Quick answer: sweat loss = weight before - (weight after - fluid drunk + urine); divide by hours run for the rate. Weighing 165 lb before and 163 lb after a 1-hour run with 16 fl oz drunk gives 1.38 L (47 fl oz) per hour, and a 1.21% drop in body weight.
How to do a sweat test
Go to the toilet, then weigh yourself nude or in minimal dry clothing just before the run. Run as normal, and note how much you drink (weigh or measure the bottle before and after). Afterwards, towel off and weigh yourself again in the same clothing before drinking anything else. If you pass urine during the run, estimate it, or leave the box empty. Type the numbers and the time you ran.
The method
This is the field method described by Lindsay Baker of the Gatorade Sports Science Institute (2017): whole-body sweat loss equals pre-exercise body mass minus post-exercise body mass, plus fluid drunk, minus urine passed. One kilogram of lost mass is treated as one litre of sweat. Sweat rate is that loss divided by the duration.
Worked example
Baker's own example: 82.5 kg before, 81.0 kg after, 1.1 L drunk, 0.3 L of urine, 1 hour 45 minutes. Sweat loss = 82.5 - (81.0 - 1.1 + 0.3) = 2.3 L. Rate = 2.3 / 1.75 = 1.31 L per hour. Body weight dropped by 1.5 / 82.5 = 1.82%.
The 2% line
The American College of Sports Medicine position stand on exercise and fluid replacement (Sawka and colleagues, 2007) says the goal of drinking during exercise is to prevent excessive dehydration, meaning a loss of more than 2% of body weight from water deficit. The same paper notes that individual sweat rates can be estimated from body weight before and after exercise, which is what this tool does. For a 165 lb runner, 2% is 3.3 lb. The result flags any loss over 2%.
Limits
Sweat rate changes with heat, humidity, pace and fitness, so test in conditions like your race and repeat in different weather. The aim is not to drink back every drop: drinking more than you sweat carries its own risks. If you have a heart or kidney condition or take medicine that affects fluid balance, follow your doctor's advice on drinking.
Planning a long race? Add carbohydrate on the race fueling calculator and prepare with the carb loading calculator.
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
How do I calculate my sweat rate?
Weigh yourself before and after a run, add the fluid you drank, subtract any urine, and divide by the hours you ran. 2.3 L lost over 1.75 hours is 1.31 L per hour.
How much weight loss during a run is too much?
The ACSM 2007 position stand aims to prevent a loss of more than 2% of body weight from water deficit. For a 150 lb runner that is 3 lb.
How much should I drink during a long run?
Use your sweat rate as the upper guide and aim to keep your weight loss under 2%. Drinking more than you sweat is not the goal.
Why is my sweat rate different on different days?
Heat, humidity, pace, clothing and how acclimatised you are all change it. Test in conditions like the ones you will race in.