So if I eat 500 calories a day, how much weight will I lose if my goal is to lose 65 lbs?

First of all, what is your goal? To lose 65 lbs in 6 months if you eat 500 calories per day might be problematic.  

A more reasonable approach would be to say that you want to lose 1 lb per week.  This means if you eat 500 calories/day for 6 months, if the most weight loss possible was 1 lb per week (which it isn’t), then you would have lost 52 pounds which probably isn’t enough if your goal is 65 lbs.  

Depending on your present body fat percentage and activity level, losing 1-2 pounds per week 4-6 months may not be impossible if you eat 500 calories/day.  

But if after 6 months if you haven’t lost anywhere close to 65 lbs, then it’s time to re-evaluate your approach as 1 lb per week is a very reasonable expectation if you stick to 500 calories/day for 4-6 months.

So if I have been eating 500 calories a day, how much weight will I lose if my goal is 1 pound per week?

If your goal is “1 pound per week”, depending on your present body fat percentage and activity level, this should be very manageable if you stick to 500 calories/day for 4-6 months because 52 pounds loss in 6 months is not unreasonable if it takes about 3,500 fewer calories than you eat to lose 1 lbs.  

So if you eat 500 calories/day for 6 months, if 52 pounds is your goal, then it’s very possible if your present body fat percentage and activity level are such that losing about 3,500 calories per week is reasonable.

Getting back to the question at hand if I eat 500 calories a day how much weight will I lose if my goal is 65 lbs?

It depends on what your present body fat percentage and activity level is.

If you’re fairly sedentary and not carrying a lot of excess fat (body fat % in the 20s), then it might be possible if you focus exclusively on eating only 500 calories/day for 6-9 months while gaining strength with resistance training if you are careful about staying within your daily calorie limit.  

Most calorically restricted diets are not sustainable if they go on too long, so if after 6-9 months if you haven’t lost anywhere close to 65 lbs, then it’s time to re-evaluate your approach as 1 lb per week is a very reasonable expectation if you stick to 500 calories/day for 4-6 months.

If I eat 500 calories a day how much weight will I lose if my goal is 90 lbs ?

Again, this depends upon present body fat percentage and activity level, but if we assume that you’re sedentary and not carrying a lot of excess fat (body fat % in the 20s), then if it takes about 3,500 fewer calories than you eat to lose 1 lbs if your goal is 90 lbs it would take about 300 days if you stick to 500 daily.  

90 lbs / 3,500 cal/lbs = 0.25 lbs/day or about 1 lb per week if you’re careful to stay within a 500 calorie limit every day if the most weight loss possible was 1 lb per week.