Weight Loss Calculator - Calories Needed to Lose Weight

Use this weight loss calculator to accurately determine the calories you need to intake daily to reach a realistic goal weight and maintain it. The calculator generates tables and graphs that will show projected changes to your weight, lean mass, and fat mass over time.

The calculations rely on mathematical models developed by the National Institute of Diabetes and Digestive Kidney Diseases of the National Institute of Health. You can utilize this calculator 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


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. Also, 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.

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 - your energy expenditure must be 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, 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.

A future version of this calculator will allow you to enter your RMR and body fat percentage values manually, should that information be available. You will get more accurate results.

Related Calculators


  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