Sweat Rate Calculator — How Much Do You Sweat, and What to Put Back
Work out how much you sweat, and what to put back. Weigh yourself before and after a run, enter the numbers, and this gives you your sweat rate in litres per hour, how much fluid and sodium to replace, and which drink actually matches your losses.
Sweat rate varies enormously between people and between conditions. Published ranges run from under 0.5 to over 2.5 litres an hour, and sweat sodium varies more than tenfold — roughly 200 to 2,300 mg per litre. That spread is why generic hydration advice fails so many runners. The only way to know your own numbers is to measure them.
The calculator
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Steady-state L/hr
plan long efforts on this
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Session average L/hr
including the warm-up
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ml per hour to replace
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mg sodium per hour (estimate)
Why sweat rate takes about 15 minutes to reach full speed
Sweating is driven by a rise in core temperature, not by the act of starting to move. For the first few minutes of a session your core temperature is still climbing, so you sweat well below the rate you will eventually settle at. Output typically begins at roughly a third of its final level, climbs steeply through the first 15 minutes, and reaches a steady state by around 20 to 30 minutes.
That matters because it means a short test underestimates the rate you will actually sustain. A 60-minute test spends a meaningful share of its time in the ramp, so dividing total loss by total time gives you a number lower than what you will hold for hours three, four and five of a long race. This calculator reports both: the session average you measured, and the steady-state rate to plan around. The chart shows how the two relate.
Be clear about what is measured and what is modelled. Your session average is measured — it comes straight off the scale. The steady-state figure is derived from a standard ramp curve fitted to your session length, not something the test measures directly. It is a better basis for planning a long effort than the raw average, but it is still a model. The longer your test, the smaller the difference between the two.
How to run the test properly
- Weigh yourself before, with as little clothing as possible, after using the bathroom. Note it to the nearest 0.1 kg.
- Run for 60 to 90 minutes at the intensity you actually race at. Sweat rate changes with effort, so an easy jog tells you very little about race day.
- Track every millilitre you drink. Weigh or measure your bottle before and after if that is easier than counting sips.
- Towel yourself dry and weigh again in the same clothing state. Sweat still on your skin will skew the result.
- Enter the numbers above. One kilogram lost is roughly one litre of sweat, and one millilitre of fluid weighs one gram, which is what makes this arithmetic work.
- Repeat in different conditions. One test gives you a number for that day. Three across cool, warm and hot give you a range you can actually plan around.
Reading your result
| Sweat rate | Category | What it means in practice |
|---|---|---|
| Under 0.5 L/hr | Low | Modest fluid needs. Check the test was not run in unusually cool conditions before planning around it. |
| 0.5–1.5 L/hr | Typical | Where most athletes sit. Standard hydration guidance will work reasonably well for you. |
| 1.5–2.5 L/hr | High | You will not be able to drink back everything you lose. Prioritise sodium and accept some deficit. |
| Over 2.5 L/hr | Very high | Plan carefully for heat. Consider a proper sweat sodium test rather than relying on the average. |
On drinking too much. More fluid is not automatically better. Sustained intakes above roughly a litre an hour raise the risk of hyponatremia — diluted blood sodium — which is more dangerous than the mild dehydration most runners worry about. If your sweat rate is high, the fix is usually more sodium per litre, not more litres.
What the numbers are based on
Published sweat sodium concentrations run from about 200 mg to over 2,300 mg per litre, averaging near 950 mg/L — this calculator uses that average unless you know your own from a sweat test. Fluid guidance for ultra-distance running sits around 450 to 750 ml per hour. The 2% body-weight threshold is where performance decrements are commonly reported. The ramp curve behind the steady-state figure assumes sweat output starts near 30% of its eventual rate and approaches steady state with a time constant of about 10 minutes, which puts it near full rate by 30 minutes. Your own sweat sodium can only be measured directly; anything estimated from an average is a starting point you then adjust by how you feel and perform.
Common questions
How do I calculate my sweat rate?
Weigh yourself with as little clothing as possible before a run, run for 60 to 90 minutes at the intensity you race at, towel off, and weigh yourself again in the same state. Sweat rate equals body weight lost plus any fluid you drank, divided by the hours you ran. One kilogram of weight lost equals roughly one litre of sweat, and one millilitre of fluid drunk weighs one gram, which is what makes the arithmetic work.
Why does sweat rate take about 15 minutes to reach full speed?
Sweating is triggered by a rise in core temperature, not by starting to move. At the beginning of a session your core temperature is still climbing, so you sweat well below your eventual rate. Sweat output typically reaches roughly a third of its final level in the first minutes and settles at a steady state by around 20 to 30 minutes. That means a short test underestimates the rate you will actually sustain over a long race, which is why this calculator reports both a session average and a steady-state figure.
What is a normal sweat rate for a runner?
Published ranges put most athletes between 0.5 and 1.5 litres per hour. Under 0.5 L/hr is a low sweater, 1.5 to 2.5 L/hr is high, and above 2.5 L/hr is very high. Heat, humidity and intensity all push the number up, so a single test tells you about the conditions you tested in, not about you in every condition.
How much sodium do I lose in sweat?
Sweat sodium varies far more between people than sweat rate does. Published values run from about 200 mg to over 2,300 mg per litre, with an average near 950 mg per litre. That is a more than tenfold spread, which is why generic electrolyte advice fails so many people. This calculator estimates your hourly sodium loss from the average unless you know your own concentration from a sweat test.
How much should I drink per hour when running?
Aim to replace most of what you lose, not all of it, and not more. For efforts over a few hours, published guidance for ultra-distance running sits around 450 to 750 ml per hour. Drinking more than about a litre an hour raises the risk of hyponatremia, which is dilution of blood sodium, and is more dangerous than mild dehydration. If your measured sweat rate is very high, the answer is usually more sodium rather than more water.
Does a sweat rate test need to be done in race conditions?
Yes, as closely as you can manage. Sweat rate changes with temperature, humidity and how hard you are working, so a test on a cool easy run tells you very little about a hot race. Test at race intensity, and ideally test more than once across different conditions so you know your range rather than a single number.
Which drink should I use to replace what I lose?
For heavy sodium losses, Electroride carries 600 mg of sodium per serving, the highest in the range. Watermelon Mint is next at 500 mg. For steady hydration without much fuel, the isotonic mixes — Honey Lemongrass and Watermelon Mint — are built to be sipped continuously through a session. If you need carbohydrate as well as fluid, the Orange Ginger and Passionfruit Mango endurance drinks carry carbohydrate from dates and evaporated cane juice alongside the electrolytes. Full nutrition panels for the drink mixes are still being verified and will be published on the nutrition data page when they are.
Sodium content for every Spring product is published on our complete nutrition data page. Plan your race fuelling with the race nutrition calculator. Last reviewed: 14 August 2026.