Mavr Sports Nutrition Team
Updated October 2, 2026. 10 min read.
To test your sweat rate, weigh yourself before and after a 45 to 90 minute workout, add the fluid you drank, subtract any urine, and divide by the time you trained. 1 kg of weight lost is about 1 liter of sweat. The whole test needs a scale, a bottle, and a towel.
Your sweat rate tells you how much fluid you lose per hour. That number is the starting point for how much to drink in long runs, rides, and races. Here is the test in six steps, a calculator, a worked example, and a table that turns your number into fluid per hour.
The test fits into one normal workout. Follow these steps in order: weigh, train, weigh, count your fluid, calculate, and repeat in different weather.
Use the toilet first. Then weigh yourself naked or in the same light, dry clothes you will weigh in after. Write down the number and the time.
Run or ride for 45 to 90 minutes at a steady effort. Pick a session close to what you want to plan for, such as long-run pace or race pace, in typical weather. Do not skip drinking to make the test "cleaner". Drink as you normally would.
Straight after the session, towel off all sweat, take off wet clothes, and weigh yourself the same way as before. Do it before you eat, drink, or use the toilet. Sweaty clothes and wet skin can add a few hundred grams and make your result look too low.
Write down exactly how much you drank during the session. Weigh or measure your bottle before and after if you are not sure. If you used the toilet during the session, estimate the volume. Most people do not, and then this is zero.
Or let the calculator do it. Enter your numbers in metric or imperial units:
Use measurements from one representative workout. The formula accounts for drinks and any urine passed during the test.
A sweat-rate estimate is a planning input, not an instruction to replace every milliliter while moving. Avoid drinking enough to gain weight during exercise, and seek qualified medical advice for a history of hyponatremia, kidney or heart conditions, or heat illness.
One test is one data point. Sweat rate changes a lot with heat, humidity, effort, clothing, and heat acclimatization, and it differs between running and cycling. Repeat the test in cool, warm, and hot weather, and at least once at race effort. Keep a simple log like this example:
| Test | Weather | Session | Sweat rate |
|---|---|---|---|
| Test 1 | 12°C, dry | 60 min easy run | 0.7 L/hour |
| Test 2 | 22°C, humid | 75 min tempo run | 1.2 L/hour |
| Test 3 | 28°C, sunny | 90 min long run | 1.6 L/hour |
These are example numbers to show how much the result can move. Yours will differ. Plan each race with the test that best matches its weather and effort.
Sam weighs 70.0 kg before a 75-minute run at 22°C. Sam drinks 400 ml during the run and does not stop for the toilet. After toweling off, Sam weighs 68.9 kg.
Now Sam plans a 2-hour half marathon in similar weather. At 1.2 L per hour, Sam will sweat about 2.4 liters. To keep weight loss under about 2% (1.4 kg), Sam needs to drink at least about 1 liter over the race, roughly 500 ml per hour. Sam should also stay well below 1.2 L per hour so there is no risk of drinking more than is lost. A plan of 500 to 700 ml per hour, tried on a long run first, fits both limits.
Use this table as a starting point for sessions longer than about 90 minutes. In shorter sessions, drinking to thirst is usually enough. The aim is the same in every row: lose less than about 2% of body weight, and never gain weight.
Under 0.5 L/hour
What it meansLight sweater
Fluid per hour to start withDrink to thirst, often 200 to 400 ml
NotesEasy to overdrink. Do not force fluid.
0.5 to 1.0 L/hour
What it meansAverage
Fluid per hour to start with300 to 700 ml
NotesKeep intake below your number.
1.0 to 1.5 L/hour
What it meansHeavy sweater
Fluid per hour to start with500 to 900 ml
NotesPlan bottles or aid stations. Think about sodium.
