The finding. The 2% body-weight line is a fixed amount for each runner, but sweat keeps adding up by the hour, so the least you must drink to stay inside it depends on how long you run. For the example in the sweat rate calculator (165 lb, 1.38 L of sweat per hour), staying under 2% needs no drinking at all on a 1-hour run, about 0.63 L per hour over 2 hours and about 1.01 L per hour over 4 hours. A sweat test gives you the rate; the duration turns it into a drinking target.
How the sweat test works
The field method comes from Lindsay Baker of the Gatorade Sports Science Institute (Sports Science Exchange 161, 2017):
Sweat loss (L) = body mass before - (body mass after - fluid drunk + urine passed)
Sweat rate (L per hour) = sweat loss / hours of exercise
One kilogram of lost body mass is counted as one litre of sweat. Weigh nude or in minimal clothing, after emptying your bladder, before and after the session. Measure what you drink by weighing or reading the bottle, and estimate any urine passed during the session.
Baker's paper works one through. The athlete starts at 82.5 kg and finishes at 81.0 kg, having drunk 1.1 L and passed 0.3 L of urine in 1.75 hours. Inside the brackets: 81.0 - 1.1 + 0.3 = 80.2 kg. Subtracted from 82.5, that leaves 2.3 L of sweat, and 2.3 divided by 1.75 hours is 1.31 L an hour. The scale itself dropped 1.5 kg, or 1.82% of the starting weight. The sweat rate calculator runs the same arithmetic in pounds and fluid ounces or in kilograms and litres.
The 2% line in pounds, kilograms and fluid ounces
The line comes from the 2007 ACSM position stand on exercise and fluid replacement by Sawka and colleagues. Its abstract frames drinking during exercise as a way to avoid excessive dehydration, which it puts at more than 2% of body weight lost to water deficit, and it adds that weighing before and after exercise is a way to estimate an individual's sweat rate: the test described above.
Two percent of body weight, computed for runners from 120 to 240 lb, and converted to fluid with the method's own 1 kg = 1 L rule:
| Body weight | kg | 2% in lb | 2% in kg | As fluid (1 kg = 1 L) |
|---|---|---|---|---|
| 120 lb | 54.4 | 2.4 lb | 1.09 kg | 1.09 L (37 fl oz) |
| 135 lb | 61.2 | 2.7 lb | 1.22 kg | 1.22 L (41 fl oz) |
| 150 lb | 68.0 | 3.0 lb | 1.36 kg | 1.36 L (46 fl oz) |
| 165 lb | 74.8 | 3.3 lb | 1.50 kg | 1.50 L (51 fl oz) |
| 180 lb | 81.6 | 3.6 lb | 1.63 kg | 1.63 L (55 fl oz) |
| 195 lb | 88.5 | 3.9 lb | 1.77 kg | 1.77 L (60 fl oz) |
| 210 lb | 95.3 | 4.2 lb | 1.91 kg | 1.91 L (64 fl oz) |
| 225 lb | 102.1 | 4.5 lb | 2.04 kg | 2.04 L (69 fl oz) |
| 240 lb | 108.9 | 4.8 lb | 2.18 kg | 2.18 L (74 fl oz) |
A useful rule of thumb falls out of the table: each pound of body weight lost is about 15.3 fl oz of fluid (0.4536 L). For a 150 lb runner the 2% line is 3.0 lb, 1.36 kg, or about 46 fl oz.
From a sweat rate to a drinking target
The 2% line is an amount; sweat is a rate. To stay inside the line over a run of a given length, what you drink must cover everything you sweat beyond the allowed deficit:
Minimum to drink per hour = sweat rate - (0.02 x body mass in kg) / hours
Take the default example in the calculator: 165 lb before, 163 lb after, 16 fl oz drunk, 1 hour. In metric that is 74.843 kg before, 73.936 kg after and 0.473 L drunk, so sweat loss = 74.843 - (73.936 - 0.473) = 1.380 L, a rate of 1.38 L per hour. The allowed deficit is 2% of 74.843 kg = 1.497 L, about 1.50 L. Here is the minimum drinking rate that keeps this runner at or under 2% as the run gets longer, assuming the same sweat rate throughout:
| Run duration | Sweat lost (L) | Allowed deficit (L) | Minimum to drink, per hour |
|---|---|---|---|
| 1 h | 1.38 | 1.50 | none needed |
| 1.5 h | 2.07 | 1.50 | 0.38 L (13 fl oz) |
| 2 h | 2.76 | 1.50 | 0.63 L (21 fl oz) |
| 3 h | 4.14 | 1.50 | 0.88 L (30 fl oz) |
| 4 h | 5.52 | 1.50 | 1.01 L (34 fl oz) |
| 5 h | 6.90 | 1.50 | 1.08 L (37 fl oz) |
On a 1-hour run, 1.38 L of sweat is less than the 1.50 L allowance, so this runner stays under 2% without drinking, which matches the calculator's 1.21% result for that example. At 2 hours the floor is 1.380 - 1.497 / 2 = 0.63 L per hour. At 4 hours it is 1.380 - 1.497 / 4 = 1.01 L per hour. As the run gets longer, the allowance is spread thinner and the floor climbs toward the sweat rate itself.
