Sweat Rate Calculator

Weigh yourself nude right before a run, then again right after, and keep track of exactly how much you drank while you were out. Your sweat rate is the body weight you lost plus the fluid you took in, divided by the time you ran. The simplest way to think about it: every pound you drop is about 16 ounces (470 mL) of sweat, so add your fluid back in and divide by the hours you were out. A 1.5 pound loss over a 1 hour run while drinking 12 ounces means you lost roughly 24 ounces of sweat plus 12 you put back, so about 36 ounces per hour. And do the test again in race-like heat, because that is your real number.

There is no one number that fits everybody, because sweat rate swings hard from runner to runner, from under 16 ounces per hour for a small runner in cool weather to over 60 ounces for a big runner in the heat. A decent starting band for a mid-size athlete is roughly 14 to 27 ounces (about 0.4 to 0.8 liters) per hour when it is cool, going up to 50 percent more in real heat. Honestly, the only good answer is to measure your own sweat rate with a before and after weigh-in, then drink to roughly match it and listen to your thirst. This calculator takes the loss you measured and turns it into that personal target.

How much sodium is in your sweat is all over the map from person to person, from about 200 to over 2,000 milligrams per liter, and most runners land somewhere around 500 to 1,000 mg/L. A salty sweater finishes a run caked in white residue with their eyes stinging, and they sit at the high end. A light sweater barely leaves a mark. The spread is so big that a calculator can only hand you a planning range, and a common one is 300 to 700 mg of sodium per liter of fluid you drink, leaning high when it is hot. If you want your exact number, a lab sweat test is the only way to get it.

Both work, it just depends on the day. On shorter, cooler efforts, drinking to thirst is safe and it keeps you from overdrinking. On long races in the heat, thirst lags behind what you are actually losing, so a lot of runners pair thirst with a loose schedule built off their measured sweat rate, sipping steadily and checking in instead of chugging at aid stations. The thing to avoid on either end is forcing fluid past thirst hour after hour, because that is how hyponatremia happens. Treat your sweat rate as the ceiling, not a quota you have to hit.

A lot. As the temperature climbs, and humidity especially, your body sweats more to dump heat, and in humid air that sweat barely evaporates, so you keep pouring it out and get almost no cooling back. Sweat rate can easily double from a cool morning to a hot, humid afternoon. That is the whole reason one sweat test is not enough, you have to measure in conditions close to your race. This calculator lets you drop in a forecast temperature so the fluid and sodium targets climb with the heat, instead of leaving you planning off a cool-weather number that will not hold up.

Hyponatremia is dangerously low blood sodium, and it usually comes from drinking way more fluid than you are sweating out over a long event, which waters down the sodium in your blood. Early signs are nausea, bloating, headache, and confusion, and it can get serious. The way you avoid it is simple: do not overdrink. Use your measured sweat rate as a guide instead of forcing fluid, take sodium with what you drink (not plain water for hours on end), and listen to your thirst. And if you finish a long race heavier than you started, that is a red flag you drank too much. When in doubt, talk to a medical professional.