FatCalc

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 a 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 try to stick to it for that period of time.

Calorie Deficit Calculator

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Enter your body parameters and a 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 what's required to power your body. When in a calorie deficit, the body breaks down fat and muscle from which it extracts the missing calories it requires, resulting in weight loss.

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 1 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 3,500 calories per pound rule. It is based on the fact that body fat contains approximately 3,500 calories of energy per pound.

Unfortunately, the rule does not take into account important contributing factors such as the physiological changes that occur during weight loss. The amounts of body fat and muscle tissue both change with an energy imbalance. When you reduce your energy intake, muscle mass is lost along with fat mass. Muscle mass is a significant contributor to BMR. As muscle mass decreases, so does the number of calories you burn daily. This is why exercising regularly and eating a balanced diet that includes protein when trying to lose weight is essential.

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 body fat and lean body mass, levels of glycogen, sodium, extracellular fluids, and changes to the thermic effects of feeding.

To illustrate, if you are a 37 year old, 6 foot sedentary male weighing 265 pounds, your body burns around 2,600 calories a day and you need to intake that same amount of food energy to maintain your 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 less calories a week. According to the 3,500 calories per pound 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 instead applied Hall’s mathematical model, it would show that it actually 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 3,500 calories per pound rule would suggest. Rather, the loss levels off in a nonlinear fashion because of body dynamics. The actual weight loss curve is much more closely approximated by the Hall’s model.

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. Additionally, 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 then 1,500 calories a day. Nutrition therapist 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. This occurs even if you are overweight and deliberately trying to lose weight. The slower the metabolic rate, the slower the calorie burn.

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References

  1. Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. Lancet. 2011;378(9793):826-837. doi:10.1016/S0140-6736(11)60812-X
  2. Jackson, A., Stanforth, P., Gagnon, J. et al. The effect of sex, age and race on estimating percentage body fat from body mass index: The Heritage Family Study. Int J Obes 26, 789–796 (2002).
  3. M D Mifflin, S T St Jeor, L A Hill, B J Scott, S A Daugherty, Y O Koh, A new predictive equation for resting energy expenditure in healthy individuals, The American Journal of Clinical Nutrition, Volume 51, Issue 2, February 1990, Pages 241–247.
  4. Manore MM. Exercise and the Institute of Medicine recommendations for nutrition. Curr Sports Med Rep. 2005;4(4):193-198. doi:10.1097/01.csmr.0000306206.72186.00
  5. Hall KD. Body fat and fat-free mass inter-relationships: Forbes's theory revisited. Br J Nutr. 2007;97(6):1059-1063. doi:10.1017/S0007114507691946