Two Formulas, Different Eras
Both formulas calculate BMR from the same inputs (weight, height, age, sex), but they were developed 50 years apart and have meaningfully different accuracy profiles.
Harris-Benedict (1919, revised 1984)
The original BMR formula, developed in 1919 and updated in 1984:
Men: BMR = 88.362 + (13.397 × kg) + (4.799 × cm) − (5.677 × age)
Women: BMR = 447.593 + (9.247 × kg) + (3.098 × cm) − (4.330 × age)
Harris-Benedict is still widely used, especially in clinical settings and older nutrition literature.
Mifflin-St Jeor (1990)
Developed in 1990 using a more diverse population:
Men: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5
Women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161
Which Is More Accurate?
A 2005 meta-analysis in the Journal of the American Dietetic Association (Frankenfield et al.) compared five BMR equations against indirect calorimetry measurements. The Mifflin-St Jeor equation predicted measured BMR within 10% for 82% of subjects — higher accuracy than Harris-Benedict.
For non-obese adults, Mifflin-St Jeor is the recommended formula in most current clinical practice guidelines.
Harris-Benedict was developed on a smaller, less representative sample and tends to overestimate BMR by 5–15% in modern populations with lower activity levels than early 20th-century study participants.
The Bottom Line
Calorie Calc uses Mifflin-St Jeor because it's more accurate for most modern adults. If you've used a calculator that seems to give you a higher BMR than expected, it was likely using the older Harris-Benedict formula.
Both are estimates — use results as a starting point and adjust based on real-world weight trends.