It’s absolutely unreasonable to think that you should maintain your weight with zero fluctuations. Unless you measure and do the exact amount of exercise, consume the exact amount of food, and live in the same exact conditions on a day to day basis…even then, there’s no point in trying to maintain the exact same weight on a daily basis. It’s nuts.
A stable weight is in the range of +/- two to five pounds in my opinion. Somewhere in that range. Here’s why:
Daily fluctuations in weight are completely normal. It's a part of being a living, breathing human being. We aren’t tight sealed cans or water bottles that can maintain a certain mass on a day to day basis. We move around, we eat, we take in food, we expel matter, it's all continuously going on.
Factors affecting body weight:
The amount of food that you’ve eaten. This affects the amount of physical matter in you which affects your body weight.
The amount of carbs that you ate the day prior.
The amount of salt in your food can affect water retention.
Carbs and salt both cause you to retain more water. Especially if it’s higher than what you normally consume.
Stress and chronic stress can affect water retention
A heavy weight training session can affect water retention
Some medication makes you retain more water
If you haven’t gone to the bathroom yet
If you’re weighing yourself after eating or drinking
For women, depending on where you are in your cycle you’ll notice large fluctuations due to water retention due to hormonal changes. That can change on a day to day basis quite rapidly. I've seen women go up by up to seven pounds in a day or two. I've experienced big changes in my body weight on a 24-hour period just based on how hydrated I was from the day prior.
You need to take these factors into account when you're looking to the number on the scale and it’s higher than you’d like. You can't just assume that you've gained body fat. Fat and weight are completely different things.
Weight can be made up of muscle, fat, water weight, food in the gut, and even clothes (remember to weigh yourself without clothes, I actually have to remind people of this lol). These are all things that can affect body weight.
If you see the number on the scale jump, remember it’s very hard to gain a several pounds of body fat in a 24-hour period - unless you absolutely ate a house full of food. It's incredibly hard to put on that much body fat in a short time span.
So what you're experiencing, if you get a sudden jump in your weight, is just most likely water weight due to the factors listed above and it will balance itself back out, so nothing to freak out about at all.
And remember, if you’re using a scale to track your weight, use it properly.