Endurance · 3 min read
Sweat rate is how much fluid you lose per hour while exercising. It varies enormously between people doing the same session in the same room, which is why a general fluid guideline is a poor substitute for your own figure.
By Chris Gkoufas ·
The reference method is gravimetric, and it is arithmetic rather than technology: body mass before, body mass after, every drink weighed. Whatever mass you have lost, plus whatever you drank, is what left you as sweat. Divided by the duration, that is a rate per hour.
It is accurate and it has one limitation. It produces a single average for the whole session and tells you nothing about what happened inside it. A wearable sensor collects sweat as it leaves the skin and reports the rate continuously, which shows the rise as intensity or heat climbs.
At DVOTION Labs both are done in the same session. The FLOWBIO S1 sensor reports continuously; body mass before and after gives an independent whole-body figure to check it against. The two agreeing is worth more than either alone, and the two disagreeing is worth knowing before a race plan is built on one of them.
Sweat rates across athletes commonly run from about 0.5 to 2.0 litres an hour, and can exceed that in heat. That is a fourfold spread, and it is not explained by how hard people are working. Two athletes at the same relative intensity in the same conditions can sit at opposite ends of it.
The practical consequence is that a fixed recommendation is wrong for most people in one direction or the other. An athlete losing 1.8 L/h told to drink 500 ml an hour finishes a long session considerably down. An athlete losing 0.6 L/h following the same instruction is drinking close to what they lose, which carries its own risk over several hours.
It does not tell you how much sodium to take, because volume and concentration are independent. Two athletes can lose exactly one litre an hour and need very different amounts of sodium, and there is no way to infer one from the other.
It also belongs to the conditions it was measured in. Sweat rate moves with exercise intensity, temperature, humidity, airflow, clothing, heat acclimatisation and hydration status. A figure recorded indoors at 20 °C is a reference point for those conditions, not a constant you carry to a hot race. If a target event is materially different, the useful answer is a second measurement in conditions closer to it rather than a coefficient applied to the first.
Multiply it by the duration you are planning for. A rate of 1.2 L/h over a three-hour ride is around 3.6 litres of fluid loss, which tells you the size of the problem, not the size of your drinks bottle.
Full replacement is not the target and is rarely practical. Guidance built around limiting body-mass loss during exercise, rather than eliminating it, is the standard position, and drinking beyond your sweat rate over a long session carries a real risk of its own. What the number gives you is a deliberate decision about how much of the gap to close, instead of a guess.
Yes. Trained and heat-acclimatised athletes tend to begin sweating earlier and distribute it better, which is a thermoregulatory advantage rather than a problem. Individual sweat rate still varies widely for reasons that have nothing to do with training status.
It moves with fitness, body composition, heat acclimatisation and season. Retesting annually, or before a target event in conditions unlike the last measurement, is a reasonable approach.
Yes, and it is worth doing. Weigh yourself with minimal dry clothing before a session, weigh your bottles, repeat both afterwards having towelled dry, and account for anything you drank. The main sources of error are wet kit, a full bladder, and forgetting to weigh a drink.
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