“I always weigh myself before, and I lost nine pounds of body weight on a five and a half hour effort.”

That’s what one athlete told me after a long effort. Nine pounds is a lot on its own. But then they added that they drank at least two liters along the way.

Two liters of water weighs about 4.4 pounds. So if they finished nine pounds down after drinking all of that, they actually lost closer to 13 pounds of sweat.

Most athletes have no idea their number is anywhere near that. And that’s exactly why knowing your sweat rate is so useful.

Why your sweat rate matters

Your sweat rate tells us how much fluid you’re losing over a given amount of time. Once you know it, you’re not just guessing and drinking randomly.

One athlete asked me, “You need a liter of water every hour?” Not necessarily. That depends on what your sweat rate is. Most people land somewhere between 750 milliliters and 1.2 liters an hour, but that range is wide. Your number might be at either end, or outside it completely.

The 2% line

Even a 2% drop in body weight from water loss can hurt performance. For a 150 lb athlete, that’s only three pounds.

Three pounds goes quickly. On a hot day, a heavy sweater can lose that in the first couple of hours without realizing it. Your sweat rate tells you how fast you’re heading toward that line and how much you need to drink to stay ahead of it.

The test, step by step

You don’t need a lab for this. The low-tech way works really well, and all you need is a scale.

1. Weigh yourself before. Ideally completely naked, because sweaty clothes add weight that isn’t you. Write the number down.
2. Go for about an hour. A normal ride or run at a steady effort is perfect. An hour is long enough to get a real number and short enough to be easy to repeat.
3. Track everything you drink. Note how much fluid you took in, in ounces or milliliters. A marked bottle makes this easy.
4. Towel off and weigh yourself again. Strip down, dry off so the sweat on your skin doesn’t count, and step back on the scale.

The math

Add the weight you lost to the fluid you drank. That total is roughly how much you sweated.

A quick conversion helps: 1 pound of weight lost is about 16 ounces (roughly 450 ml) of fluid.

Here’s an example:

• You weighed 150 lb before and 148.5 lb after, so you lost 1.5 lb, which is about 24 oz.
• You drank one 16 oz bottle during the hour.
• 24 oz + 16 oz = about 40 oz of sweat in an hour, which is about 1.2 liters.

One athlete asked whether they needed to account for the food they ate too. For the first round, don’t worry about it. The goal is simply to measure total water weight loss. You can get more precise later, but a rough number you actually test beats a perfect number you never get around to.

Reading your number

The goal is to get the difference between your before and after weight as close to zero as possible. Here’s what your result is telling you:

• You lost weight, even though you drank. You’re not keeping up with what you’re sweating out. There are a couple of options here. One is to bring more water, which sometimes isn’t realistic, because who wants to carry an extra 10 pounds of water? The other, and often the better fix, is to increase the sodium in your bottles. Sodium helps you hold on to the fluid you’re already drinking and drives your thirst so you drink a little more. Salty Sweater or Heavy Sweater? walks through how much.
• You came out close to even. Nice. Whatever you did on that ride is working for those conditions.
• You gained weight. You overdid it on fluid. That’s worth knowing, because drinking well past what you need can cause problems too.

And remember, the goal isn’t necessarily to replace every ounce during the effort. Ultimately, the goal is to drink to thirst, and your sweat rate tells you whether your thirst, and what’s in your bottle, is keeping up.

Why conditions change everything

Your sweat rate isn’t one number. It’s a number for a specific set of conditions.

If you did a run in December in Arizona, that’s going to be very different from July in Arizona. The same rider will probably need more water in Florida than in Denver. Heat, humidity, altitude, effort and even what you’re wearing all change how much you sweat.

So one test gives you one data point. The real value comes from testing across the conditions you actually train and race in.

Me-search, not research

I call this me-search instead of research. A lot of my athletes build their own little database of sweat rates, so eventually they can say, “When it’s this temperature and these conditions, I know my sweat rate is around a liter an hour.”

A simple note on your phone is plenty. For each test, log:

• Date and duration
• Temperature and humidity (hot and dry, hot and sticky, cool, etc.)
• Type of effort (easy endurance, tempo, race)
• Weight before and after
• Fluid you drank, and what was in it
• Your sweat rate

It would be really helpful to do this for all of your races too, so you get a sense of your sweat rate in race conditions and not just in training. After a season, you’ll have a pretty clear picture of what a hot race day actually asks of you.

Test the fix

Once you know your number, you can use the same scale to check whether your plan works.

Say you know you lose about a liter an hour in certain conditions. Head out for an hour-long run or ride, drink a liter of water with about 800 milligrams of sodium in it, and weigh yourself after. You should come out close to zero.

If you’re still coming out lighter, keep weighing yourself before and after, and add a little more sodium each time until that number gets close to zero. If you’re starting to gain weight, pull it back. That’s the whole process: test, adjust, test again.

The bottom line

You don’t need a lab or special equipment to figure out how much you sweat. A scale, an hour-long ride and a little math will get you most of the way there. Test in the conditions you actually train and race in, keep a log, and use it to decide how much you drink and what goes in your bottle.

If you want help turning your sweat rate into a full hydration and fueling plan for your next race, that’s exactly what we work on inside the Gravel Performance Nutrition Course and in 1:1 sessions.