How to measure your sweat rate with a bathroom scale

One litre of water weighs one kilogram. That single fact turns a cheap scale into the most accurate hydration tool most athletes will ever own.

Most hydration advice is a number someone else picked. "Drink 500 ml before training." "Drink every 15 minutes." The trouble is that sweat rates between two people doing the same session can differ several-fold. The American College of Sports Medicine puts typical exercise sweat rates at roughly 0.3 to more than 2.4 litres per hour, depending on the person, the effort and the weather.¹

Generic advice can't cover a range that wide. Your own number can, and you can get it with a bathroom scale.

The idea

Over an hour of exercise, almost all the weight you lose is water. Fat and glycogen losses over that window are small by comparison. And since one litre of water weighs about one kilogram, the change on the scale is close to a direct readout of fluid lost.

The test, step by step

1. Empty your bladder, then weigh yourself in minimal clothing. Write it down.
2. Train as normal for about an hour. Use a bottle you can measure, and note how much you drink.
3. Avoid peeing during the session if you can. If you do go, estimate it and subtract it in the formula.
4. Towel off completely straight after, then weigh yourself again in the same minimal clothing. Sweat-soaked kit can hold a surprising amount.
5. Run the formula.

Worked example

Before: 78.0 kg. After 75 minutes of training: 76.9 kg. Drank 0.6 L. No bathroom break.

Sweat loss = 78.0 - 76.9 + 0.6 = 1.7 L. Sweat rate = 1.7 / 1.25 = 1.36 L per hour.

The same session also shows the deficit: 1.1 kg lost is about 1.4% of body mass.

What throws the number off

• Different clothes on the two weigh-ins. Weigh in the same minimal kit both times.
• Not towelling off. Sweat on the skin counts as weight you haven't lost yet.
• Eating during the session. Gels and bars add mass. Add their weight to "fluid drunk" if you want to be exact.
• One test only. Sweat rate changes with temperature, humidity, intensity and fitness. Repeat the test in hot and cool conditions and at different intensities, and keep a small table.
• Very long sessions. Over several hours, fuel use and the water released when it's burned make the scale less exact. For sessions of an hour or two it's fine.

What to do with your number

Once you know roughly how much you lose per hour in given conditions, you can plan intake so the deficit stays under that 2% line without drinking past your losses. If your sweat rate is 1.4 L per hour and you can comfortably drink 0.8 L per hour, you know a 90-minute session in the heat will put you down about 0.9 kg. That's information a generic rule can't give you.

The scale tells you what you lost. It doesn't tell you what you drank across the rest of the day, which is the other half of fluid balance. We cover that in Drink to thirst or drink to a plan?