How to Work Out Your Own Sweat Rate
Sweat rate is one of the few things in this area you can measure yourself, with equipment you already own, in about the time it takes to do a normal session. It is simple arithmetic: what you weighed before, what you weighed after, and what you drank in between. The number that falls out tells you roughly how much fluid you lose per hour under the conditions you actually train or work in, which is far more useful than any general figure. Do it once on a mild day and once on a hot one and you will have a range rather than a guess. Here is the method, step by step.
What You Need: bathroom scales, a drink bottle with markings or a known capacity, a towel, and a session of a known length. Digital scales that read to 100 g are ideal, but anything consistent will do, because you are measuring a difference rather than an absolute. Use the same scales, on the same floor, both times. If you plan to eat during the session, weigh the food too, or simply do the first measurement on a session where you do not.
Weigh In Before: strip down to minimal dry clothing, towel off if you have just showered, and weigh yourself immediately before you start. Write the number down to whatever precision your scales give you. Go to the toilet first, and note that too, because anything that leaves the body between the two weigh-ins ends up in the number.
Track What You Drink: over the session, keep a running total of everything you drink, in millilitres. A bottle with markings makes this easy, and so does simply counting refills of a bottle whose capacity you know. One litre of water weighs one kilogram, so millilitres and grams are interchangeable here, which is what makes the arithmetic straightforward at the end.
Weigh Out After: as soon as the session finishes, towel yourself dry, take off wet clothing, and weigh again on the same scales. Sweat still sitting on your skin or soaked into a shirt has left your body but has not left the scales, so drying off properly matters more than it sounds. Write the second number down.
Do the Arithmetic: sweat loss equals the weight you lost plus the fluid you drank. If you started at 80.0 kg, finished at 78.8 kg and drank 500 mL, then 1.2 kg plus 0.5 kg gives 1.7 kg, or about 1.7 litres over the session. Divide by the length of the session to get an hourly rate: over two hours that is roughly 850 mL an hour. That hourly figure is the thing worth keeping.
What you do with the number is up to you. It tells you what volume you are working with over a shift or a long ride, which makes it easier to plan what to carry and how often to refill rather than working it out on the run. It also tells you how much your own figure moves: repeat the measurement on a cold morning, on a humid afternoon, in a different set of clothing or at a different workload and the rate will change, sometimes considerably. Two or three measurements across different conditions give you a range that is genuinely yours rather than an average of other people. If you use an electrolyte powder or capsule, the same arithmetic tells you how many serves your session actually spans, since each serve states its directed water volume on the pack. Measuring beats estimating, and it costs you one session and a set of scales.
These products are foods, not medicines. If you have been advised to follow a sodium-restricted diet, or you are pregnant or managing a medical condition, speak with your healthcare professional before use.