Quick question on about post-workout weight gain

Great question

If you're weighing food on a scale, and logging/tracking meticulously, you should have a pretty clear idea whether you've eaten enough calories above your maintenance calories to gain X pounds (estimating that 3500 calories over maintenance will approximate 1 pound gained). One can gain a little water weight from a new workout, temporarily; whether one will & how much seems to be based on how big a change it is (I believe muscle repair activity is what's holding onto the water), and some variation in individual bodies.

But, water weight can occur for a lot of reasons: Time of month; higher level of carbs or calories on a particular day compared to usual (even if not over maintenance calories sometimes); extra sodium eaten.

Usually just drinking more water won't increase water weight, if you're weighing at a time of day that's allowed enough time for surplus water to be eliminated by sweating/urination. Extra water even can potentially reduce the possibility of sodium-related water weight.

Furthermore, you can have non-fat gain that isn't water, for example, if you've got a substantially greater weight of food/liquids still somewhere in the digestive process at the time you weigh in.

If you weigh yourself frequently, but tend to worry about daily ups & downs, it can be helpful to use a weight-trending app like Happy Scale, Trendweight, Libra, etc., to track a longer-term trend line. Over time, if you weigh daily at the same time & keep a record, you'll get a feel for what circumstances give you a higher scale reading absent actual fat gain, and how long it'll take to drop off.