The Sweat Test: The simplest way to personalize your hydration strategy

How much should I drink during a run?

It’s one of the most common questions runners ask.

Ask ten runners and you’ll probably get ten different answers.

Drink every 15 minutes.

Drink when you’re thirsty.

Drink 500 ml per hour.

Drink 750 ml per hour.

The problem is that most hydration advice starts with a number.

But your body didn’t get that memo.

Hydration needs are highly personal. Two runners running side by side on the same route can lose very different amounts of fluid. One may finish with a slightly damp t-shirt, while the other looks like they just stepped out of a swimming pool.

If hydration is personal, shouldn’t your hydration strategy be personal too?

Fortunately, there is a simple way to find out exactly how much fluid your body loses during a run.

I call it the Sweat Test.

No fancy gadgets.

No laboratory equipment.

Just a weighing scale and one hour of running.

The Sweat Test

Step 1: Plan a 60-minute run

Choose a run that lasts approximately one hour. Ideally, run at the intensity at which you most often train or race, as sweat rates vary with effort.

The objective is to create a realistic amount of sweat that reflects your normal running conditions.

Step 2: Weigh yourself before the run

Before stepping out, weigh yourself naked and note down your weight.

This is your starting point.

Step 3: Carry a measured quantity of hydration

Carry a fixed amount of fluid, say 250 ml.

You may consume this during the run if needed.

However, do not consume any additional fluids or food during the test. Keeping things controlled is important if you want an accurate result.

Step 4: Complete the run

Run for 60 minutes at the planned effort.

Try to keep the conditions as close as possible to your normal training environment.

Step 5: Wipe off the sweat

Once the run is complete, wipe away any sweat from your body using a towel.

This helps improve the accuracy of the measurement.

Step 6: Weigh yourself again

Now weigh yourself naked once again and note down your weight.

This is your finishing weight.

Step 7: Calculate your hourly fluid loss

The calculation is simple:

Fluid Loss = Weight Lost During the Run + Fluid Consumed During the Run

For example:

• Pre-run weight: 70.0 kg
• Post-run weight: 69.2 kg
• Weight lost: 0.8 kg
• Fluid consumed during the run: 250 ml

Total fluid loss:

800 ml + 250 ml = 1,050 ml

This means your body lost approximately 1.05 litres of fluid during that one-hour run.

For best results, perform the test during a run where you do not need a bathroom break, as urination can affect the accuracy of the calculation.

What does this number mean?

This number is your personal hourly sweat rate under those conditions.

In simple terms, it tells you how much fluid your body is losing every hour while running under similar conditions.

Of course, sweat rates are influenced by temperature, humidity, pace, fitness level and even the clothes you wear. You may notice different results in summer and winter, or during an easy run versus a hard workout.

If you train or race in very different conditions through the year, it may be worth repeating the test in different seasons.

But even with those variations, the sweat test provides something that generic hydration advice never can: A hydration strategy built specifically for you.

Why getting hydration right matters

When you run, your body cools itself by producing sweat. As fluid loss increases, your heart has to work harder to deliver oxygen to working muscles and regulate body temperature. The result is a higher heart rate, greater perceived effort and often a drop in performance.

But hydration is not a case of “more is better.” Drinking too little can hurt performance, while drinking too much can leave you feeling bloated and uncomfortable.

The goal is simple: replace fluids at a rate that is reasonably close to what your body is losing. And that is exactly what the sweat test helps you determine.

Your body has already given you the answer

Many runners spend years searching for the perfect hydration strategy.

Sports drinks.

Hydration packs.

Electrolyte tablets.

Special bottles.

Yet very few spend one hour finding out how much fluid they actually lose.

The sweat test changes that.

It replaces guesswork with data.

And once you know your sweat rate, you can stop relying on generic hydration advice and start making decisions based on what your body actually needs.

The next time someone tells you exactly how much you should drink during a run, smile politely.

Then go do the sweat test.

The answer isn’t in a hydration chart. It’s already inside you.

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