Sweat Rate Calculator: How to Measure Your Sweat Rate at Home
Your sweat rate is the weight you lose during a run, plus what you drank, minus anything you peed, divided by the hours you ran. Put your numbers into the sweat rate calculator below; most runners land somewhere between 0.5 and 2 litres an hour, and in Malaysian heat the top half of that range is common.
The maths is the easy part. What makes the number useful is testing in the right conditions, knowing which errors you can ignore, and knowing what to do once you have it. That is what the rest of this page covers.
Sweat rate calculator
Please enter both weights and the duration.
‘;return;} var lossKg=pre-post; var sweatL=lossKg+fl/1000-ur/1000; var hrs=mins/60; var rate=sweatL/hrs; var pct=lossKg/pre*100; if(sweatL<=0){out.innerHTML='These numbers give zero or negative sweat. Check the weights and the fluid figure.
‘;return;} var band=rate<0.8?'lower end':(rate<=1.5?'middle':'upper end'); var html='‘+rate.toFixed(2)+’ L per hour
‘; html+=’Total sweat: ‘+sweatL.toFixed(2)+’ L over ‘+Math.round(mins)+’ min. That sits at the ‘+band+’ of the usual 0.5 to 2 L/h range for athletes.
‘; if(lossKg<0){html+='You finished ‘+Math.abs(lossKg).toFixed(1)+’ kg heavier than you started. You drank more than you sweated, which is the direction to avoid on long runs.
‘;} else{html+=’Body weight change: -‘+pct.toFixed(1)+’%. ‘+(pct>2?’That is more than the 2% line in the ACSM fluid guidance, so you drank too little for this session.’:’That is within the 2% line in the ACSM fluid guidance.’)+’
‘;} out.innerHTML=html; }); })();The calculator treats 1 kg of body weight as 1 litre of sweat. It gives an estimate for the session you tested, not a fixed property of your body.
How do you measure sweat rate at home?
You need a bathroom scale that reads to 0.1 kg, a bottle you know the size of, and a run of about an hour in the conditions you care about. Weighing yourself before and after exercise is the simplest and most accurate way to get a whole-body sweat rate, according to a methodology review by Lindsay Baker of the Gatorade Sports Science Institute (Baker, 2017).
- Empty your bladder, then weigh yourself naked or in the same minimal kit you will wear after. Write it down.
- Run for 45 to 90 minutes at the effort you plan to race at. Drink as you normally would, and note every millilitre. Mark your bottle in 100 ml steps if it has no scale.
- Avoid eating during the test. A gel is fine if you note it, but a bowl of noodles at the 7-Eleven halfway will skew the result.
- If you have to stop to pee, estimate the volume. Most people underguess; a full bladder is often 300 to 500 ml.
- Straight after, towel off, strip off the wet kit, and weigh again on the same scale on the same floor.
That last step matters more than people think. Sweat trapped in a soaked singlet, shorts and shoes led to an 8 to 10% underestimate of sweat loss in a treadmill study of female distance runners (Cheuvront et al., 2002). In Malaysian humidity your kit can hold a surprising amount of water; weighing in a dripping cotton T-shirt is the easiest way to spoil the test.
What does the calculator leave out?
It leaves out two small sources of weight loss that are not sweat: water breathed out, and the mass of fuel your body burns. Together they make up roughly 5 to 15% of the weight lost during exercise (Baker, 2017). Baker suggests correcting for them when sessions run past two or three hours, are very intense, or happen somewhere cool and dry.
For a one-hour test in Malaysian weather, you can mostly let them go. In the Cheuvront study, the simplest field method, body mass change corrected only for what runners drank, came within about 7% of the fully corrected value in warm conditions (30°C). In cool conditions (14°C) every field method missed by 14 to 41%. The errors that grow in the cold partly cancel out in the heat. A Malaysian morning, which sits at 29 to 31°C with 80 to 95% relative humidity around most events here (Singh, 2003), is the kind of setting where the bathroom-scale method behaves best.
Your own day-to-day noise is real too. Whole-body sweat rate varies by about 5 to 7% from one day to the next in the same person under the same conditions, so Baker treats a change of more than about 5% as the point where a new test is telling you something (Baker, 2017). If two tests a week apart give 1.20 and 1.25 L/h, that is the same number.
A worked example from a Putrajaya long run
Here is the kind of test we would ask a runner preparing for the Borneo International Marathon in Kota Kinabalu to do. These numbers are illustrative, not from a real athlete.
