Endurance · 3 min read
You can measure your own sweat rate with a set of scales and some care about the details. Here is the method, the arithmetic, and the four things that most often make a home measurement wrong.
By Chris Gkoufas, DVOTION Labs ·
Empty your bladder. Weigh yourself with as little dry clothing as possible, to the nearest 0.1 kg or better. Weigh every bottle you intend to drink from.
Train for a known length of time at an intensity you actually want the answer for. Note the temperature and, if you can, the humidity.
Afterwards, weigh the bottles again. Remove wet kit, towel yourself and your hair dry, and weigh yourself the same way as before. If you urinated during the session, that has to be collected and weighed, or the measurement is not usable.
Sweat loss in litres is the mass you lost, plus the mass of fluid you drank. A kilogram of body mass lost is treated as a litre of sweat, which is close enough for this purpose.
Worked through: you start at 74.2 kg and finish at 73.1 kg, so you have lost 1.1 kg. You drank 600 g of fluid. Total sweat loss is 1.1 + 0.6 = 1.7 litres. Over 75 minutes that is 1.7 ÷ 1.25 = 1.36 litres an hour.
That figure belongs to that intensity in those conditions. Write both down beside it, or in six months you will have a number and no idea what it describes.
Wet clothing is the largest and most common error. Sweat held in a soaked top has left your body but has not left the scales, so it does not appear as loss and your rate reads low. Towel dry and change completely before the second weighing.
A full bladder is the second. Urine produced during exercise is mass lost that is not sweat, so it inflates the figure unless it is collected and weighed. Empty beforehand and, if you can, do not go during.
Forgetting a drink is the third — any fluid taken and not weighed is subtracted from your apparent sweat loss.
The fourth is subtler. Substrate oxidation and respiratory water loss also reduce body mass, so a gravimetric figure is very slightly high as an estimate of sweat alone. Over an hour the effect is small, and over a long session it is worth knowing about.
A wearable sweat sensor collects sweat as it leaves the skin and reports the rate as the session runs. That gives you the shape of the curve rather than one average across it, which matters when intensity or heat rises through a session — the rate at the end can be well above the mean.
Sensor and scales answer slightly different questions, so a lab session that runs both gives an average for the whole body and a continuous trace to read it against. A wearable sensor of this kind has been evaluated against body-mass change for sweat loss and against flame photometry for sodium, in dry and humid heat.
Long enough to lose a mass your scales can resolve, which in practice means at least 45 minutes to an hour. A shorter effort puts the measurement error close to the size of the thing being measured.
Drink as you normally would, and weigh it. Deliberately not drinking does not improve the measurement and makes the session less representative of what you actually do.
One measurement gives one condition. Two or three across different temperatures and intensities give you a range, which is far more useful for planning than a single figure treated as fixed.
Not one that is reliable. Estimating from how wet your kit feels, or from thirst, is not a measurement — both are poor guides to volume. A wearable sensor removes the weighing but still needs the session itself, because the figure only means anything at an intensity you actually train at.
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