In a nutshell, weight loss is simple mathematics. The calories you consume minus the calories you burn per day equals your weight loss or weight gain. In real world, other factors come into play as well, but for an average person the above mentioned equation works in most cases.
This equation can have 3 outcomes. Either the number you get is negative, positive or it can be zero.
Calorie deficit or surplus?
If the number is negative (lower than zero) it represents a caloric deficit. If the number is positive it represents a caloric surplus. In case equation equals zero (is nor positive, nor negative, but exactly "0") this represent a maintenance rate. In other words, if the result you are getting is positive it means that you are gaining weight, if result is negative it means that you are losing weight. If the result is zero you are nor gaining nor losing weight - you are maintaining your current weight.
How do you calculate your caloric expenditure?
So how do you calculate how much calories your body consumes every day? First of all you must realize that your body needs energy for every operation it does. And this does not include physical activity only (like running, jogging, playing tennis), but also vital functions of the body.
Your body needs energy for performing functions like thinking, breathing, keeping proper body temperature, digesting. Did you know that an average human brain consumes about 20W of energy? That is more than an average energy-saving light bulb.
Basal metabolic rate
Body's essential need for energy is also known as BMR (short for Basal Metabolic Rate). BMR is the amount of calories you need per day if you were to stay inactive all day - for example lying in bed all day, not moving a muscle. You can calculate your own BMR rate using this calculator to see how much calories your body needs just to maintain vital functions.
In addition to BMR (basal metabolic rate) your body also needs energy to perform regular functions like walking around, crouching, standing up. For an average inactive person this represents about 10 to 15 percent of daily calorie needs. So if your BMR is for example 1500 calories your total daily calorie consumption is about 10 to 15 percent more, roughly around 1650 to 1725 calories. For a more accurate calculation, use this tool and simply enter your body weight and the duration of your daily activities to get a more reliable number.
Like already mentioned in the beginning of the article you should aim for a negative number when calculating daily calorie consumption versus daily calorie expenditure.
A simple maths
Now that you know how to calculate your daily calorie needs it is time to calculate how many calories you consume per day. It is simple to do this. Every food that is bought in the store has the nutritional data printed on the packaging. The informational chart usually tells you how much protein, carbohydrates and fats is contained in the food. It also tells you how much calories on average is contained in 100 grams of food (that is 3,5 ounce). To break this number down keep in mind that 1 gram (about 0,04 ounce) of protein contains 4 calories, 1 gram of carbohydrates contain 4 calories and 1 gram of fat contains 9 calories.
Sum all food calories in all your meals and there is your daily calorie consumption. Now you have both numbers and you can finally do the subtraction. Is you number positive or negative?
Remember, if you want to lose weight the number should be negative. You can achieve that by reducing calorie intake or by increasing caloric needs (by increasing physical activity). You cannot affect BMR significantly.