Weight Loss Calculator - Calories Needed to Lose Weight

This weight loss calculator accurately determines the calories you need daily to reach a realistic goal weight within your desired time frame and maintain it. It predicts the expected changes in your weight, lean mass, and fat mass over time and displays the results in charts and graphs. The calculator also calculates macro intake amounts for balanced, low fat, low carb, and high protein diets based on the calories needed to reach your goal weight.

Weight Loss Calculator


The calculations rely on mathematical models developed by the National Institute of Diabetes and Digestive Kidney Diseases of the National Institute of Health. It is intended for individuals 18 years and over who are not pregnant or breastfeeding. This calculator can also be utilized for weight gain goals.

Enter your body parameters 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 that the required food energy intake to reach your goal weight and 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 calories for men. Nutrition therapists will tell you that, below those levels, you will not meet food group targets and nutrient recommendations. You can later change your target date to increase or decrease your required calorie intake.

Consult a doctor for guidance and support if you consider going on a diet of fewer than 1,000 kcal or 4,200 kJ per day.

The Body Dynamics of 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. Energy intake must be less than energy expenditure for a weight loss.

A popular rule-of-thumb states that a cumulative energy deficit of 3,500 calories is required to lose 1 pound of body weight. This rule dates back to a calculation that assumes a loss of fatty tissue consisting of 87% fat. At that time, there was limited understanding of fundamental metabolic processes. The rule is now generally accepted as an oversimplification. Followers of diet programs based on this static 3,500 calorie rule usually fail to reach their goal weight. Unfortunately, many weight loss calculators and programs still follow this rule.

Predicting weight changes requires a dynamic assessment of how the body's energy expenditure changes over time and the proportions of that energy coming from body fat vs. lean tissue. For the same amount of calorie intake change, a person with more body fat will have a more significant portion of weight change from body fat vs. lean tissue compared to an individual with less body fat. Additionally, people with a higher initial body fat lose more weight, but reaching a weight plateau takes longer.

When your body requires more energy than you intake, it gets the extra needed energy by metabolizing stored energy in body fat and the protein and glycogen in lean tissue. The energy deficit required to effect a given weight loss equals the energy density of the lost tissues.

Lean tissue stores less energy than body fat but requires more energy than body fat for maintenance. It contributes more to the body's total energy expenditure rate because it impacts your resting metabolic rate, RMR. As your body's lean tissue decreases with weight loss, the reduction in RMR makes it more challenging to lose weight.

When you lose weight, where does the fat go?

When your body burns fat, 84 percent of it turns into carbon dioxide, which your lungs exhale. The remaining 16 percent becomes water and is expelled mainly through sweat and urine.

What causes water weight loss when dieting?

Your water weight loss is the weight of water released from glycogen. Low glucose levels in the blood from dieting can trigger the release of needed glucose contained in glycogen molecules stored mainly in the liver and muscles. Each gram of glycogen is bound to 3 or 4 grams of water. Glycogen stores depleted during the first few days of dieting release that water and excreted as urine.

Ideal Body Weight

Your body fat percentage may be a better indicator of health than bodyweight alone. Ideal bodyweight would put you in a healthy body fat percentage category. Use the Body Fat Calculator found on this site. It can calculate what percentage of your body is fat and find a target weight corresponding to a healthy percentage of body fat for you.

Your Body Shape is Important Too

Numerous studies have found that having an apple-shaped body or carrying excess belly fat is riskier than having a pear-shaped body or heavy bottom. Even normal-weight people with excess belly fat have an increased risk of health problems. You can use the Body Shape Index calculator found on this site to assess your mortality risk related to your body shape.

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 formulas are much more accurate in determining energy expenditure and energy requirements for weight management. They challenge the popular 3,500 calories per pound rule by considering the physiological changes during weight loss. It includes the changes in body fat, muscle mass, the thermic effects of feeding, glycogen, sodium, 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 your 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 the value for you through a predictive Mifflin-St Jeor formula based on your height, weight, age, and sex.

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

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