With no drinking at all, this runner reaches 2% after 1.497 / 1.380 = 1.08 hours, about 65 minutes. Baker's example runner (82.5 kg, 1.31 L per hour) gets there after 1.65 / 1.31 = 1.26 hours.
The ceiling matters too. The ACSM goal is to prevent losses beyond 2%, not to replace every drop: a runner who weighs more after a run than before has drunk more than they sweated, and the calculator flags that result.
How long until 2% with no drinking
Minutes to the 2% line = 0.02 x body mass in kg / sweat rate x 60. The sweat rates in the columns are example inputs chosen to show the arithmetic, not typical or recommended values; I have not found a published norm I could confirm, so use your own measured rate.
| Body weight | At 0.5 L/h | At 1.0 L/h | At 1.5 L/h |
|---|---|---|---|
| 120 lb | 131 min | 65 min | 44 min |
| 150 lb | 163 min | 82 min | 54 min |
| 180 lb | 196 min | 98 min | 65 min |
| 210 lb | 229 min | 114 min | 76 min |
| 240 lb | 261 min | 131 min | 87 min |
Body weight and sweat rate pull in opposite directions: a heavier runner has a bigger allowance, a faster sweater uses it up sooner. Two runners with the same weight can reach the line at very different times, which is why your own test beats any table.
Small errors in the test, computed
The method is simple, but a few details move the answer a lot on a short test.
- Scale resolution. A bathroom scale that reads to 0.2 lb can be off by up to 0.2 lb on each weighing, so up to 0.4 lb (0.18 kg) on the difference. On a 1-hour test that is up to 0.18 L per hour of error in the rate; on a 2-hour test, up to 0.09 L per hour. Longer tests are more forgiving.
- Forgetting what you drank. Leaving out a 16 fl oz bottle understates sweat loss by 0.47 L. On a 1-hour test that is the whole 0.47 L per hour.
- Clothing. Wet clothes add weight after the run and hide sweat. Baker's method weighs nude or in minimal clothing for that reason.
Why one test day is not every day
A sweat rate is a measurement of one session: that day's weather, pace, clothing, duration and how used you were to the heat. Change any of those and the rate changes, so a number measured on a cool easy run will not describe a hot race. The practical answer is more tests, not a correction factor: test in conditions close to the race you are planning for, at race effort, and keep a short log of the results with the temperature. I have not used any published average sweat rates here, because I could not confirm a source for them; the only rates on this page are worked examples.
Fluid and fuel are planned together on race day. For the carbohydrate side, carb loading and race fueling in grams works through the hourly targets, and the race fueling calculator turns them into gels; a sports drink counts toward both totals. The marathon pace chart gives your expected finish time, which is the duration you need for the drinking floor above.
Limits and health
These are estimates from a field method and a population guideline, not medical advice. The drinking floor assumes the sweat rate stays the same for the whole run, which it may not. People with heart or kidney disease, or on medicines that change fluid balance, should take their drinking targets from their own doctor rather than from this arithmetic.
Sources: Baker (2017), Sweat testing methodology in the field: challenges and best practices, GSSI Sports Science Exchange 161; Sawka et al. (2007), American College of Sports Medicine position stand: Exercise and fluid replacement, Med Sci Sports Exerc. Rules checked . Found a problem? Tell us.
Questions
What is the formula for sweat rate?
Sweat loss = weight before - (weight after - fluid drunk + urine), counting 1 kg as 1 L, using nude or minimal-clothing weights. Sweat rate is that loss divided by the hours of exercise.
What is 2% of body weight for a runner?
2.4 lb at 120 lb, 3.0 lb at 150 lb, 3.6 lb at 180 lb and 4.8 lb at 240 lb. As fluid, 3.0 lb is about 1.36 L or 46 fl oz.
How much should I drink per hour while running?
Enough to keep weight loss under 2%: sweat rate minus 2% of your body mass divided by the hours you run. For a 165 lb runner sweating 1.38 L per hour that is about 0.63 L per hour over 2 hours and 1.01 L per hour over 4.
Is one sweat test enough?
No. The result belongs to that day's weather, pace and clothing. Repeat the test in conditions close to your race.