This is a video I made going over some of the basics of hydration, water and electrolyte loss during exercise, and strategies to deal with that loss in order to try to perform your best.
To summarize if you want to read text:
How much do we sweat?
Usually 1-2L or 2-4 lbs per hour. But anywhere from .5L to 3.5L per hour (or about 1-7 lbs.) for very elite athletes training intensely in hot weather.
How much sodium do we lose?
Usually, .5-2g per L or 250-1000mg per pound of fluid lost. On average, about 1g per L, but this differs from person to person.
How much can we drink?
Up to 2 bottles, about 1.5 L or about 45-50 oz per hour. In cool weather, .5 L or about 16 oz per hour may be plenty, but in warm weather, aim for 1.5-2 bottles per hour, or about 1-1.5 L per hour. Be sure to include 300-600 mg of sodium per bottle. Almost all drink mixes include a reasonable quantity of sodium.
Don’t overconsume just water without electrolytes in your drinks or food. Too much water and too little salt leads to sub-optimal hydration and hyponatremia in extreme cases.
Remember to keep taking water with electrolytes for hours after any long, hard training or racing session that leaves you dehydrated.