Sweat Rate Calculator: Weigh-In Method for Fluid Loss per Hour

Your sweat rate is how much fluid you lose per hour of exercise. A bathroom scale measures it better than any rule of thumb: the weight you lose during a workout, plus anything you drank, is almost all sweat. Both the American College of Sports Medicine (Sawka et al. 2007) and the National Athletic Trainers' Association (McDermott et al. 2017) recommend this weigh-in, because sweat rates differ too much between people for any single drinking rule to fit everyone.

This calculator runs the NATA equation with your numbers, including any urine you passed, and turns the result into three practical answers: the share of your body weight you lost against the 2 percent line both position stands use, how much to drink per hour in a session like this one, and how much to drink afterward. It also shows how far scale rounding can move the result, because on a short test that can be the largest error of all.

How to calculate sweat rate

The weigh-in method needs four numbers: your weight before, your weight after, what you drank and how long you trained. The NATA position statement gives the equation in its Table 2:

Sweat loss = weight before − weight after + fluid drunk − urineSweat rate = sweat loss ÷ hours of exercise

The NATA equation counts each kilogram of weight lost as a liter of sweat, so in metric a gram lost is a milliliter of sweat. In US units, a pound of sweat is about 15.3 fluid ounces. Many sweat rate charts use 16 ounces per pound, which overstates the loss by about 4 percent.

1. Find the weight you lost. A runner who weighs 170 lb before an hour's run and 168 lb after it has lost 2.0 lb, which is about 31 oz of fluid (2 × 15.3).
2. Add what you drank. The runner finished a 16 oz bottle on the way, so the sweat loss is 31 + 16 = 47 oz.
3. Subtract any urine. There was no bathroom stop, so nothing comes off. Urine is weight you lose without sweating it, so leaving it in makes the sweat rate read high.
4. Divide by the hours. The run lasted 60 minutes, so the sweat rate is 47 oz an hour, or 1.38 liters.

Metric is simpler. Athlete B in the NATA statement, a recreational runner, weighed 80 kg before a 10 km race, weighed 79.2 kg after it and drank 0.4 liters during the 45 minutes: (80 − 79.2 + 0.4) ÷ 0.75 = 1.6 liters an hour.

How to do a sweat rate test at home

A sweat test at home takes one normal workout and a digital bathroom scale. These steps follow the best-practice list in Baker's 2017 review of sweat testing methods.

1. Start normally hydrated. Eat and drink as usual on the day. The NATA statement asks athletes to begin exercise in fluid balance and notes that a deficit of more than 2 percent lowers sweat rate.
2. Empty your bladder, then weigh yourself nude. If that is not practical, weigh in light clothes that you can put back on dry for the second weigh-in. Weigh twice, as Baker recommends, and use the average.
3. Train as you normally do, for about an hour. Use your usual pace, kit and conditions, because the result applies only to sessions like the one you tested.
4. Measure every drink. Note how much you drink in ounces or milliliters. Pouring water over your head or spitting it out spoils the count, so keep a second bottle for cooling.
5. Towel off and weigh again before you pee. Use the same scale on the same spot of floor, in the same state as the first weigh-in. If you did pee during the session, enter how much in the calculator.
6. Repeat in other conditions. A hot run, a cool run and an indoor session can give very different numbers. The NATA statement recommends testing several times, in realistic conditions and at different intensities.

What the 2 percent line means

Both position stands treat 2 percent of body weight as the most to lose during exercise. In the research the NATA statement reviews, endurance performance was consistently reduced once losses reached 2 percent in temperate and hot conditions, although it held up in cool conditions. Strength, power and anaerobic endurance also suffered consistently beyond 2 percent, and mental performance could slip past a 1 percent loss, although NATA describes most of those mental effects as small.

The NATA statement also describes what happens in the body. For every 1 percent of body weight lost, core temperature rises about 0.15 to 0.20 °C and heart rate climbs 3 to 5 beats a minute. Control of body temperature starts to suffer above a 1 percent loss, the risk of heat illness rises past about 3 percent, and thirst usually kicks in near 2 percent. When people drink as they please during exercise, they replace only about two-thirds of what they sweat, so on longer sessions a deficit can build even when you are drinking.

The line is a percentage, so it moves with your size. For a 170 lb person it is 3.4 lb, and for a 130 lb person it is 2.6 lb. The calculator flags any loss above 2 percent of your starting weight. It flags a weight gain too, because the NATA statement advises against finishing heavier than you started.

How much to drink per hour of exercise

The calculator gives a window rather than one number. The low end is the least you can drink and still finish under the 2 percent line, and it depends on how long the session lasts. The high end is your sweat rate, because drinking more than you sweat means gaining weight, and the NATA statement names that as the cause of exercise-associated hyponatremia, a dangerous drop in blood sodium.

