Your baseline calorie needs are often referred to as your BMR or basal metabolic rate (also called your resting energy expenditure, REE). This is how many calories your body needs to maintain your organs in a state of rest (1).
Knowing your BMR is a good starting place in determining what calorie level you should strive for in your average calorie consumption. Using average calories is more efficient than only tracking daily intake, as this allows you to include fasting windows and calculate your consistent intake over multiple weeks. Your average calorie consumption is the amount of calories you consume on average over a given period of time.
Your BMR depends on your height, weight, age, body composition, and overall health, and does not account for any movement or daily activity.
For the average individual, it takes roughly 1,320 calories to support your liver, brain, heart, and kidneys each day - this amount falls below many of the popular weight loss diet plans that recommend 1,200 calorie diets (2,3).
Majority of these calorie needs come from your heart and kidneys, requiring about 440 calories per day each. And your brain accounts for almost 20% of your daily energy requirements, utilizing anywhere from 300 to 500 calories a day just to think.
It is possible to follow a very low diet below your BMR needs, but it is not recommended long term or without the support of a physician regularly monitoring your health and nutrition status.