Built by Tommo, Verus founder — free to use, no account required.
Sweat Rate Calculator
Measured, not estimated — because no formula knows how much you sweat.
Sweat Rate Calculator
Measure what you actually lose in an hour, then turn it into a race fluid plan.
Your sweat rate
0.87L/hour
1.30L lost over the session
Body mass lost
1.1%
Acceptable
Between 1% and 2%. Normal for a hard or hot session, and below the point where performance usually suffers.
Plan a race at this sweat rate
You would sweat roughly 3.5L over 4 hours. To stay inside a 2% body-mass loss, drink about 0.49–0.87L per hour.
The lower figure keeps losses inside 2%; the upper replaces sweat litre for litre. Full replacement is rarely necessary and drinking to it over several hours carries its own risk, so most people should aim at the lower end of that range.
Sweat loss is weight lost plus fluid drunk minus urine passed, treating one kilogram as one litre. Your rate is not a fixed property — it rises with heat, humidity and intensity, so measure it again in conditions like the ones you will race in rather than assuming one test covers every day. This is general guidance, not medical advice.
Use It On A Real Race
About This Tool
Most hydration calculators ask your height, weight and the weather, then produce a number. That number is close to meaningless, because the variation between two people of the same size in the same conditions is larger than anything a formula can capture — published sweat rates for endurance athletes span roughly 0.5 to 1.5 litres an hour and beyond. So this tool does not estimate. It measures, from a session you weigh yourself before and after, and then turns the measurement into a fluid plan for a race of any length.
How To Use It
- Weigh yourself with minimal clothing immediately before a run, ideally 60-90 minutes long.
- Track every millilitre you drink during the session, and any urine passed.
- Towel dry and weigh yourself again straight afterwards, in the same clothing.
- Enter the numbers — the tool computes sweat loss as weight lost plus fluid drunk minus urine passed.
- Enter a race length to convert your rate into an hourly drinking target with a sensible floor and ceiling.
Why this measures instead of estimating
Search for a hydration calculator and most of them will ask your height, weight, the temperature and how hard you are working, then hand you a number. That number is close to useless, and it is worth being clear about why.
Sweat rate varies enormously between people. Published rates for endurance athletes run from roughly half a litre an hour to well over one and a half, and the spread between two people of the same size, running the same pace, in the same weather, is larger than any formula's sensitivity to its inputs. A model that takes your height cannot know that you are the person in your running club who arrives back looking like they swam there.
So this tool does not guess. You weigh yourself, you run, you weigh yourself again, and the arithmetic is exact.
The whole formula
Sweat loss = weight lost + fluid drunk − urine passed, treating one kilogram of body mass as one litre of fluid. Divide by the hours you were out. That is the entire method, and it is more reliable than anything more sophisticated that skips the weighing.
Running the test properly
The measurement is only as good as the protocol. Five things matter:
- 1 Weigh yourself with minimal clothing, immediately before you go out. Clothing that absorbs sweat will otherwise be counted as body mass you still have.
- 2 Towel dry thoroughly before weighing afterwards, in the same minimal clothing. Sweat sitting on your skin is fluid you have already lost.
- 3 Track your drinking accurately. Estimating it defeats the point — use a bottle you know the volume of.
- 4 Run 60 to 90 minutes. Shorter than that and normal scale variation is a large fraction of the result.
- 5 Note the conditions. A rate measured on a cool morning tells you very little about a hot race, and this is the single most common way people mislead themselves with this test.
The number that actually matters is the percentage
Sweat rate is the input; body mass lost is the output that predicts a bad day. The endurance literature is expressed in percentage of body mass, and the widely cited threshold is around 2% — beyond that, endurance performance measurably declines in most people.
| Body mass lost | What it means | What to change |
|---|---|---|
| Under 1% | Well replaced | Nothing. Your intake broadly matched your losses. |
| 1-2% | Acceptable | Normal for a hard or hot session and below the point where performance usually suffers. |
| 2-4% | Performance-limiting | Drink more during sessions of this type, and review whether you started already down on fluid. |
| Over 4% | Too much | A large deficit. Review the drinking strategy and whether the session suited the conditions. |
Note that the goal is not to replace every litre. Aiming to match sweat loss exactly over several hours is neither necessary nor risk-free — drinking well beyond your losses carries its own serious danger. Keeping losses inside about 2% is the target, which is why the race plan this tool produces gives you a floor and a ceiling rather than a single figure.