For the default runner, who weighs 170 lb and sweats 47 oz an hour, a 1-hour session needs no minimum at all: even with nothing to drink, the loss stays under 3.4 lb. A 2-hour session at the same rate needs at least 21 oz an hour, and a 3-hour session at least 29 oz an hour. The drinking plan on the result card works this out for your own numbers.

If you have not measured your sweat rate, a common guideline for marathon runners, from the International Marathon Medical Directors Association, is to drink to thirst at 0.4 to 0.8 liters an hour, about 14 to 27 oz, with the lower rates for slower runners in cooler weather. A model by Montain, Cheuvront and Sawka (2006) shows why one rate does not fit everyone. A 50 kg (110 lb) runner at 8.5 km/h who drinks 0.8 liters an hour finishes a cool marathon 3.6 percent heavier, while a 90 kg (198 lb) runner at 15 km/h who drinks 0.4 liters an hour finishes 3.9 percent lighter.

The NATA statement says about 200 mL (7 oz) every 15 to 20 minutes may be ideal for how quickly fluid leaves the stomach, and that drinking to thirst is a safe choice when you do not know your sweat rate. For sessions under an hour, it says water is all you need. Past an hour, a drink with carbohydrate or electrolytes may help.

Heat also changes what kind of session makes sense. Sweat rate climbs with the temperature at every pace, as the chart below shows, so an indoor session in a cool room is one way to keep training through a heat wave. Our page on the home workout app with no equipment shows how FitCraft builds bodyweight strength, cardio, yoga and mobility sessions you can do indoors.

How much to drink after exercise

After exercise the goal is to replace what is left of the deficit. If you have plenty of time before your next session, normal eating and drinking will do it. The NATA statement puts the threshold at 12 hours: past that, food and drinks taken as you like restore the balance, and the water and electrolytes in meals help you hold on to fluid. For a whole-day target, the water intake calculator adds an exercise allowance to the official intake values.

When the next session is close, drink more than you lost. The NATA statement recommends 100 to 150 percent of the loss when recovery is under 4 hours. The 150 percent end is 1.5 liters for each kilogram lost, or about 23 oz for each pound. For the default runner, who lost 2 lb, that is 31 to 46 oz.

The extra is there because a large drink also makes you pee. In a study by Shirreffs and colleagues (1996), 12 men lost about 2 percent of their body weight cycling, then drank 50, 100, 150 or 200 percent of the loss as a lower-sodium (23 mmol/L) or a higher-sodium (61 mmol/L) drink. Urine output rose with the volume they drank: in the lower-sodium group, the median was 493 mL after the 100 percent drink and 1,361 mL after the 200 percent drink. The authors concluded that you have to drink more than you sweated to restore fluid balance, and that without enough sodium the extra mostly comes back out as urine.

So have the drinks with a meal or a salty snack rather than on their own. The NATA statement suggests eating and drinking within 2 hours of finishing, to replace fluid, electrolytes, carbohydrate and protein together.

Sweat rate chart for runners

How much you sweat depends mostly on how hard you work, how big you are and how hot it is. Montain, Cheuvront and Sawka (2006) predicted sweat rates from the energy cost of running for runners of three body weights at four paces, in cool (18 °C, 64 °F) and warm (28 °C, 82 °F) weather. Use the chart to sanity-check your own result rather than replace it. The model assumes every drop of sweat evaporates, so real sweat rates run higher when sweat drips off, and your own rate can sit well above or below it.

Read across a row and pace matters: the 154 lb70 kg runner sweats 22 oz0.65 L an hour at an easy pace in the cool and 46 oz1.36 L at the fastest pace in the warm. Read down a column and size matters: at the same pace and weather, the heaviest runner sweats about 1.8 to 2 times as much as the lightest. The step from cool to warm adds the least in this model, about 3 to 4 oz (0.09 to 0.12 liters) an hour at each pace, although full sun, humidity and heavy clothing push real sweat rates higher.

How accurate is this sweat rate calculator?

The calculator does the arithmetic exactly, so the accuracy question is about the weigh-in. The best test of it comes from Cheuvront, Haymes and Sawka (2002), who accounted for every part of the weight change in 8 women running 30 km on a treadmill, once at 30 °C (86 °F) and once at 14 °C (57 °F). In the warm run, a simple weigh-in corrected for drinks, or for drinks and urine, landed within 0.2 to 6.9 percent of the true sweat loss. Correcting for sweaty clothes but not for urine made it worse, off by 15.3 percent, because those two errors had been offsetting each other. In the cool run, every simple method missed by 14 to 41 percent.

