How to calculate your sweat rate
To calculate your sweat rate, weigh yourself nude before and after a session. Take the weight you lost, add everything you drank, subtract any urine you passed, then divide by the hours you exercised. The result is liters of sweat per hour. Our sweat rate calculator does the arithmetic for you.
The formula
Baker’s 2017 review in Sports Medicine sets out the standard field method [1]:
sweat rate = (weight before − weight after + fluid drunk − urine passed) ÷ hours of exercise
It works because each gram of weight you lose during exercise is treated as one milliliter of sweat. The ACSM position stand on fluid replacement states the same assumption: sweat has a specific gravity close to water [6]. So a kilogram lost is about a liter of sweat.
Fluid you drank during the session gets added back, because it replaced sweat that the scale never saw. Urine gets subtracted, because it left your body without being sweat.
Step by step
- Empty your bladder, then weigh yourself nude: Clothes hold sweat, and a soaked shirt and shorts can hide a real share of your loss. The ACSM recommends nude weights for this reason [6]. Write the number down.
- Weigh your bottles: Weigh or measure every bottle before you start. You need to know exactly how much you drink, not roughly.
- Do a normal session: Pick a session that matches the conditions you want to plan for: race pace in the heat, a long easy ride, a two-hour practice. Time it.
- Track what goes in and out: Note the volume you drank. If you pee during the session, estimate or collect the volume. Try not to eat much, since food also adds weight.
- Towel off and weigh yourself nude again: Dry off fully first. Sweat sitting on your skin still counts as weight you have not lost.
- Run the numbers: Enter everything into the sweat rate calculator. It gives liters per hour and the share of your body weight you lost.
If you use pounds and fluid ounces, that’s fine. The calculator converts using 29.5735 mL per US fluid ounce.
A worked example in words
Say you weigh yourself before a run, drink one standard bottle on the way, don’t stop to pee, and weigh yourself again an hour later. If the scale dropped by exactly the weight of that bottle’s contents, you sweated two bottles’ worth: one that the scale shows and one that your drinking replaced. Divide by one hour and that’s your hourly rate.
Skip the fluid correction and you will badly underestimate your rate. A small weight change can hide a lot of sweat if you drank most of it back.
What throws the result off
Clothing: Wet kit adds weight to the post-session reading and makes your sweat rate look lower. Weigh nude or in the same dry clothes both times.
Scale precision: A bathroom scale that only reads in coarse steps can blur a short session. Longer sessions give bigger changes, which a basic scale handles better. Use the same scale, on the same hard floor, both times.
Long sessions: For sessions longer than two to three hours, or very hard efforts, or dry air, you also lose weight through breathing and from the fuel you burn. Baker notes this makes the simple method less exact in those cases [1]. The ACSM lists the same non-sweat losses [6]. For most training sessions the error is small.
Food: A gel or bar adds weight. Either skip food in your test session or weigh what you eat and treat it like fluid in.
One test: Sweat rate changes with pace, heat and humidity. A single number from a cool morning run won’t hold for a humid afternoon race. Test a few times across seasons and keep a log.
What your sweat rate tells you
Your sweat rate is the fluid side of your hydration plan. It also sets how much sodium you lose, since sodium loss per hour is sweat rate multiplied by the sodium in your sweat. Our guide to how much sodium per hour covers that part. Whole-body sweat usually carries 230 to 1,610 mg/L of sodium [1].
The percent body weight lost matters too. The ACSM position stand sets the goal of drinking during exercise as preventing water loss of more than 2% of body weight [6]. If your weigh-ins show you routinely go past that on long sessions, your drinking plan is worth a look.
Feed your sweat rate into the hydration calculator and it returns fluid, sodium and carbohydrate ranges per hour, plus drinks that fit.
Don’t try to drink your whole sweat rate back
A high sweat rate does not mean you should force down that much fluid every hour. Drinking more than you sweat is the main cause of exercise-associated hyponatremia, a drop in blood sodium that can be dangerous [5]. The 2015 consensus statement recommends drinking to thirst [5].
Here is where your weigh-in helps in the other direction. If you ever finish a session heavier than you started, you drank more than you sweated. That’s a clear signal to drink less next time.
FAQ
What is a normal sweat rate?
There isn’t one normal figure. Sweat rate depends on body size, fitness, pace, heat and humidity, and it varies widely between athletes in the same sport [2]. The only number that matters for your plan is yours, measured in the conditions you train and race in. Use the calculator after a typical session.
How do I calculate sweat rate in pounds and ounces?
Use the same formula. Take the weight lost, convert it to fluid ounces, add ounces drunk, subtract urine, and divide by hours. The sweat rate calculator accepts pounds and ounces directly and converts using 29.5735 mL per fluid ounce, so you don’t need to do it by hand.
How long should a sweat rate test be?
Long enough for the weight change to be clear on your scale, and similar to the sessions you want to plan for. An hour at your usual effort works for most people. For sessions over two to three hours, breathing and fuel burn add small errors that the basic method does not correct [1].
Does sweat rate change in hot weather?
Yes. Heat, humidity, pace and clothing all push sweat rate up or down, so a winter test won’t match a summer race. The ACSM recommends measuring weight before and after exercise to build a personal plan [6]. Test in each season or condition you plan for and keep a log of the results.
Should I replace all the fluid I sweat?
Not during exercise. Drinking to thirst is the recommended approach, and drinking more than you sweat is the main cause of exercise-associated hyponatremia [5]. The ACSM goal is to keep losses under 2% of body weight [6], not to hit zero. If you gain weight during a session, drink less.