Fat Calc  

Body Weight Planner / Weight Loss Goal Calculator

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Body Weight PlannerWeight Goal Calculator

Use this app as a body weight planner and weight loss goal calculator to calculate the amount of food energy (Calories) you need to consume on a daily basis to reach a weight goal within your desired period of time and maintain it.

A change in body weight results from the difference between your food energy intake and the amount of energy expended by your body. Energy is expended in maintaining your body functions and in performing physical activities. For successful weight loss to occur, there must be an energy imbalance such that energy expenditure is greater than your energy intake.

There is a widespread myth that by reducing your energy intake by about 500 Calories (2.1 MJ) per day, you will slowly lose about 1 lb (0.45 kg) of weight a week. The National Institutes of Health and the American Dietetic Association have both erroneously stated this. This rule is sometimes referred to as the 3500 Calories per pound rule and is used in many weight loss formulas. It is overly simplistic and it's accuracy can be called into question especially in the long term. Unfortunately, it continues to be used in weight loss programs.

The rule's inaccuracy stems from the fact that it does not take into account the physiological changes that occur during weight loss, your sex, age and initial weight. The amounts of body fat and lean tissue both change with an energy imbalance. Pound for pound, the energy content of body fat is about 5 times that of lean tissue. When you reduce your energy intake, muscle mass is lost along with fat mass. This reduces your resting metabolic rate, causing you to burn less energy.

As an example, if you were to take in 500 less calories a day (3500 Calories a week), you would lose 1 lb a week according to the rule. So if you were planning on losing say 25 lbs, it would take you (25 x 1) or 25 weeks to do so. This is clearly wrong. According to the Hall's model, it would actually take over 37 weeks if you were a 34 year old, 6 foot male initially weighing 250 lbs. Actual times would vary somewhat depending on sex, age, height and initial weight.

About the Calculations

Calculations incorporated into this app are based on the model developed by Kevin Hall, Ph. D., and a team of researchers at the National Institute of Health. It is a much more accurate model in determining energy expenditure and energy requirements for the purpose of weight management. It challenges the 3500 Calories per pound rule by taking into consideration the physiological changes that take place during weight loss. This includes changes in body fat, muscle mass, the thermic effects of feeding, glycogen and sodium intake.

Along with your sex, age and basic body measurements, the calculator requires your body fat percentage. If your body fat percentage is unknown, the calculator can roughly estimate it for you through a formula derived from research by Jackson et al on estimating percentage body fat from body mass index.

Your resting metabolic rate (RMR) or resting energy expenditure (REE) value is also required. The terms RMR and REE are generally used interchangeably and is a measurement of the energy burned by your body to keep it functioning while at rest. Both RMR and REE are measured by means of direct or indirect calorimetry gas analysis. Such measurements can be taken at health clubs and some medical clinics but can be expensive and inconvenient. If your RMR or REE value is unknown, the calculator can roughly estimate it for you through a predictive Mifflin-St Jeor formula based on your height, weight, age and sex.


Enter your body parameters, activity level, body fat, RMR/REE, your weight goal and the period of time (start date to end date) in which you want to achieve your weight goal. If you don't know your activity level, body fat percentage or RMR/REE, click the Estimate buttons for an estimation.

Click Calculate to calculate the amount of food energy you need to consume on a daily basis to achieve your goal and maintain it.

The results are presented in tabular and graphic formats. The graphic view includes a display of the weight projections at a starvation level (0 Calories) and at 1,000 Calories per day. Consult a doctor for guidance and support if you are considering a diet of less than 1,000 Calories per day. Food group targets and nutrient recommendations will not be met below that level.

  1. Quantification of the effect of energy imbalance on bodyweight.
    Hall KD, Sacks G, Chandramohan D, Chow CC, Wang YC, Gortmaker SL, Swinburn BA.
    Lancet (2011 Aug 27) 27;378(9793):826-37.

  2. The effect of sex, age and race on estimating percentage body fat from body mass index.
    Jackson et al
    International Journal of Obesity (2002) 26, 789–796

  3. A new predictive equation for resting energy expenditure in healthy individuals.
    M D Mifflin, S T St Jeor, L A Hill, B J Scott, S A Daugherty, Y O Koh
    The American Journal of Clinical Nutrition, Volume 51, Issue 2, February 1990, Pages 241–247