A home weigh-in has four sources of error, and you can control three of them: the scale, the urine and the clothes. The fourth, breathing and burning fuel, is small in sessions under 3 hours. Beyond all four, one test describes only one set of conditions.

The scale

Each reading on a scale is rounded to its step, so the difference between two readings can be off by up to one full step. Yorkin and colleagues (2013) tested 61 home bathroom scales: 59 percent read in steps of 0.1 or 0.2 lb, 13 percent in steps of 0.15 kg and 28 percent only in whole pounds. Dial scales were less precise than digital ones and, before anyone touched them, sat on average 0.95 kg (2.1 lb) away from zero. On a scale with 0.2 lb steps, rounding can move a one-hour result by 3 oz. On a whole-pound scale it can move it by 15 oz, close to half of a typical liter-an-hour sweat rate.

So use a digital scale, zero it, put it on the same spot of hard floor both times, weigh twice and test for an hour or more. The range under the result shows this rounding band for a scale that reads to 0.2 lb or 0.1 kg.

Urine and food

Urine is weight you lose without sweating it. Taken on its own, as Baker's review breaks down the Cheuvront study, leaving urine out of the equation inflated the sweat loss by 16 percent in the heat and 37 percent in the cool. Empty your bladder before the first weigh-in, weigh again before you pee afterward, and enter any urine you passed during the session. Food and gels work the other way: add their weight to what you drank, as Baker recommends.

Sweat trapped in clothes

Weighing in sweaty kit makes the sweat loss read low, because your clothes still hold sweat that left your body. In the same study, a singlet, shorts, socks and shoes held back 8 to 10 percent of the sweat, and Baker notes that no equation can correct for it across different clothing. That is why Baker recommends weighing nude or in minimal clothing. If you weigh in clothes, change into the same dry set for the second weigh-in.

Breathing and burning fuel

Some of the weight you lose leaves as water vapor in your breath, and burning fuel trades the oxygen you take in for the carbon dioxide you breathe out, which shifts your weight a little more. Baker puts the combined effect at about 5 to 15 percent, which makes the sweat rate read slightly high, and in the Cheuvront study it came to 9 percent in the heat and 20 percent in the cool. Baker suggests correcting for it mainly when a session lasts longer than 2 to 3 hours, is very hard or takes place in cool, dry air, and Maughan, Shirreffs and Leiper (2007) note that breathing losses can be substantial during hard work in a dry environment. This calculator does not subtract it, so for long, hard or cold sessions, read your result as up to 15 percent high.

One test is one set of conditions

Even a perfect weigh-in describes one session. Baker reports that whole-body sweat rate varies by about 5 to 7 percent from day to day in the same conditions, so two tests that differ by less than about 5 percent may not differ at all. Change the pace, the weather, the clothing or your heat acclimatization and the rate can change far more. Baker advises testing in conditions like the ones you train and compete in, repeating the test in different conditions, and planning around a low, moderate or high category and a range of drinks rather than an exact number.

How much sweat rates vary between people

The spread between people is far larger than any weigh-in error. Baker's review puts typical sweat rates at about 0.5 to 2.0 liters an hour (17 to 68 oz), and in the 461 athletes it shows, only about 2 percent sweated more than 3 liters an hour. The NATA statement gives 0.5 to 4.0 liters an hour as the adult range. Men tend to sweat more than women, mostly because they are larger and make more heat (Baker 2017), children sweat less than adults, and heat acclimatization raises the rate (NATA 2017), which our review of heat acclimation training covers in detail. Size, heat and effort add up: in the NATA examples, the 120 kg college football player at a 2.5-hour practice, at a wet-bulb globe temperature of 29 °C, sweated 3.2 liters an hour, two to two and a half times the rate of the runner, the tennis player and the soccer player.

Worked examples: seven sessions through the calculator

Four of these are the sample athletes in the NATA position statement (Table 3). They were weighed in kilograms, so their pound values are conversions. The other three are our own examples. Drink during is per hour for a session as long as the one tested, and drink after is 100 to 150 percent of the weight lost.

Three patterns stand out. NATA athlete C, the football player, sweated 3.2 liters an hour and lost 5 percent of his weight despite drinking 2 liters, so a practice that long would take at least 2.24 liters an hour to stay under the line. Athlete D lost 2.5 percent in a soccer game with drinks only at halftime, and NATA's suggestions for her were to start the game very slightly over-hydrated and drink a little more at the break. The slow marathoner drank about 18 oz an hour against a sweat rate of 14 oz and finished a pound heavier, the pattern the NATA statement warns about.