Your rate is not a fixed property
It rises with heat, with humidity and with intensity, and it falls as you acclimatise — though acclimatisation is a subtle case, because adapting to heat actually makes you sweat earlier and more freely while making that sweat more dilute. The practical consequence is the same either way: one measurement does not cover every day.
Humidity matters as much as temperature and is more often ignored. Sweat that cannot evaporate still leaves your body — you get the fluid loss with none of the cooling — so a humid day can produce a high sweat rate and a poorly cooled runner at the same time. The heat-adjusted pace calculator on this site works in dew point rather than relative humidity for exactly that reason.
Turning a rate into a race plan
Once you know your rate, a race plan is arithmetic. A runner losing 0.9 litres an hour over a four-hour marathon sweats about 3.6 litres. Replacing all of it means drinking 900ml an hour, which most people cannot absorb while running. Keeping losses inside 2% of a 75kg body mass means giving up about 1.5 litres, so the target becomes roughly 2.1 litres across the race — a little over 500ml an hour, which is realistic.
That is the calculation the tool does for you. It is worth doing before race day rather than during it, because the alternative is discovering at twenty miles that your plan was never physically possible.
- Practise drinking at your target rate on long runs. Absorbing 500ml an hour while running is a trainable skill.
- Take sodium as well as fluid on long efforts — sweat carries it, and replacing volume alone dilutes what is left.
- Start the race properly hydrated rather than trying to catch up during it.
- Adjust for the day. A hot race is not the day to run the plan you built in March.
Verus is a running site, not a medical one. If you have a condition affecting fluid balance, heat tolerance or blood pressure, this is a conversation for a doctor rather than a calculator.
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Frequently Asked Questions
How do you calculate your sweat rate?
Sweat loss is weight lost during the session plus fluid drunk minus urine passed, treating one kilogram of body mass as one litre of fluid. Divide by the session length in hours to get litres per hour. Weigh yourself with minimal clothing before and immediately after, towelled dry, and track your drinking accurately for it to mean anything.
What is a normal sweat rate for a runner?
Most endurance athletes lose somewhere between 0.5 and 1.5 litres an hour, but the spread is genuinely wide and depends on body size, intensity, heat, humidity and how acclimatised you are. That range is exactly why measuring beats estimating — your own number is far more useful than the middle of a population range.
How much should I drink during a marathon?
Enough to keep body-mass loss inside about 2%, which is where endurance performance starts to decline measurably for most people. That is usually less than full replacement of sweat loss. This tool gives a range: the floor keeps you inside 2%, the ceiling matches sweat litre for litre, and most people should aim at the lower end.
What percentage of body weight loss is dangerous when running?
Beyond roughly 2% of body mass, endurance performance measurably declines. Larger deficits compound that and are worth actively correcting. Drinking far in excess of sweat losses carries its own serious risk in the other direction, which is why this tool caps its recommendation at full replacement rather than encouraging more.
Does sweat rate change with the weather?
Substantially. It rises with heat, humidity and intensity, so a rate measured on a cool morning will understate what a hot race demands. Measure it again in conditions like the ones you will race in — and remember that humidity matters as much as temperature, because sweat that cannot evaporate still leaves your body.
Do I need electrolytes or just water?
Sodium as well as water on longer efforts, because sweat carries sodium and replacing volume alone dilutes what is left. Concentrations vary widely between individuals — roughly 200 to 2,000mg per litre — which is why "salty sweater" is a real category rather than a marketing term. The race fuel calculator estimates a sodium target from your fluid plan.