I have couple of questions.
Why do people say to eat 2000 Calories.
What happens when you eat 300-500 (just an example)calories above your BMI.
Can someone calculate BMI for 19, 120 pounds, 5'2. and lowest level of activity.

I have been doing well dieting but just recently i had alot of guest stayed over at my house for about 2 weeks and i have been wayyyy off my diet. Cuz my brain kept telling myself "its vacation, just go back to your oldself after these 2 weeks but for now eat whatever you want." But someone told me if you were dieting and then you start to eat alot then you will gain double the weight. Is that true?

1 - don't know cos everyone's different and have different calorie requirements
2 - assuming you meant above your base maintenance intake of cal's you will put on wt...muscle if your doing wts hard enough and fat if your not
3 - about 2400 cals i would say but only a quick guess...a good ryule of thum for mainenace is 12 x bw...bulking 16 x bw and fat loss 10 x bw but again everyone is different (metabolism, stress etc)
what yiou should do is weight yourself tomorrow night before bed then keep a food diary for 7 full days noting everything you eat and drink and their portions...wt yourself again next week at the same time...add up all cals and divide by 7 (days)...try to eat as normal as you would and don't think about eating more healthy as normal...if you put on wt than you ate above your maintenance level, if lost wt then below and if stayed the same there's your base level

you are confusing BMI with BMR. BMI is body mass index, and is a useless number IMO.
BMR is basal metabolic rate...the amount of calories your body needs to maintain its current weight at its current activity level.

http://www.bmi-calculator.net/bmr-ca...dict-equation/

Keep in mind that BMR is still just an estimate. sometimes its close enough...sometimes you need to track your calories for a week on fitday.com and that'll give you a real life maintenance intake to go off.

A couple of things can happen when you suddenly start eating more than you have been:
1. if you were in "starvation mode", then your body says OMG excess food, I don't know when the starvation thing will happen again so I better store as much fat as possible today in case we are starving again tomorrow I will have some reserves to call on.
2. your body says: OK now I have the fuel I need and I can pack on muscle for a few days, so release HGH and make good use of the extra calories to add some muscle
The theory is, if you are strenght training on a regular basis you tend to do the 2nd one more. I have seen some diets that even recommend 1 day a week or 2 days every 2 weeks of over eating to both induce this effect and keep your sanity. One diet plan I read ever recommended alternating 2 week periods of under-eating and over-eating as a way to induce the second effect and add muscle mass.

Smart folks here....I like the answers given.
My 2 cents:
2000 calorie mumbo jumbo comes from the basic old dogmas of the healthy american diet in the 80-90s. In reality, it is very much individual.
Definitely agree you are confusing BMI with BMR. BMR is your basic metabolism on an average including the average work you do a day. Since a calorie is not just simply a calorie, it all depends on what the sources of 300 - 500 calories are. Once again this is individual. 300-500 high protein/veg and fruit rich calories would probably increase the BMR a bit and muscle size.
Forget BMI....it's a joke of a scale. BMR is easy to calculate. Take your weight divide it by 2.2 to get your weight in kg......Next calculate your RMR....simply take the kg number and multiply by 22. This number is your RMR....what it takes to keep your homeostasis body alive and functioning properly. BMR is a ballpark average. My way of handling this.... figure out how active in life you are. Very active (1.6)/ Moderate active (1.5)/ Slightly active (most people 1.4).......take the number that corresponds with your activity level and mutiply it with your RMR number.....this will be your average BMR

The other thing is too haphazard to say it is true. What are you eating (macro nutrient types)? How much of each thing are you eating? How active are you being? How much sleep do you get? You see there are too many questions that need to be answered before guessing that eating alot will make you double weight.

Goodluck to you

Everyone made good responses. Calorie intake is someting that is extrememly individual. No one can just look at you and tell you how many calories you should eat. RMR is definately a gross estimate too. It might be right on for you, or it could be a few hundred calories off.

Don't worry about how you ate on your vacation. Just worry about the now and eating healthy foods and properly fueling your body.

Sarah