Over 1.5 L/hour
What it meansVery heavy sweater
Fluid per hour to start with700 to 1,000 ml if your gut allows
NotesYou cannot replace it all. Pace for the heat and plan sodium.
| Sweat rate | What it means | Fluid per hour to start with | Notes |
|---|---|---|---|
| Under 0.5 L/hour | Light sweater | Drink to thirst, often 200 to 400 ml | Easy to overdrink. Do not force fluid. |
| 0.5 to 1.0 L/hour | Average | 300 to 700 ml | Keep intake below your number. |
| 1.0 to 1.5 L/hour | Heavy sweater | 500 to 900 ml | Plan bottles or aid stations. Think about sodium. |
| Over 1.5 L/hour | Very heavy sweater | 700 to 1,000 ml if your gut allows | You cannot replace it all. Pace for the heat and plan sodium. |
Thirst, gut comfort, and fluid access still matter. If your race has aid stations every 5 km, check that your plan is realistic before you commit to it.
Sweat rate varies a lot between athletes and within the same athlete. Commonly reported ranges run from roughly 0.5 to 2.5 liters per hour, and sweat sodium from roughly 20 to 80 mmol per liter (about 460 to 1,840 mg). Two runners side by side in the same race can need completely different plans:
Sweat rate
Athlete A0.5 L/hour
Athlete B2.0 L/hour
What it meansAthlete B loses about 4x more fluid
Sweat sodium
Athlete A30 mmol/L (about 690 mg/L)
Athlete B70 mmol/L (about 1,610 mg/L)
What it meansEach liter of Athlete B's sweat is more than twice as salty
Sodium lost per hour
Athlete AAbout 15 mmol (about 350 mg)
Athlete BAbout 140 mmol (about 3,200 mg)
What it meansCompletely different hydration plans
| Measure | Athlete A | Athlete B | What it means |
|---|---|---|---|
| Sweat rate | 0.5 L/hour | 2.0 L/hour | Athlete B loses about 4x more fluid |
| Sweat sodium | 30 mmol/L (about 690 mg/L) | 70 mmol/L (about 1,610 mg/L) | Each liter of Athlete B's sweat is more than twice as salty |
| Sodium lost per hour | About 15 mmol (about 350 mg) | About 140 mmol (about 3,200 mg) | Completely different hydration plans |
Guessing means either underdrinking (higher heart rate, early fatigue) or overdrinking (sloshing, and in the worst case low blood sodium, which is dangerous). A measured number helps you avoid both.
A scale cannot measure how salty your sweat is. White salt marks on clothes hint at salty sweat, but they are not a real test. A lab test or a validated sweat patch gives a number. Sodium loss per hour is your sweat rate times your sweat sodium, so you need both to plan.
If you want to add salt during long sessions, our guide to salt tablets for runners covers how much sodium per hour, when to use tablets, and when to skip them.
In the Mavr app you can save each test to your sweat profile, either linked to a workout or entered by hand, and Mavr uses it for fluid and sodium guidance that scales with heat, intensity, and session length. The habit matters more than the tool: measure carefully, retest when the weather changes, and never confuse loss with a drinking order.
Subtract your weight after exercise from your weight before, add the fluid you drank, subtract any urine, and divide by the hours you exercised. 1 kg lost is about 1 liter of sweat.
About 45 to 90 minutes at a steady effort. Use an effort and weather close to the race or session you are planning, and repeat in different conditions.
Most athletes fall somewhere between about 0.5 and 2.5 liters per hour. Around 0.5 to 1.0 L per hour is common in mild weather. Heavy sweaters in the heat can pass 2 L per hour.
You drank more than you sweated, or the measurements were off. Recheck the numbers and repeat the test. In long events, gaining weight is a warning sign for low blood sodium.
Not for most training. Consider a validated test if you race long, sweat heavily, or have a medical history that makes sodium planning important. White marks on clothes do not measure it.
Mavr does not measure sweat from a wearable. You do the scale test above, then save it to your sweat profile in the app, and Mavr uses it for your fluid and sodium guidance.
General guidance for healthy adults, not medical advice. Talk to a doctor or registered dietitian if you have a health condition, take medicines or have special dietary needs.
Ask Mavr: "Use my sweat rate to build a hydration plan for my next long session."
Ask Kai, the coach in the app