How to calculate your sweat rate
A sweat-rate test gives you an idea of how much you lost during one workout. Here’s how to run the test, work out the result and compare it with another session.
On this page
Key points
- Weigh yourself on the same scale just before and after a typical workout.
- Add what you drank, subtract any urine you passed and divide the result by the number of hours you exercised.
- Repeat the test in different conditions so you can pick a result that suits the session you’re planning.
- Use the result as a starting point. Plan to replace only part of the fluid you lose while exercising.
At a glance
- Core output
- Litres of sweat lost per hour
- Useful test duration
- Long enough to produce a measurable change, often 45–120 minutes
- Key equipment
- Reliable scale, measured bottles and a timer
- Most common error
- Failing to count drinks or wet clothing
- Best practice
- Record several comparable tests and use a range
The sweat-rate formula
Start with the weight you lost during the session, add what you drank and subtract any urine you passed. Divide by the number of hours you exercised to get litres per hour.
For this estimate, count each kilogram of weight lost as roughly one litre of water. Convert your figures to kilograms and litres before you start.
- Sweat loss (L) = weight loss (kg) + drink (L) − urine (L)
- Sweat rate (L/hour) = sweat loss ÷ duration (hours)
How to get a useful measurement
Weigh yourself on the same scale just before and after a typical workout. Wear as little clothing as you can at both weigh-ins, towel off first and record what you drink and any toilet stops.
Choose a workout like the one you’re planning. You may sweat a very different amount on a hard indoor ride than on an easy run in cool weather.
- Record weather, intensity, clothing and route.
- Avoid comparing results from different scales.
- Repeat unusual results before changing a plan.
Worked example
A runner starts at 70.0 kg and finishes at 69.4 kg after 90 minutes. They drank 600 mL and did not urinate. Estimated sweat loss is 0.6 + 0.6 = 1.2 L. Dividing by 1.5 hours gives 0.8 L/hour.
The 0.8 litres per hour is what this runner lost on that day. When planning what to drink, allow for thirst, the time you’ll be out and where you can get a refill. A practical plan usually replaces part of the loss.
What can distort the result?
Wet clothing, inaccurate scales, unrecorded drinks, food eaten during the session and a very short test can all distort the estimate. Other changes during exercise can also affect what you weigh afterwards.
If the calculation produces a negative loss or an implausibly high value, check the units and inputs before using it.
Pick a session like the one you’re planning
If you’re training for a summer half marathon, try the sweat test on a warm run in similar kit and at a similar effort. A gentle winter treadmill walk will tell you much less about race day.
Drink as you normally would during the test. Record what you drink so it can be included in the result.
Prepare the scale and bottles
Place a reliable scale on a hard, level surface. Use the same scale before and after and check that it returns to zero. Household scales have limited precision, so very short sessions may produce changes smaller than the measurement error.
Mark bottle volume or weigh bottles before and after. If you refill, record every refill. Estimating “about half a bottle” after the session is a common source of error.
Weigh yourself in dry kit
Wet clothes can make you weigh more at the end and hide some of the weight you lost. Wear as little clothing as you can at each weigh-in and towel off before the second one.
Rain, swimming and water poured over the body make field testing harder. Those sessions may still be useful for experience, but they are poor choices for a precise body-mass calculation.
Save a few details with your result
A result of 0.85 litres per hour means more when you know how hard you worked, what you wore and what the weather was like. Save the activity, effort and conditions with each test so you can find a similar one later.
Try an easy session and a harder one in cool and warm weather. A few well-chosen tests give you more to work with.
Check the result before saving it
Recalculate the arithmetic by hand when a result is surprising. Confirm that pounds were not entered as kilograms, minutes were converted correctly and every drink refill was counted. A bathroom stop should reduce estimated sweat loss, while fluid consumed should be added because it replaced part of the loss before the final weigh-in.
Compare the result with nearby tests rather than rejecting it solely because it differs. A genuine hot or hard session can be higher, but a sudden extreme value is a prompt to inspect the scale surface, wet clothing and recorded units before using it in another tool.
