FatCalc

Weight Loss Calculator

This weight loss calculator can help you reach your weight loss goal. Enter how much weight you want to lose and by when. It will calculate the daily calories needed to reach your desired weight loss. It will show the expected changes to your body composition over time and suggest macronutrient intake amounts for different diet types. Accurate for individuals 18 years and over who are not pregnant or breastfeeding.

Weight Loss Calculator

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Enter your age, sex, height, current weight, and goal weight. Click the Physical Activity field to find your physical activity level. Physical activity levels can range from 1.4 (little activity) to 2.3 (very active). The default is 1.4. If you are unsure of your physical activity level, start with the default - the worst-case scenario. Enter a time frame for your goal weight (start date and target date).

The calculator will only produce results if it determines the required food energy intake to reach your goal weight by the target date is more than 1,000 calories per day. Otherwise, it will automatically modify your target date for a minimum daily intake of 1,200 calories for women and 1,500 for men. Nutrition therapists will tell you that, below those levels, you will not meet food group targets and nutrient recommendations. You can change your target date later to increase or decrease your required calorie intake. Consult a doctor for guidance and support if you consider consuming a diet of fewer than 1,000 kcal or 4,200 kJ per day.

Body Dynamics of Weight Loss

Your body requires energy to power its essential functions to maintain life and perform physical work. Glucose (blood sugar) is the body's principal energy source from food. When your body gets more glucose than it needs for energy, the surplus gets sent to the liver and muscles and stored as glycogen for later use. The liver and muscles have a limited storage capacity for glycogen, so once the glycogen stores are full, the excess glucose is converted to triglycerides and stored in fat cells.

To lose weight, you need to consume fewer calories than your body burns, putting your body in an energy deficit state. In that state, your body will compensate for the difference by extracting the extra energy from its energy stores to meet its daily calorie needs, resulting in weight loss from the lost fat and lean body mass tissues.

A popular rule-of-thumb for weight loss says that to lose 1 pound of body weight a week, eat 500 fewer calories daily (3,500 fewer calories weekly). This rule dates back to when there was limited understanding of fundamental metabolic processes. It takes the simplistic view that the energy content of weight loss is 3,500 calories per pound because fat tissue contains approximately 3,500 calories per pound, so burning that equivalent number of calories will result in a 1-pound weight loss.

The reality is that when you lose weight, you lose a combination of fat and lean tissue - not just fat. Lean mass consists of everything that's not fat, including muscle, bones, organs, ligaments, tendons, other tissues, and water. The rule also ignores other body dynamics of weight loss, such as the reduction in basal metabolic rate and decreased energy cost of physical activity. Your total daily energy expenditure (TDEE) changes with weight loss. Consequently, followers of diet programs based on this simple rule usually fail to reach their weight loss goal in the long run.

Lean Mass Loss from Weight Loss

Predicting weight changes requires a dynamic assessment of how the body's energy expenditure changes over time and how much of that energy is associated with body fat vs. lean tissue. A non-linear relationship exists between changes in body fat and lean mass as a function of body weight change. The following table illustrates an example of the predicted fat and lean body mass percentage losses for a 6-foot, 30-year-old sedentary male on a daily calorie-restricted diet of 1,500 calories. It shows the percentage losses of fat and lean tissues for various initial body weights and the time required for a subject to reach a target 175-pound goal weight. As shown in the table, the fat loss goes hand-in-hand with lean mass loss. The lean mass lost is primarily in the form of muscle mass.

Weight, fat and lean mass losses for a 6-foot, 30-year-old sedentary male on a 1,500 calorie diet.
Initial
Weight
Goal
Weight
Weight
Loss
Fat
Loss
Lean
Loss
Time to
Goal Weight
400 lb175 lb56%88%26%1.7 yrs
350 lb175 lb50%83%24%1.44 yrs
300 lb175 lb25%76%20%1.12 yrs
250 lb175 lb30%62%15%8.6 mos
200 lb175 lb13%29%8%2.7 mos

Losing Weight Reduces Your BMR

Basal Metabolic Rate (BMR) is the energy your body uses for vital functions such as breathing, cell growth, digestion, and the maintenance of body tissue. For most people, BMR accounts for 60-75% of the daily calories burned, making it an essential factor in weight loss. BMR is affected by many factors, especially lean body mass. When there is a reduction in lean body mass, your body's primary energy requirements also decrease.

Lower Thermic Effect of Food

Digesting, absorbing, transporting, metabolizing, and storing ingested nutrients from the food you eat requires energy and is referred to as the Thermic Effect of Food (TEF). For most people, on a balanced diet, TEF accounts about 10% of daily calories burned. Of course, eating less food reduces your TEF accordingly.

Lower Thermic Effect from Physical Activity

The thermic effect of physical activity (TEF) is the energy expenditure in response to physical activity. It typically accounts for 15 to 30% of daily calorie burn. It can rise to as much as one-half or more in active individuals, heavy laborers, and athletes. As you lose weight, your body size gets smaller, and you have less mass to move around. Thus, for the most part, less energy is required to perform the same physical activities.

About the Calculations

This calculator uses the mathematical body model developed by Kevin Dennis Hall, Ph. D., and his team of researchers at the National Institute of Diabetes and Digestive Kidney Diseases of the National Institute of Health. It has been shown that his mathematical model can accurately determine an individual's energy requirements for weight management. It challenges the popular 3,500 calories per pound rule by accounting for the body dynamics discussed earlier, including body adaptations that oppose weight change and the weight change associated with changes to glycogen and extracellular fluid levels.

Hall's model requires your body fat percentage value along with your sex, age, and basic body measurements. The calculator roughly estimates the percentage through an equation derived from Jackson et al. research on estimating percentage body fat from body mass index.

The model also requires a resting metabolic rate (RMR) or resting energy expenditure (REE) value. Both RMR and REE can be measured usually through indirect calorimetry gas analysis. Such measurements can be taken at health clubs and some medical clinics but can be expensive and inconvenient. This calculator roughly estimates your value through a predictive Mifflin-St Jeor formula based on height, weight, age, and sex.

The calculator suggests intake amounts, in grams, for macronutrients (carbs, protein, and fat) based on the required caloric intake to reach your goal weight. According to the Food and Nutrition Board of the Institutes of Medicine (IOM) dietary guidelines, an acceptable macronutrient distribution range is 45%-65% of calories for carbohydrates, 10%-35% for protein, and 20%-35% for fat. The suggested amounts put each macro's percentage within the IOM acceptable range for each diet type.

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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