She runs a 70-minute loop around Putrajaya Lake at marathon effort, starting at 6.45 am. She weighs 58.6 kg before and 57.5 kg after, in dry kit. She drank 400 ml from a handheld and did not stop for the toilet.
- Weight lost: 58.6 − 57.5 = 1.1 kg
- Total sweat: 1.1 + 0.4 = 1.5 L
- Sweat rate: 1.5 ÷ (70 ÷ 60) = 1.29 L per hour
- Body weight change: 1.1 ÷ 58.6 = 1.9%
Two weeks later she repeats the same loop after an overnight downpour, with cloud cover and a breeze. This time she gets 0.98 L/h. The 0.3 L gap is far bigger than the 5 to 7% day-to-day noise, so it is a real effect of the weather. Her race-day plan now has two versions, and she picks one after checking the Kota Kinabalu forecast the evening before.
Note what she did not do. She did not decide to drink 1.29 L an hour in the race.
What should you do with your number?
Use it to set an upper limit and a rough target, not a volume to match. The ACSM position stand on fluid replacement suggests keeping weight loss during exercise under about 2% of body mass, and says plans should be individual (Sawka et al., 2007). For our runner, 2% is 1.17 kg. Over a four-hour marathon at 1.29 L/h she would sweat about 5.2 L, so to stay inside the line she needs to drink around 4 L, roughly 1 L an hour. That is a lot of cups, and it tells her to carry a bottle between stations rather than rely on two sips at each one.
The opposite mistake is the dangerous one. Drinking more than you sweat is the main cause of exercise-associated hyponatraemia, and the international consensus advises drinking to thirst and watching for weight gain during an event (Hew-Butler et al., 2015). Your sweat rate gives you a ceiling: if you drink more per hour than you sweat, you will gain weight.
Humidity adds a wrinkle. In thick air, some sweat drips off without evaporating, so you sweat more than the cooling you actually get (Baker, 2017). A Malaysian sweat rate can look high compared with figures from European studies. That does not mean you are broken; it means a drier-climate drinking plan will undershoot here.
Once you know your fluid number, sodium becomes a sum you can do. Our guide to how much sodium runners need per hour uses sweat rate as its starting point, and how much sodium you lose running in Malaysian heat explains the sweat-saltiness side. If your longer runs pass two hours and your numbers point to planned sodium, our Elec Stamina caplets list 220 mg of sodium each, and the pack pairs one caplet with every 500 ml of water drunk during exercise. If your runs are shorter than that, water and normal meals are enough.
When should you test again?
Retest whenever the conditions you race in change. Sweat rate moves with exercise intensity, air temperature, sun, wind, clothing and heat acclimation (Baker, 2017). Heat acclimation in particular improves sweating, which usually means a higher sweat rate once you have adapted (Périard et al., 2015). A runner who has just returned from a month in a cooler country, or who has switched from 6 am to midday sessions, should not trust last quarter’s number.
Three tests make a decent profile: a hot, still morning; a cloudy or post-rain morning; and one at the effort and time of day of your target race. Trail runners heading for a hilly event should add a climb-heavy test, because slow uphill hiking and fast running produce very different numbers. Cyclists can use the same calculator on the bike; we look at what changes on long rides in Cycling Hydration in Malaysian Heat.
For the full picture on fluids, heat and electrolytes in one place, see our running hydration and electrolytes guide for Malaysia.
References
- Baker LB (2017). Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability. Sports Medicine, 47(Suppl 1), 111-128. doi:10.1007/s40279-017-0691-5
- Cheuvront SN, Haymes EM, Sawka MN (2002). Comparison of sweat loss estimates for women during prolonged high-intensity running. Medicine & Science in Sports & Exercise, 34(8), 1344-1350. doi:10.1097/00005768-200208000-00017
- Singh R (2003). Fluid balance and exercise performance. Malaysian Journal of Nutrition, 9(1), 53-74. PubMed 22692532
- Sawka MN, Burke LM, Eichner ER, et al. (2007). American College of Sports Medicine position stand: exercise and fluid replacement. Medicine & Science in Sports & Exercise, 39(2), 377-390. doi:10.1249/mss.0b013e31802ca597
- Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. (2015). Statement of the 3rd International Exercise-Associated Hyponatremia Consensus Development Conference. British Journal of Sports Medicine, 49(22), 1432-1446. doi:10.1136/bjsports-2015-095004
- Périard JD, Racinais S, Sawka MN (2015). Adaptations and mechanisms of human heat acclimation: applications for competitive athletes and sports. Scandinavian Journal of Medicine & Science in Sports, 25(Suppl 1), 20-38. doi:10.1111/sms.12408
