Basal metabolic rate
We use the Mifflin-St Jeor equation by default, because comparison studies found it the most accurate of the common equations for healthy adults, predicting resting energy within 10% of measured values for most people.
Women: 10 × kg + 6.25 × cm − 5 × age − 161
You can also choose the revised Harris-Benedict equation (Roza and Shizgal, 1984) or Katch-McArdle, which uses lean body mass (370 + 21.6 × lean mass in kg) and needs a body fat percentage.
Activity factors
Total daily energy expenditure is BMR multiplied by an activity factor: 1.2 (sedentary), 1.375 (lightly active), 1.55 (moderately active), 1.725 (very active) or 1.9 (extra active). These widely used multipliers are approximations and the largest source of error, so we suggest choosing the lower level when unsure.
Goals and energy balance
We use about 7,700 kcal per kilogram of body weight change (about 3,500 kcal per pound). A goal of 0.5 kg a week therefore means roughly 550 kcal a day below maintenance. This static rule overestimates long-term loss because energy needs fall as weight drops, so we advise recalculating every 4 to 5 kg.
Safety limits
- We do not display weight-loss targets below 1,200 kcal a day for women or 1,500 kcal a day for men. These are commonly cited minimums for unsupervised dieting.
- We flag weekly goals above about 1% of body weight as hard to sustain.
- Inputs are limited to adults aged 18 to 100, because the equations were developed for adults.
Macronutrients
Protein and carbohydrate are counted at 4 kcal per gram and fat at 9 kcal per gram (Atwater factors). On the main calorie calculator, protein is set at 1.8 g/kg when losing weight, 1.4 g/kg when maintaining and 1.6 g/kg when gaining, capped at 40% of calories. Fat is 28% of calories and carbohydrate makes up the rest. Fibre is shown at 14 g per 1,000 kcal, the guideline used in the Dietary Guidelines for Americans.
Protein ranges
Protein ranges follow the RDA of 0.8 g/kg for inactive adults and the International Society of Sports Nutrition position stand, which supports 1.4–2.0 g/kg for people who exercise, with higher intakes during fat loss.
BMI
BMI is weight in kilograms divided by height in metres squared. Standard categories follow the World Health Organization. Optional Asian cut-offs (healthy 18.5–22.9, overweight 23–27.4, obesity 27.5 and above) follow the WHO expert consultation for Asian populations, published in The Lancet in 2004.
Calories burned
Energy cost of activity uses MET values from the Compendium of Physical Activities. Calories per minute = MET × 3.5 × body weight in kg ÷ 200. Net calories subtract one MET (resting energy).
Main sources
- Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition, 1990.
- Roza AM, Shizgal HM. The Harris Benedict equation reevaluated. American Journal of Clinical Nutrition, 1984.
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association, 2005.
- Hall KD, et al. Quantification of the effect of energy imbalance on bodyweight. The Lancet, 2011.
- Jäger R, et al. International Society of Sports Nutrition position stand: protein and exercise. Journal of the International Society of Sports Nutrition, 2017.
- WHO Expert Consultation. Appropriate body-mass index for Asian populations. The Lancet, 2004.
- Ainsworth BE, et al. 2011 Compendium of Physical Activities. Medicine & Science in Sports & Exercise, 2011.
- U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2020–2025.
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