So we see this question now and then. I was reading through Primal Blueprint, and he addresses it pretty well.

Let's just say that you have been going strong on keto for a while, and have no glycogen in your body. You then eat a bunch of carbs. How much will you gain? How much is water weight?

So your muscles store carbs as glycogen. Per the Primal Blueprint, an individual can store up to 400 grams of glycogen, with athletes able to store up to 600 grams. Let's assume that most of us aren't athletes and use the 400 grams. For those of us in the US and unable to deal with the metric system, that's about .88 pounds. Remember - we aren't talking fat, just stored glycogen (energy) in your muscles.

For every gram of glycogen, your body will also hold 3-4 grams of water. So with 400 grams of glycogen, that's 1200-1600 grams of water. So for just water gain, that's 2.65 - 3.5 pounds of water gain. Combined, you have 3.5 - 4.4 pounds of total gain potentially from eating those carbs.

So if you go off the wagon and see a gain in this range, don't stress it. The glycogen and water weight will come off quickly. But, if you see a gain much higher than that, you've probably back tracked a bit, and regained some fat.

If you've tended to eat a higher carb level anyway, you probably already have some stored glycogen in your muscles, and therefore wouldn't add this full amount. It also can take time to digest that food, so don't expect all of this to show up the next morning - you could have a bigger surprise waiting a day later.

Exercise - burn it off!
Hope that helps!