Methodology and Sources

Every calculator on this site uses the same small set of published equations. This page lists them, explains the choices we made, and cites the sources. Last updated November 11, 2026.

Medical review

Step 1: Basal metabolic rate (BMR)

Weight W in kilograms, height H in centimeters, age A in years. US inputs are converted with 1 kg = 2.2046226 lb and 1 in = 2.54 cm.

Mifflin-St Jeor (default)

Men: 10W + 6.25H − 5A + 5 · Women: 10W + 6.25H − 5A − 161

Used by default because a systematic review found it the most reliable of the common equations for healthy adults with and without obesity (Frankenfield et al., 2005).

Katch-McArdle (when body fat is entered)

BMR = 370 + 21.6 × LBM, where LBM (lean body mass, kg) = W × (1 − body fat % ÷ 100). With the formula set to Auto, entering a body-fat percentage switches to this equation.

Revised Harris-Benedict (option)

Men: 88.362 + 13.397W + 4.799H − 5.677A · Women: 447.593 + 9.247W + 3.098H − 4.330A (Roza and Shizgal, 1984). The original 1918 coefficients are shown on the Harris-Benedict page for reference only.

Step 2: Activity factor

TDEE = BMR × factor. These five multipliers are a long-standing convention used with BMR equations across nutrition and fitness calculators; they are not derived in the BMR papers. They sit on a similar scale to the physical activity level (PAL) categories in the Institute of Medicine’s Dietary Reference Intakes for energy (2005).

Step 3: Maintenance and gentle goal ranges

• Maintenance = TDEE.
• US units: lose about 1 lb/week = TDEE − 500; about 0.5 lb/week = TDEE − 250; gain about 0.5 lb/week = TDEE + 250. Metric: ±275 and −550 for about 0.25 and 0.5 kg/week. These use the familiar approximation of about 3,500 calories per pound (7,700 per kg), which is a rough rule for the first weeks and overstates change over longer periods.
• We do not show larger deficits. If any reduced target would fall below 1,200 calories a day for women or 1,500 for men, levels commonly cited as minimums without medical supervision (Harvard Health Publishing), the calculator shows a note recommending a doctor or registered dietitian instead of the number.
• The calculator accepts ages 18 to 100. The equations were developed in adults; for under-18s it shows a note instead of a result.

Rounding

Calculations use full precision. Results are rounded to the nearest whole calorie for display, which makes them easy to check against other calculators, but the estimates are not meaningful to that level: treat them as approximate.

Verification

The math lives in one module with automated tests. We checked it against the original coefficients above and against an independent online calculator (calculator.net BMR calculator, checked November 11, 2026):

Limitations

Prediction equations describe averages. Individuals can differ from them by 10% or more, especially people with very high or low muscle mass, older adults, and groups underrepresented in the original studies. The equations do not account for pregnancy, breastfeeding, illness, medications or hormonal conditions. Measured values (indirect calorimetry for resting rate, doubly labeled water for total expenditure) are more accurate than any equation.

Sources

1. Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241–247. doi:10.1093/ajcn/51.2.241
2. Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. Am J Clin Nutr. 1984;40(1):168–182. doi:10.1093/ajcn/40.1.168
3. Harris JA, Benedict FG. A biometric study of human basal metabolism. Proc Natl Acad Sci USA. 1918;4(12):370–373. doi:10.1073/pnas.4.12.370
4. McArdle WD, Katch FI, Katch VL. Exercise Physiology: Nutrition, Energy, and Human Performance. Lippincott Williams & Wilkins (source of the Katch-McArdle equation).
5. Cunningham JJ. A reanalysis of the factors influencing basal metabolic rate in normal adults. Am J Clin Nutr. 1980;33(11):2372–2374. doi:10.1093/ajcn/33.11.2372
6. Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc. 2005;105(5):775–789. doi:10.1016/j.jada.2005.02.005
7. Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. National Academies Press; 2005. doi:10.17226/10490
8. U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2020–2025, Appendix 2: Estimated calorie needs. dietaryguidelines.gov
9. Harvard Health Publishing. Calorie counting made easy. health.harvard.edu

Who runs this site

Draft: the owner's legal business name will be added here before launch.

FreeTDEECalc.com is operated by FreeTDEECalc.com. Found an error? Email hello@freetdeecalc.com and we will check and correct it.