FatCalc

Weight Loss Calculator

This weight loss calculator can accurately determine how many calories you need to consume daily to reach your goal weight within a realistic timeframe and maintain it.

Calculations are based on a mathematical model developed by the National Institute of Diabetes and Digestive Kidney Diseases of the National Institute of Health. This calculator can be utilized for both weight loss and weight gain goals. It is intended for individuals 18 years and over who are not pregnant or breastfeeding.

Weight Loss Calculator

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

Enter a time frame for your goal weight (start date and target date), and the calculator will calculate how many calories to intake daily to reach it. The results of the calculations will show your projected weight loss or gain in a tabular and graphical format. It will also project your body composition of lean body mass and fat mass.

The calculator will only produce results if it determines that the required food energy intake needed to reach your goal weight and target date is greater 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 calories for men. Nutrition therapist will tell you that food group targets and nutrient recommendations will not be met below those levels. You can later change your target date to increase or decrease your required calorie intake.

Consult a doctor for guidance and support if you are considering a diet of less than 1,000 kcal or 4,200 kJ per day.

What is the science behind weight loss?

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 your energy expenditure is greater than your energy intake.

When you lose weight, where does the fat go?

When your body burns fat, 84 percent of it turns into carbon dioxide and is exhaled by your lungs. The remaining 16 percent becomes water and is expelled mostly by sweat and urine.

What causes water weight loss when dieting?

Your water weight loss is the weight of water released from glycogen. Low levels of glucose in the blood from dieting can trigger the release of needed glucose contained in glycogen molecules stored in your body. Each gram of glycogen is bound to 3 or 4 grams of water. As the glycogen stores deplete during the first few days of dieting, that water is released and excreted in the urine.

About the Calculations

This calculator uses the mathematical model developed by Kevin Dennis Hall, Ph. D., of the National Institute of Health, and is based on the NIH Body Weight Planner. The calculations are much more accurate in determining energy expenditure and energy requirements for the purpose of weight management. They challenge the popular 3,500 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, sodium and extracellular fluid levels.

Along with your sex, age and basic body measurements, the formulas requires your body fat percentage value. The calculator roughly estimates it through an equation 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 usually measured by means of 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 the value for you through a predictive Mifflin-St Jeor formula based on your height, weight, age and sex.

A future version of this calculator will allow you to optionally enter your RMR and body fat values manually should you have that information available.

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References

  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

Last Update: September 21, 2021 17:57 UTC