What you need

You only need a few things:

• bathroom scales
• your running bottle or bottles
• a towel
• somewhere to note your numbers

Optional but helpful:

• kitchen scales if you want to weigh bottles before and after
• a spreadsheet or notes app to track results in different conditions

How Do You Calculate Your Sweat Rate?

Calculating your sweat rate is surprisingly simple.

1. Weigh yourself before your run.
2. Record how much fluid you drink.
3. Weigh yourself again after the run.
4. Add together your body weight lost and fluid consumed.
5. Divide the result by the number of hours you ran.

The answer gives you your estimated sweat rate in litres per hour (L/hr).

Once you know this number, you can build a much more personalised hydration strategy instead of simply guessing how much to drink.

How to measure your sweat rate step by step

Step 1: Weigh yourself before your run

Go to the toilet first if you can.

Then weigh yourself just before you head out.

For the most accurate result, do this with minimal clothing or no clothing at all. That way you are not adding the weight of sweaty kit afterwards.

Write down your starting weight.

Step 2: Do a normal training run

You do not need to make this complicated.

Just do a session that reflects the kind of running you want to plan for.

A steady run of around 60 minutes is ideal for most runners. Longer sessions can work too, but once you get very long, fuelling, glycogen use, and other variables start muddying the numbers a bit.

While you run, keep track of how much you drink.

The simplest way is to use one bottle and know how much was in it to begin with.

Step 3: Weigh yourself after the run

When you finish, towel yourself off and weigh yourself again in the same way as before.

Consistency matters here.

If you weighed yourself before the run in minimal kit, do the same afterwards.

Write down your finishing weight.

Step 4: Add up what you drank

Work out how much fluid you consumed during the run.

If your bottle started with 500ml and finished empty, you drank 500ml.

If you had two bottles, add them together.

If you prefer to weigh bottles, that works too. A 1 gram change is roughly equal to 1 ml of fluid.

Step 5: Account for any toilet stop

Ideally, do the test on a run where you do not need the toilet.

If you did stop for a wee during the session, include an estimate of urine loss as well. That makes the result more accurate.

Step 6: Calculate your sweat rate

The basic idea is:

Sweat loss = body weight lost + fluid consumed − urine passed

Then:

Sweat rate = sweat loss ÷ time in hours

So if you lost 1.0 kg, drank 500 ml, ran for 1 hour, and did not go to the toilet:

• sweat loss = 1.0 L + 0.5 L = 1.5 L
• sweat rate = 1.5 L/hr

That means you were losing fluid at roughly 1.5 litres per hour in those conditions.

Worked example

Here is a simple example.

Before run: 75.0 kg
After run: 74.2 kg
Fluid consumed: 600 ml
Run time: 60 minutes
Toilet stop: none

Step 1: Work out body weight lost

75.0 − 74.2 = 0.8 kg

Step 2: Add fluid consumed

0.8 + 0.6 = 1.4 L

Step 3: Divide by time

1.4 ÷ 1 = 1.4 L/hr

So the estimated sweat rate is:

1.4 litres per hour

That would put you firmly in heavy sweater territory for that session.

What Is Considered a High Sweat Rate?

There is a wide range of perfectly normal sweat rates among runners.

As a general guide:

• Under 0.75 L/hr = lighter sweater
• 0.75–1.25 L/hr = moderate sweater
• 1.25–1.75 L/hr = heavy sweater
• 1.75 L/hr and above = very heavy sweater

These are only guidelines.

The important thing isn’t whether your number looks high or low—it’s understanding what your body loses in different running conditions.

Am I a Heavy Sweater?

If your sweat rate is above around 1.25 litres per hour, you’ll generally fall into the heavy sweater category.

Many marathon and ultra runners lose between 1.2 and 2.0 litres per hour, with some losing even more during hot races.

You may also be a heavy sweater if you regularly notice:

• soaked running clothes
• sweat dripping down your arms or legs
• white salt marks on your clothing
• constantly feeling thirsty
• finishing long runs noticeably lighter than when you started

Knowing your sweat rate removes the guesswork and helps you create a hydration strategy that matches your body.

How Much Fluid Do Runners Lose Per Hour?

Most runners lose somewhere between 0.5 and 2.0 litres of sweat per hour depending on:

• temperature
• humidity
• running intensity
• body size
• heat adaptation
• clothing

Heavy sweaters can exceed 2 litres per hour during long races or very hot conditions.

The only reliable way to know your own number is to measure it.

How Much Should You Drink During a Run?

One of the biggest misconceptions is that you need to replace every drop of sweat you lose.

For most runners, that isn’t necessary or practical.

Instead, many runners perform well replacing around 50–70% of their hourly sweat loss, although individual needs vary.

For example:

• Sweat rate: 1.4 L/hr
• Target fluid intake: approximately 700–1000 ml/hr

Your ideal intake will depend on temperature, pace, race duration and stomach comfort.

A Few Important Things to Remember

Your sweat rate changes

Don’t expect one test to cover every run.

Your sweat rate will vary between:

• winter and summer
• easy runs and hard sessions
• road races and trail races
• short runs and long ultras

That’s completely normal.

Repeat the test

One test is useful.

Three or four tests in different conditions are much better because they reveal patterns.

For example:

• Cool easy run: 0.9 L/hr
• Long steady run: 1.3 L/hr
• Hot race effort: 1.7 L/hr

Now you’re building your hydration plan using real data rather than guesswork.

Frequently Asked Questions

Is 2 litres of sweat per hour normal?

Yes. Many heavy sweaters lose around 2 litres or more per hour during hot weather or long races.

Should I replace all my sweat losses?

Usually not. Most runners perform well replacing a proportion of their losses rather than every drop.

Does sweat rate change?

Absolutely. Heat, humidity, effort and fitness all affect how much you sweat.

Do heavy sweaters need electrolytes?

Heavy sweaters often lose large amounts of sodium alongside fluid, making electrolyte replacement an important part of hydration on longer runs.

Next Step: Build Your Hydration Strategy

Your sweat rate tells you how much fluid you’re losing.

Combine that with your estimated sodium loss and you’ll have a much clearer picture of how to fuel long runs, marathons and ultras.

Read next:

 👉How to Estimate Your Sodium Loss

 👉Hydration Strategy for Sweaty Runners

 👉 Best Electrolytes for heavy Sweaters

Final thought

If you’re a heavy sweater, measuring your sweat rate is one of the simplest and most valuable things you can do.

It turns hydration from guesswork into a practical strategy built around your own body.

You don’t need to be perfect—you just need better information than you had before.

That alone can make a huge difference on long runs and race day.