Use your result to plan drinks
Your result shows roughly how much you lost on that session. For a similar outing, plan to drink less than you expect to lose and avoid gaining weight while you exercise. Try the plan with the bottles and refill stops you’ll use.
Save each result with a descriptive label in your training notes—for example, “warm long run, 24°C, easy pace” or “indoor bike, strong fan”. When planning, choose the closest conditions and keep the difference visible rather than averaging unrelated sessions into false precision.
What to do in different situations
| Situation | What to consider | Why it matters |
|---|---|---|
| Body mass fell and no drink was taken | Convert the mass loss to litres and divide by hours | This is the simplest field-test case |
| Fluid was consumed | Add the measured drink volume to body-mass loss | The fluid replaced part of the sweat before the final weigh-in |
| Urine was passed | Subtract its measured volume | Urine loss is not sweat loss |
| Post-exercise weight increased | Check inputs and consider overdrinking; do not report a negative sweat rate | Intake may have exceeded measured loss or the test may contain error |
| Result exceeds about 3–4 L/hour | Recheck units, clothing, scale and records; seek specialist interpretation | Extreme values are possible but input error is common |
| Conditions differed from the target event | Label the result and repeat in representative conditions | Sweat rate is situation-specific |
Worked example with drinks and a bathroom stop
A cyclist weighs 76.4 kg before a 90-minute indoor ride and 75.8 kg afterwards. They drink 750 ml and pass 100 ml of urine.
- Body-mass loss: 76.4 − 75.8 = 0.6 kg, treated as approximately 0.6 L.
- Add drink consumed: 0.6 + 0.75 = 1.35 L.
- Subtract urine: 1.35 − 0.10 = 1.25 L estimated sweat loss.
- Convert duration: 90 minutes = 1.5 hours.
- Divide: 1.25 ÷ 1.5 = 0.83 L/hour.
Save about 0.83 litres per hour for this indoor ride, along with the room conditions and fan setup. If it’s a good match for a session you’re planning, choose it in the planner to get a starting range.
How to use the calculator
- 1
Choose kilograms or pounds and enter pre- and post-session measurements from the same scale.
- 2
Enter the complete session duration, excluding a long unrecorded break if body mass changed during it.
- 3
Add every measured drink and any measured urine volume.
- 4
Check the displayed equation before accepting the result.
- 5
Save the result for the electrolyte and race planners only when it is plausible.
- 6
Repeat in another representative condition and compare the range.
Common mistakes
Mixing units
Entering pounds as kilograms or millilitres as litres creates dramatically incorrect results.
Using wet kit
Retained water masks body-mass loss.
Forgetting a refill
Uncounted intake makes estimated sweat loss too low.
Testing for only a few minutes
Scale precision may overwhelm a small true change.
Treating the result as permanent
Intensity, clothing, heat, humidity and acclimatisation change sweat rate.
Replacing the entire loss automatically
A measurement describes loss; it does not prescribe full replacement during exercise.
Use the result as guidance
These tools support planning and education. They do not diagnose dehydration, heat illness or electrolyte disorders. Stop exercise and seek appropriate medical help for severe or worsening symptoms.
Frequently asked questions
Is one kilogram of weight loss exactly one litre of sweat?
It is a practical field approximation. Body-mass change also reflects respiration, fuel use, food and measurement error, which is why repeat tests matter.
How long should a sweat-rate test be?
Often 45–120 minutes is useful because it creates a measurable change while resembling training. The best duration is representative and safe for the activity.
Can I calculate sweat rate without weighing?
Not reliably with this method. Clothing salt marks, perceived sweat and wearable estimates can provide context but are not the same measurement.
Should I stop drinking during the test?
No. Follow a normal, safe training routine and measure what you drink so it can be included.
Why did I gain weight?
You may have consumed more than the measured losses, or the test may include scale, clothing or recording error. Do not convert a negative result into a sweat rate.
How often should I repeat it?
Repeat when season, environment, training intensity, clothing or event demands change. Several representative tests are better than frequent testing without a purpose.