A quick primer on Basal Metabolic Rate(BSM) and Total Daily Energy Expenditure(TDEE)

BMR (or basal metabolic rate) is the amount of calories your body burns just to stay alive. If you were in a coma - you would need this many calories just to sustain life. This includes organ function, hair growth, nail growth, etc.

So can u eat below your BMR???, not for too long if you aren't giving your body enough calories to just sustain life (and if you are likely doing more than laying in a coma)) like walking, talking, eating, digesting food, etc ...; Then your body needs more than your BMR ! )
Some people think that eating a 500 calorie deficit from their BMR is the way to lose weight. I'm sure for a short time, yes you would see a weight loss. But, long term, not really ..This will not be sustainable long term, at some point your weight loss will slow way down & stall.

TDEE (or total daily energy expenditure) is the amount of calories your body burns in a day).
In order to figure out your TDEE, you take your BMR and you multiply it by your daily activity level.
Each level has a different number that you use. Since this number takes into account not only your basic caloric needs but also your activity level this is the number from which you should deduct your 500 calorie deficit.

So the different from your BMI, BMR (Basal Metabolic Rate) is how it measures the number of calories you expend per day while your body is at rest. Vs how your TDEE tells you how many calories you burn per day, taking into account your activity levels.

Women:

BMR = 655 + (4.35 x weight in pounds) + (4.7 x height in inches) – (4.7 x age in years)

Men:

BMR = 66 + (6.23 x weight in pounds) + (12.7 x height in inches – (6.8 x age in years)

Now that you have your BMR, u can multiply it by your activity calculator to find your TDEE.
Figure out which of the activity levels below describes you best...

So:

Sedentary (little or no exercise) = 1.2

Lightly Active (light exercise/sports 1-3 days/week) = 1.375

Moderately Active (moderate exercise/sports 3-5 days/week) = 1.55

Very Active (hard exercise/sports 6-7 days/week) = 1.725

Extra Active (hard exercise/sports & physical job or 2x daily training) = 1.9

Example: A female age 27 5’5” 140lbs, lightly active

BMR = 655 + (4.35 x 140) + (4.7 x 65) – (4.7 x 27) = 1,442

TDEE = 1442 x 1.375 = 1,984

So in conclusion, your TDEE tells you how many calories you burn in a day. In general, you should try to match the number of calories you consume with your TDEE in order to give your body the energy it needs to operate properly.

In order to lose weight, you need to expend more energy than you consume. In other words, your TDEE needs to be higher than the number of calories you eat.... And for bulking you need to consume more calories than u expend... Pretty simple concept !

The safest approach is to cut 500-1,000 calories per day (1-2lbs per week) by manipulating your diet, cardio, and strength training.
And Remembering that 1lb of fat is equal to 3,500 calories. If you want to gain some weight, you should consume more than your TDEE.