How much water per hour? Hydration, heat, and sweat rate
There is no single "drink X ml per hour" number that works for every rider. Sweat rate varies enormously between individuals — roughly 0.5 to 2.5 liters per hour, sometimes more. It depends on your body size, your fitness and heat acclimatization, the air temperature and humidity, and how hard you're riding. Two riders on the same route, at the same pace, can lose very different amounts of fluid.
The classic way to find your own number is a simple weigh-in test. Weigh yourself right before a steady-effort hour and again right after, without drinking anything during that hour (or, if you do drink, subtract what you drank from the result). The weight you lost is roughly your sweat loss for that hour, since 1 kg of body weight lost is close to 1 liter of fluid. Do this on a warm day and a cool day and you'll see how much the number moves.
Temperature and humidity both push your sweat rate up, and they do it in different ways. Higher temperature simply makes your body produce more sweat to stay cool. Humidity makes that sweat less useful: cooling depends on sweat evaporating off your skin, and in humid air it evaporates more slowly. That's why a hot, humid day can feel harder to manage than a hot, dry day at the same reading on the thermometer — your body is sweating heavily but getting less cooling benefit from it.
This creates a real tradeoff with your carb mix. If you mix your bottle strong to hit a high carb target, you're limited in how much plain fluid that bottle can also carry. On a hot ride, that concentrated mix may not give you enough water to keep up with sweat losses. Many riders solve this by carrying a separate plain-water bottle alongside the carb mix on hot days, rather than expecting one bottle to cover both hydration and fuel.
You don't need to replace every gram of sweat loss in real time — running a small fluid deficit over a few hours is normal and well tolerated. But letting that deficit grow too large has costs. Significant dehydration hurts performance directly, and it also slows gastric emptying, which means the carbs you are drinking or eating get absorbed more slowly right when you need them fastest.
The natural unit for that deficit is percent of body mass, because that is the unit every study reports its findings in — and because a litre means something very different to a 55 kg runner than to a 95 kg rider. For a 75 kg cyclist, one litre short is about 1.3% of body mass. Up to roughly 2%, the evidence for any reliable performance cost is weak, and the 2% figure itself is genuinely disputed: several blinded trials find nothing at 2–3%, while others find a clear drop at the same number. Treat it as a soft warning zone, not a cliff edge.
What matters more than the number is the weather you collect it in. Sawka, Cheuvront and Kenefick (2015) found the cost of a deficit negligible until skin temperature passes about 27 °C — in their tally of the literature, no study in the cold (2–10 °C) showed impairment, while 8 of 9 above 25 °C did. The same 2% deficit costs you close to nothing on a cold day and several percent of your performance in real heat. That is why Carb Fueling grades the same shortfall differently depending on the temperature you set: at 20 °C and below the bar stays green out to 2.5% of body mass, and at 30 °C and above only to 1.2%.
The other end of the scale is rarer but more dangerous. Drinking past your sweat losses dilutes the sodium in your blood and leads to exercise-associated hyponatraemia — the one acute failure mode in this whole subject with a documented path to a hospital bed, and one that typically catches slower finishers who drink at every aid station "to be safe". So the hydration bar turns dark red for a surplus too: there is no prize for drinking more than you lose.
Carb Fueling takes the temperature you set for your route and your riding effort, and uses them to estimate your fluid need in ml per hour alongside your carb plan — so you don't have to guess or run your own weigh-in test mid-ride. The figure above the hydration bar is that balance: a minus is a shortfall, a plus means the plan has you drinking more than you sweat.
Sources: Sawka, Cheuvront, Kenefick, Sports Med 2015 (the 27°C skin-temperature threshold above which dehydration starts affecting performance).