How Do I Figure Out My Calorie Deficit?

When your calorie intake is lower than what your body needs to complete these three functions, it leads to a calorie deficit. You can manually calculate your calorie deficit by first knowing your TDEE.

A calorie is a unit used to measure the energy that you get from anything you eat. Your body “burns” these calories or, in other words, breaks down the food to give you energy for all the activities you do. If you’ve ever tried to lose weight, you're probably familiar with the term “calorie deficit.” This means that your body burns more calories in a day than what you eat.

The calories that you burn every day are the sum of three processes that happen within your body. The three processes together give you an idea about the total daily energy expenditure (TDEE) of your body or the total calories that your body burns every day to complete all of its activities.

When your calorie intake is lower than what your body needs to complete these three functions, it leads to a calorie deficit. An extended period of calorie deficit leads to weight loss. On the other hand, if your calorie intake is more than what your body needs, it’s known as a calorie surplus, which will lead to weight gain. The three main processes are:

Resting energy expenditure (REE) 

This refers to the calories that your body burns when you’re not doing any activity and includes essential life functions like breathing and blood circulation.

Activity energy expenditure (AEE)

This refers to the calories you spend while doing physical activities like exercise, playing sports, and activities that are not linked to any form of exercise, like household chores.

Thermic effect of food 

Your body also uses some energy to process the food that you eat, which includes digestive, absorptive, and metabolizing activities.

You should know how many calories your body needs in a day to carry out its basic functions. This is known as the body’s maintenance calorie count. It’s easier to calculate your calorie deficit once you know how many calories your body needs in a day.

A good way to determine your body's maintenance calorie needs is by evaluating your basal metabolic rate (BMR), which is the number of calories your body uses in a day to perform life-sustaining functions. A widely used formula to calculate BMR is the Harris-Benedict formula, which has two separate versions for men and women.

For men:

66 + (6.23 × weight in pounds) + (12.7 × height in inches) − (6.8 × age in years)

For women:

655 + (4.35 × weight in pounds) + (4.7 × height in inches) − (4.7 × age in years)

For example, for a 30-year-old female who stands at 5’7” (67 inches) and weighs 123.5 pounds, the BMR would be around 1,366 calories. This is the energy that the person would need every day to sustain their normal physiological functions.

The Mifflin-St Jeor formula, proposed in the 1990s, is a widely accepted alternative method to calculate your BMR. This method also suggests separate formulas for men and women.

For men:

(10 × weight in kilograms) + (6.25 × height in centimeters) − (5 × age in years) + 5

For women: 

(10 × weight in kilograms) + (6.25 × height in centimeters) − (5 × age in years) − 161

For this formula, if we take the example of a 38-year-old man who weighs 165.3 pounds (75 kilograms) and stands at 5'11" (180.3 centimeters), the BMR would be around 1,692 calories.

Your daily calorie needs are determined by several factors, like your gender, age, height, weight, the amount of physical activity you do, your BMR, and whether you have any underlying health conditions. Another important factor is your body composition, which includes your muscle mass and fat percentage.

Once the food you eat enters your body, it’s converted to calories. There are two ways your body uses the calories. They either get burned by the body’s metabolism during various activities or get stored in the form of body fat. If your intake of calories is more than what you burn, you’ll put on weight no matter what kind of diet you follow.

There are two ways to achieve a calorie deficit. The first is to make sure that your calorie intake is lower than the minimum number of calories that your body needs for maintenance. The second way is to increase your physical activity which leads to a higher number of calories being burned than what you eat every day.

You can manually calculate your calorie deficit by first knowing your TDEE. For this, multiply your BMR by a factor based on your physical activity level (PAL):

• Sedentary (little or no exercise and have a desk job): BMR × 1.2
• Lightly active (some exercise or sports 1 to 3 days a week): BMR × 1.375
• Moderately active (moderate exercise or sports 6 to 7 days a week): BMR × 1.55 
• Very active (heavy exercise every day or twice a day): BMR × 1.725
• Extra active (heavy exercise at least twice a day or training for a marathon, etc.): BMR × 1.9

To make sure that you’re losing weight safely, you can cut your calorie intake by 500 to 1,000 calories every day from your TDEE. For instance, if you need 2,325 calories every day to maintain your present weight, you can reduce your intake to 1,325 to 1,825 calories. If your usual daily routine doesn't involve much movement, try to include some physical activities and lower your daily calorie intake by around 500 calories.

But you should keep in mind that your calorie intake shouldn't go below 1,200 per day if you're a female or 1,500 per day if you're a male unless your doctor says otherwise.

Usually, if you reduce anywhere between 500 and 1,000 calories a day, you’ll lose around 1 pound (500 grams) every week while maintaining the same levels of physical activity. If you start doing other physical activities, you could lose more weight.