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 or weight loss calculator. If your plan is to reach a target weight within a certain time frame, the calculator can tell you the amount of food energy or Calories you would need to consume on a daily basis to reach it. Alternatively, you can specify a daily dietary energy intake that's workable for you and it will calculate how long it would take to reach your weight goal.

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 burned 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.

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 sedentary 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 levels and sodium intake.

Along with your sex, age and basic body measurements, the model requires your body fat percentage. The calculator roughly estimates it 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. The calculator roughly estimates it for you through a predictive Mifflin-St Jeor formula based on your height, weight, age and sex.


Enter your body parameters and your weight goal. If you don't know your activity level, click the Estimate button for an estimation.

Select the Energy option and enter a start date and end date if your want to calculate the average daily energy intake you would need to reach your weight goal within that time frame.

Select the Time option and enter a start date and your planned daily energy intake to calculate an estimation of when your weight goal will be reached with that dietary intake.

Click Calculate and the results will be presented in tabular and graphic formats. If your weight goal is less than your current weight, the graphics view will also include a chart series of your weight projections at 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.

Related Site:

  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