiment on myself keeping temperature steady and varying power in 10 watt increments. Bumping the temp up 10 degrees and repeating the experiment, then bumping the temperature up another 10 degrees. The curve at a given temperature is roughly linear. At 70 degrees in Zone 1, my sweat rate is 0.25 L/hr. At the same temperature at the top of Zone 2, my sweat rate is 0.8 L/hr. There curve moves up a small amount from 70 to 80 degrees, but there’s a big jump from 80 degrees to 90 degrees.
At 90 degrees at my same Zone 1 power level as above, my sweat rate is 0.5L/hr. At the top of my Zone 2, my sweat rate is 1.1 L/hr.
Everyone is different, your numbers will vary, but my point is sweat rates vary by at the very least temperature and power output, probably heat acclimation as well. Keep that in mind as you run your own experiments.