The Mifflin-St Jeor equation
The calculator estimates resting energy expenditure, the calories your body uses lying still, with the equation published by Mifflin, St Jeor and colleagues in the American Journal of Clinical Nutrition in 1990. They measured 498 healthy men and women aged 19 to 78, both normal weight and obese, with indirect calorimetry, and found that weight, height and age predicted resting energy best: 10 × weight in kg + 6.25 × height in cm − 5 × age in years, plus 5 for men or minus 161 for women. In the same study the older Harris-Benedict equations from 1919 overestimated measured resting energy by about 5%.
Activity level: from resting calories to a day's calories
To turn resting energy into a full day, the calculator multiplies it by a physical activity level (PAL), the method used by the joint FAO/WHO/UNU expert consultation on human energy requirements. Its report defines PAL as energy used over 24 hours divided by basal metabolic rate, and groups lifestyles into three bands: sedentary or light activity, 1.40 to 1.69; active or moderately active, 1.70 to 1.99; and vigorous, 2.00 to 2.40, noting that higher values are hard to keep up for long. The three choices here use the report's own worked examples: 1.53 for a day spent mostly sitting, 1.76 for a day on your feet with some exercise, and 2.25 for hard manual work. The report uses basal metabolic rate where this tool uses resting energy; the two are close but not identical.
Losing or gaining weight
Weight changes when the energy you take in differs from the energy you use. The NICE guideline on overweight and obesity, updated in 2025, says diets for adults should keep intake below expenditure, and that low-energy diets of 800 to 1,200 kcal a day should only be used with long-term support from a specialist service, for no more than 12 weeks; diets under 800 kcal are only for people with a clinical need to lose weight quickly. The earlier 2014 NICE guideline recommended a deficit of 600 kcal a day with expert support, so the calculator starts there; the 2025 version gives no fixed number, and you can set anything up to 1,000. There is no official figure for gaining weight, so the surplus is your choice, ideally with a dietitian.
How accurate the estimate is
Prediction equations describe groups well and individuals less well. In the 1990 study, weight, height, age and sex together explained about 71% of the differences in resting energy between people, so two people with the same numbers can differ by a few hundred calories a day. Body composition matters too: the study found fat-free mass was the single best predictor. Treat the result as a starting point. If your weight keeps drifting over several weeks while you eat close to the maintenance figure, your real need is different and you can adjust.
Who should not use this calculator
The equation was built from healthy adults, so the calculator only accepts ages from 19 and should not be used for children or teenagers, whose growth needs extra energy. Energy needs in pregnancy and breastfeeding are worked out differently, as the FAO/WHO/UNU report explains, so it is not for those either. If you have an eating disorder, diabetes, kidney or heart disease, or take medicine that affects weight, ask your doctor or a registered dietitian. For other views of your weight, try the BMI and ideal weight calculators, and the water intake calculator for fluids.
- Mifflin MD, St Jeor ST et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr 1990;51(2):241-7
- FAO/WHO/UNU: Human energy requirements (2004 report of the 2001 consultation, PDF)
- NICE NG246: Overweight and obesity management, physical activity and diet (2025)
- NICE CG189 (2014): Obesity, dietary approaches (NCBI Bookshelf archive)
- BMI calculator
- Ideal weight calculator
- Water intake calculator