How much hydration is enough hydration during long rides? I have been doing some experiments to see if I'm hydrating enough during rides of moderate length and intensity. Basically, I weigh myself (naked) and my water bottles before and after a ride, to measure my weight loss and water consumption. Also keeping track of ride time, average speed of the ride, and the outside temp (per my phone's weather app), and any snacks I eat.
My questions is: if I'm hydrating enough, should I expect my body weight to stay approximately the same before and after a ride? Or would I expect to lose some measurable amount of weight independent of water loss (i.e. from respiration of CO2). For example, this weekend I did a 48km ride in ~2:00 hours, over which I consumed ~44oz of water (with electrolytes and sugar mixed in, no other snacks). After the ride, I was about 1 lb (0.45kg) lighter than when I started, though I wasn't feeling especially parched or exhausted. Is this difference mostly water weight? And if so, should I be drinking an extra half liter of water on similar rides? Or is it possible this weight difference was mostly from calorie burn, and my hydration level was adequate?
Obviously it isn't too much trouble to make up a small water deficit post ride, but I'm training for my first 200k brevet, and trying to establish a good baseline for staying hydrated over a substantially longer time-frame. For context, I'm a fairly overweight dude at ~225lbs. So I may need more water than other riders would on a similar ride.
When I first started doing longer distance rides, the rule-of-thumb I was told was &quot;drink a bottle every hour.&quot;
Obviously, that's a very rough figure, and it's going to depend on a lot of factors, including how much you're sweating (a combination of workload and heat+humidity), how much you weigh, and just generally how much fluid you personally need.  The first time I did a century (160km/100mi) I was loaded touring and it was very hot; that day I drank 9l of water on the road, and still needed to keep rehydrating that evening.
But to get to my point.  After many years of riding, I've found one of the best indicators that I'm getting adequate hydration is that I keep regularly needing to urinate; if I find I'm going more than 2.5h without needing to relieve myself, I'm getting dehydrated.  Again, there's a certain amount of knowing your own body here, and my numbers won't be the same as yours.  But if you're doing a long ride, you're going to need nutrition; metabolizing nutrients (especially protein) produces water and metabolic waste products you need to get rid of.  If you're not doing so regularly, it means that your body is starting to hoard water and you need to drink more.