Calorie Deficit Calculator for Weight Loss

Use this calorie deficit calculator to discover how much weight is realistic for you to lose and the calories needed to achieve that weight loss. Enter your body details and goal weight. The calculator will generate tables and graphs showing daily calorie intakes and estimated times to reach your goal weight. Calorie intakes are shown in descending units of 50 calories. For each unit decrease, you can see how much sooner it would take to reach your goal weight. You can then choose a calorie intake level that you think is doable and stick to it for that time.

Enter your body parameters and goal weight. Click the Physical Activity field to find your physical activity level. Typical physical activity level numbers range from 1.4 (sedentary) to 2.3 (very active). The default is 1.4.

What is a calorie deficit?

A calorie deficit occurs when you consume fewer calories than the amount required to fuel your body's essential functions and daily activities. When in a calorie deficit, your body breaks down body fat and muscle to get the additional energy it needs, resulting in weight loss.

For instance, if yesterday you burned 2,000 calories but only took in 1,500 calories for the day, that's 500 fewer calories than your body required, putting it in a 500-calorie deficit. Your body would have broken down enough of its energy stores to release the 500 calories of extra energy needed for the day. The weight loss associated with releasing 500 additional calories would have depended on several factors, including the percentage of energy released from fat vs. muscle.

Your body continuously burns energy to perform life-sustaining functions such as breathing, keeping your heart beating, supporting the nervous system, circulation, and body temperature regulation. It's referred to as the Basal Metabolic Rate (BMR). Most of the energy used by your body goes towards your BMR and can account for 60 to 75 percent of the daily caloric burn for the average adult.

The remaining caloric burn comes from physical activity, like going to the gym or simply moving around throughout the day. Physical activity can account for 20 to 30 percent of the daily caloric burn. The last 5 to 10 percent of the daily caloric burn comes from digesting food. Together, these three factors - BMR, physical activity, and digesting food - make up the total number of calories you burn in a day, known as your Total Daily Energy Expenditure (TDEE).

Eating fewer calories than your TDEE, a calorie deficit, is necessary for weight loss. Weight gain occurs when you eat more calories than your TDEE, a calorie surplus, and the extra calories are stored as fat.

Do you lose a pound a week with a 500 calorie deficit?

It is a myth that by eating 500 fewer calories a day, you will slowly lose a pound of weight a week. This amounts to 3,500 fewer calories a week (7 days times 500 calories) and is sometimes referred to as the 3500-kcal rule. It is based on the assumption that body fat contains approximately 3,500 calories of energy per pound.

According to an article published by the International Journal of Obesity, the researchers explain that the 3500-kcal rule "grossly overestimates" actual weight loss and leads people to unrealistic expectations. Despite this evidence, the rule continues to appear on many health-related websites.

The 3500-kcal rule does not consider important factors such as the physiological changes that occur during weight loss. Eating 3,500 fewer calories does not mean you're exclusively burning 3,500 calories worth of body fat. Muscle mass is also lost, depending on your initial body composition. People with higher initial body fat burn more fat from the energy imbalance versus muscle tissue than those with lower amounts of body fat. Muscle mass is a significant contributor to BMR. As muscle mass decreases, so does the number of calories you burn daily.

The potential loss of muscle mass is why exercising regularly and eating a balanced diet that includes sufficient protein when losing weight is essential. When you engage in muscle-strengthening activities, your body will increase the loss of fat over muscle.

About the Calculations

Formulas incorporated into this online calculator are based on a mathematical model developed by Kevin Dennis Hall, Ph. D., and a team of researchers at the National Institute of Health. The model is much more accurate in predicting weight loss because it factors in the body dynamics and physiological changes that occur when the body is in a calorie deficit state. These factors include changes in the amounts of body fat and lean body mass, glycogen, sodium, extracellular fluid levels, and changes in the thermic effects of feeding.

To illustrate, if you're a 37-year-old, 6-foot sedentary male weighing 265 pounds, your body would burn around 2,600 calories daily, and you would need to intake that same amount of food energy to maintain your current weight. Say you then go on a 1,600 calorie a day diet, a 1000 calorie deficit, to try and lose 85 pounds for a healthy BMI. You would be eating 7,000 fewer calories a week. According to the 3500-kcal rule, you would lose (7000 / 3500) or 2 pounds a week and expect to lose 85 pounds after (85 / 2) or about 43 weeks. If we applied Hall's mathematical model instead, it would show that it takes almost 70 weeks for that same weight loss.

Body weight loss through caloric restriction does not continue downwards indefinitely in a linear fashion as the 3500-kcal rule would suggest. Rather, the loss levels off nonlinearly because of body dynamics. Hall's model much more closely approximates the actual weight loss curve.

This calculator also calculates your Total Daily Energy Expenditure (TDEE) and Resting Metabolic Rate (RMR) at your starting and goal weights. RMR is the amount of energy your body burns while at rest. RMR is factored into TDEE. The calculator also suggests intake amounts for macronutrients (carbs, protein, and fat) based on the required caloric intake to reach your goal weight. Macros are set within an acceptable range according to IOM dietary guidelines.

How low of a calorie deficit should I go?

As a general rule, women should not eat less than 1,200 calories a day and men not less than 1,500 calories a day. Nutrition therapists will tell you that food group targets and nutrient recommendations will not be met below those levels.

Eating too few calories will make weight loss slower and more difficult by slowing your metabolism. Your body will sense that food is in short supply and will slow down your metabolic rate to try and protect it from starvation. It occurs even if you are overweight and deliberately trying to lose weight - the slower the metabolic rate, the slower the calorie burn.