If I have a caloric deficit, how am I gaining weight?

It's probably water retention. I fluctuate as much as 5 lbs throughout the day and up to 3 lbs from one day to the next. It's not true gain. There are many factors to scale weight. It's why I trust measurements and mirror over scale and only take weekly averages of my weight and not day to day fluctuations.

Though, you should be eating more than 1200 calories. It Sounds like you're not sedentary and are quite active. Fueling your body is important for muscle retention and overall health.

ETA: and it looks like you only have 13 lbs to lose and are only 18. Your body is still finishing growing and you're being overly aggressive with your deficit for such little weight left to lose. With less weight to lose, taking weight loss slower will be much more beneficial than rushing.

It's probably water retention. I fluctuate as much as 5 lbs throughout the day and up to 3 lbs from one day to the next. It's not true gain. There are many factors to scale weight. It's why I trust measurements and mirror over scale and only take weekly averages of my weight and not day to day fluctuations.

Though, you should be eating more than 1200 calories. It Sounds like you're not sedentary and are quite active. Fueling your body is important for muscle retention and overall health.

ETA: and it looks like you only have 13 lbs to lose and are only 18. Your body is still finishing growing and you're being overly aggressive with your deficit for such little weight left to lose. With less weight to lose, taking weight loss slower will be much more beneficial than rushing.

exactly this.

100% this.

I'm not gaining but I am not losing either. It has taken me just about a month to lose 8lbs. I think that is nothing expecially when you are a bigger person but I digress.

By going up and down as to food intake (not deficit) Your body does know how to interpret when it is getting food. At your age you still have physical maturing to do. The above comments seem to be spot on. Gradual is better than vascillating so much.

Earlier this week, I had a pretty big caloric deficit each day – around 1000 calories, since I was doing a lot of hiking as well as working out and was following a very restricted diet (I tried not to meet my 1200 calories, since I can do so without being hungry or tired). However, yesterday, I didn't exercise and I ate way, way past the point of satiety, and I've gained half a pound. I only ate 1100 calories and still had a deficit of 700 calories. Since my mom doesn't want me to be on a diet (she thinks I'm anorexic, but I'm not), it can be really hard for me to count calories. Do you think I'm miscalculating my caloric intake?

I've bolded the bits that leap out to me. 1200 Net calories is recommended as the level to get a sufficient nutrients the body needs. The Net bit is important as it means eating back the calories you use exercising so you will actually be eating more than 1200 calories. I'm not saying you have an ED but the way you are approaching this is worrying and I can understand your mothers concerns

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F off

I would say go up to maintenance for a bit of time, till you are actually maintaining (you might even lose a little) because this will even out your body fluctuating. Then once it evens out a bit, try to aim for only 1 lb of weightloss a week (only drop calories 500 calories below maintenance)

poop...

^this