Estimated basal metabolic rate and daily energy needs.

What it is and when to use it

The Harris-Benedict equation estimates the basal metabolic rate (BMR), the energy (kcal/day) the body expends at complete rest to maintain vital functions. It uses sex, weight, height and age, based on the original 1919 formulas or, more commonly, the Roza and Shizgal (1984) revision. Multiplying the BMR by a physical activity factor yields total daily energy expenditure, the basis for planning weight-maintenance, weight-loss or weight-gain diets. It is not a score endorsed by a specific clinical society, but a predictive equation in general nutritional use.

How to interpret it

The result is an absolute value in kcal/day, not a scale with risk bands. As a rough guide, adult BMR typically falls between 1,200 and 1,900 kcal/day depending on body build and sex (often higher in men owing to greater lean mass). To estimate total needs, multiply the BMR by an activity factor: sedentary ×1.2; light activity ×1.375; moderate ×1.55; intense ×1.725; very intense ×1.9. An approximate 500 kcal/day deficit relative to that total expenditure is associated with about 0.5 kg/week of weight loss.

Limitations and when not to use it

It was derived in healthy adults of relatively normal weight and tends to overestimate BMR in people with obesity, being less accurate at the extremes of age; in those cases the Mifflin-St Jeor equation is usually preferred. It has not been validated in children, pregnant women, critically ill patients, or hypermetabolic states (burns, sepsis, major trauma), where indirect calorimetry is required. It estimates resting expenditure only: it does not measure body composition, true nutritional status or macronutrient requirements, and the activity factor introduces substantial variability.

Frequently asked questions

What is the difference between Harris-Benedict and Mifflin-St Jeor?

Both estimate BMR from sex, weight, height and age, but Mifflin-St Jeor (1990) is generally more accurate in the general population and in obesity, whereas Harris-Benedict tends to overestimate calories slightly.

How do I calculate my daily calorie needs with Harris-Benedict?

First calculate the BMR with the formula, then multiply it by your activity factor (from 1.2 sedentary to 1.9 very active) to obtain total daily energy expenditure.

Can Harris-Benedict be used for children or pregnant women?

No. The equation was validated in healthy adults and is unreliable in paediatric patients, during pregnancy, or in critically ill patients, where specific methods or indirect calorimetry are needed.