Why Your Sweat Rate Is Worth Knowing
Every runner sweats at a different rate. Two people on the same run in the same weather can lose wildly different amounts of fluid, which is why generic advice like "drink 500 ml an hour" fits almost nobody. The only way to know your number is to measure it, and the measurement is simple: weigh in, run, weigh out, do a little arithmetic. Once you know your sweat rate, you can plan how much to carry, how often to drink, and how much sodium to replace, instead of guessing and hoping.
This calculator does the arithmetic for you. Give it your before and after weights, what you drank, and how long you ran, and it returns your sweat rate, your body-weight loss percentage, and a per-hour fluid and sodium plan.
The Formula
The field method the ACSM and NATA both recommend is straightforward. Any weight you lost during the run was fluid, plus you have to add back whatever you drank and subtract anything you peed out:
Sweat loss = (weight before − weight after) + fluid drunk − urine
A kilogram of body-weight change is treated as one liter of sweat, since sweat is mostly water. Divide the sweat loss by how long you ran, and you have your sweat rate in liters per hour. Say you weigh 70.0 kg before and 69.0 kg after, drank half a liter, and ran for an hour. Sweat loss is (70.0 − 69.0) + 0.5, which is 1.5 liters, so your sweat rate is 1.5 L/hr. Your body-weight loss for the session is the raw 1 kg drop, about 1.4% of 70 kg, comfortably inside the safe window.
The Two Numbers That Matter: 2% and Sodium
Two guardrails come out of the sports-medicine research:
- Keep body-weight loss under about 2%. That's the point where endurance performance and heat tolerance start to slip for most runners. For a 70 kg runner it's about 1.4 kg of sweat. The calculator flags where you landed so you can see whether your drinking kept pace.
- Don't drink past your sweat rate. Overdrinking dilutes blood sodium and can cause hyponatremia, which is genuinely dangerous. If you're gaining weight on a run, you're drinking too much. The ACSM guidance is to limit losses to around 2%, not to prevent every gram of loss.
Sodium is the other half of the story. You lose roughly 500 to 1500 mg of sodium per liter of sweat, and salty sweaters live at the top of that range. On runs longer than about two hours, or in the heat, replacing some sodium with an electrolyte drink or tabs helps you actually hold onto the fluid you take in, and cuts cramping for those prone to it.
How to Use It
- Weigh in and out. Nude and towel-dried is most accurate, before and straight after the same run. Same scale both times.
- Track what you drink. Note the fluid you take in during the run; a bottle you can measure makes this easy.
- Repeat in different conditions. Your sweat rate in a cool 10K is not your sweat rate in a hot marathon. Test on a hot day to plan for a hot race.
- Build the plan. Use your rate and the 2% guardrail to set how much to carry and how often to sip.
Heat is the biggest driver of sweat rate, and it slows your pace at the same time. Pair this with the heat-adjusted pace calculator to plan both fluid and effort for a warm race. Fluid and fuel go hand in hand on long efforts, so build your carb plan alongside it with the race fueling calculator. To set the goal pace all of this supports, use the VDOT calculator, and to carry your splits on race day, the marathon pace band generator. For the bigger picture of how these long runs fit together, see the running training plan guide.
A Worked Example
Imagine a runner who weighs 68.0 kg, heads out for a 90-minute long run in mild heat, drinks 750 ml along the way, and comes back at 66.5 kg. Sweat loss is (68.0 − 66.5) + 0.75, which is 2.25 liters, over 1.5 hours, so the sweat rate is 1.5 L/hr. The raw weight drop of 1.5 kg is about 2.2% of body weight, just past the 2% line, so this runner drifted slightly too dry and should carry or drink a bit more next time. With average-salty sweat at 1000 mg sodium per liter, that 2.25 liters of sweat carried about 2250 mg of sodium out, which is well worth topping up on an effort this long. The calculator lays all of this out from the same three weigh-in numbers.