When to ignore this calculator

The weigh-in method is built for healthy adults testing a normal training session. In the cases below, follow a clinician's plan instead.

• Children and teenagers. Children sweat less than adults and need their own hydration plans, according to the NATA statement. Ask a pediatrician or an athletic trainer.
• Heart, kidney or liver disease, or a fluid limit. If a doctor has limited your fluids or salt, drinking to a sweat-rate target can work against that plan. The NATA statement asks people with chronic medical conditions to get hydration advice from their own physicians.
• Medicines that change sweating or fluid balance. Some antidepressants and anticholinergic drugs reduce or increase sweating (Baker 2017), and diuretics raise urine output. Ask the prescriber how yours affect exercise in the heat.
• Pregnancy. Fluid needs and heat tolerance change during pregnancy. Your obstetric care team is the right source.
• Events that last several hours. Small mismatches between sweat and drinks add up over long events, and the NATA statement notes that low blood sodium turns up more often after events longer than 4 hours. Treat the drinking plan as a starting point, retest on a long session and never finish heavier than you started.
• Anyone who has needed medical care for heat illness or low blood sodium. The NATA statement says a formal assessment of sweat rate and sweat sodium may be needed for athletes with that history, rather than a home test.

If you feel dizzy, confused, sick or unusually tired during or after exercise in the heat, stop and get medical help. Dehydration and low blood sodium share many of the same symptoms, and the NATA statement notes that telling them apart can take a blood sodium test, so neither you nor this calculator can sort them out from a weigh-in.

Related reading

References

1. American College of Sports Medicine, Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. "American College of Sports Medicine position stand. Exercise and fluid replacement." Med Sci Sports Exerc. 2007;39(2):377-390. doi:10.1249/mss.0b013e31802ca597, PubMed 17277604
2. McDermott BP, Anderson SA, Armstrong LE, Casa DJ, Cheuvront SN, Cooper L, Kenney WL, O'Connor FG, Roberts WO. "National Athletic Trainers' Association position statement: fluid replacement for the physically active." J Athl Train. 2017;52(9):877-895. doi:10.4085/1062-6050-52.9.02, PMC5634236
3. Baker LB. "Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability." Sports Med. 2017;47(Suppl 1):111-128. doi:10.1007/s40279-017-0691-5, PMC5371639
4. Shirreffs SM, Taylor AJ, Leiper JB, Maughan RJ. "Post-exercise rehydration in man: effects of volume consumed and drink sodium content." Med Sci Sports Exerc. 1996;28(10):1260-1271. doi:10.1097/00005768-199610000-00009, PubMed 8897383
5. Cheuvront SN, Haymes EM, Sawka MN. "Comparison of sweat loss estimates for women during prolonged high-intensity running." Med Sci Sports Exerc. 2002;34(8):1344-1350. doi:10.1097/00005768-200208000-00017, PubMed 12165691
6. Montain SJ, Cheuvront SN, Sawka MN. "Exercise associated hyponatraemia: quantitative analysis to understand the aetiology." Br J Sports Med. 2006;40(2):98-105. doi:10.1136/bjsm.2005.018481, PMC2492017
7. Maughan RJ, Shirreffs SM, Leiper JB. "Errors in the estimation of hydration status from changes in body mass." J Sports Sci. 2007;25(7):797-804. doi:10.1080/02640410600875143, PubMed 17454547
8. Yorkin M, Spaccarotella K, Martin-Biggers J, Quick V, Byrd-Bredbenner C. "Accuracy and consistency of weights provided by home bathroom scales." BMC Public Health. 2013;13:1194. doi:10.1186/1471-2458-13-1194, PMC3890563

How this calculator differs from typical sweat rate calculators

Most online sweat rate calculators stop at one number. They subtract the after weight from the before weight, add the drinks, divide by the time and print an hourly rate, often converting at 16 ounces per pound, which runs about 4 percent high. Few count urine, say whether to weigh in clothes or explain what the number means for how much to drink. Fewer still mention that a whole-pound scale can move a one-hour result by 15 oz, or that a weigh-in on a cool day can miss by 14 to 41 percent.

This calculator uses the NATA equation with urine, converts at 15.3 fluid ounces per pound and shows a rounding range for your session length. It puts the share of body weight you lost against the 2 percent line in the ACSM and NATA position stands, then gives a drinking window for during exercise, with a floor that keeps you under that line and a ceiling at your sweat rate so you never plan to gain weight, laid out for 1, 2 and 3 hours. It adds an after-exercise target of 100 to 150 percent of the loss, and it shows no number for impossible inputs, such as a negative sweat loss or a rate above the 4 liters an hour that tops the NATA adult range.