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I'm pretty comfortable with my technique to stay hydrated, but I am curious about estimating how much liquid is actually involved. I generally am just concerned with getting rid of it, but occasionally (especially when drips become a steady trickle) I wonder at how much I can make.

I guess if I weighed the easier outputs and inputs and did calorie math to account for my breath I could get something, but that sounds like a lot of (unappealing) work.

Weighing clothes twice might give me a lower bound, but working in plastic wrap (to avoid drip and evaporation loses) isn't pleasant.

Is there a casual way to estimate sweat? Preferably usable in the field with minimal prep-work.

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  • More fit people sweat better. How could you do a test without VO2 base lines?
    – stormy
    May 15, 2018 at 5:52
  • Are you accounting for evaporation? Which is BTW how our bodies cool the core. More visible sweat where there aren't fans, breezes and air moving.
    – stormy
    May 15, 2018 at 5:53
  • I get you can ask any question you like. I would kind of argue this is not an outdoor question but close enough "in the field". But I ask what will you do with this information. Your body does a pretty good job of telling you when to hydrate. If you over hydrate your body will expel as urine.
    – paparazzo
    May 15, 2018 at 18:14
  • It was just idle curiosity that came up while hiking and lasted long enough to get to a computer. fitness.se probably would have been a better place to put it.
    – user8348
    May 15, 2018 at 18:32
  • Health is another option. And can just leave it here.
    – paparazzo
    May 15, 2018 at 18:45

2 Answers 2

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You basically have it, the formula is

  Your Weight at start
- Your Weight at end
+ Weight of water consumed
- Weight of water urinated
---------------------------
= Water lost via sweat

Then you divide the water lost via sweat by the amount of time to find your sweat rate.

Water lost via sweat/time spent exercising = sweat rate.

Of course, this will depend on the person and on the weather conditions, and a scale isn't practical for the field, but at the same time knowing your sweat rate under certain conditions will help you determine how much water you need to drink.

Also see

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  • 8
    Minus water exhaled, plus the C in the CO2 exhaled (let's assume O2 is in equilibrium). And you'd need very good scales to get any decent estimate. Wikipedia says up to 2-4 litres per hour, which is measurable,but at typical hiking exertion levels it will be a lot less.
    – Chris H
    May 14, 2018 at 16:40
  • @ChrisH I don't think any measurements are quite that precise
    – Charlie Brumbaugh
    May 14, 2018 at 16:47
  • You might well be right, but I'm not going to work through the literature and chemistry on my phone with a patchy signal. Maybe I will later
    – Chris H
    May 14, 2018 at 17:02
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    ncbi.nlm.nih.gov/m/pubmed/22714078 you'll exhale more than you might think.
    – whatsisname
    May 15, 2018 at 0:44
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    Also bmj.com/content/349/bmj.g7257 states that in fat burning most of the lost weight is in the form of exhaled CO₂. If burning glucose (glycogen) the C:H ratios are similar but there's more O; also glycogen is stored in a hydrated form; CO₂ mass-loss will still be significant.
    – Chris H
    May 15, 2018 at 8:07
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I doubt it's possible to get a sensible answer with realistic equipment, as removing evaporation will make you sweat more. So weighing your clothes while tightly sealed would give an overestimate.

The best way to get a rough idea might be to weigh clothes on a fairly still, humid, overcast day, using those same clothes to dry yourself. A gym session would be a decent substitute. This might be a good time to wear cotton so it absorbs the sweat. Weighing yourself would be interesting (combining sweating with respiration and other losses) but most scales aren't accurate enough - try changing your stance while on them.

When it comes to respiration (and urine production) don't forget that different energy sources use water differently, and the use of these sources changes over the course of prolonged exercise as well as with the starting state of your glycogen stores. Glycogen in particular stores a fair bit of water with it.

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  • Btw I was intrigued enough to have a quick look at the amount of salt etc. dissolved in sweat, for the effect on your error margin. It's a few percent at most so a smaller error than the other sources, I reckon. And the value for urine is similar. NASA have looked into this sort of thing, as have other researchers, and I doubt you'll do better than taking their values and deciding whether the high or the low end is better for you. But an interesting question and I like the idea of experiments.
    – Chris H
    May 14, 2018 at 16:38

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