Mifflin-St Jeor vs Harris-Benedict: Formula Comparison
Mifflin-St Jeor and Harris-Benedict both estimate resting energy needs. The bigger mistake is not choosing the wrong formula. It is treating any formula as a perfect measurement.
What both formulas are trying to estimate
Both equations estimate BMR or resting energy needs from body size, age, and sex. They are shortcuts for a value that would be more accurately measured in a lab.
After BMR is estimated, a TDEE calculator still has to apply activity. That second step can introduce as much practical error as the formula choice.
Why TDEETools uses Mifflin-St Jeor by default
Mifflin-St Jeor is widely used in modern calorie calculators because it tends to be a strong general starting point for many adults. It uses the same basic inputs users can enter easily: weight, height, age, and sex.
That does not make it perfect. It does make it a sensible default when body fat percentage is unknown.
Where Harris-Benedict fits
Harris-Benedict has a long history and revised versions are still discussed in nutrition contexts. Depending on the person, it may produce a slightly higher or lower estimate than Mifflin-St Jeor.
The difference matters less once you remember that food logging, step counts, training consistency, and activity selection can move the real-world result by more than the equation difference.
How to choose in practice
Use one formula consistently, then compare the result with your weight trend. If you maintain weight around the calculated calories, the estimate is close enough. If not, adjust the target rather than endlessly switching equations.
If you know body fat percentage with reasonable confidence, a lean-mass-based method can be useful. If that number is a guess, Mifflin-St Jeor may be cleaner.
Example: If one formula says 2,300 calories and another says 2,420, start with one number and track two to four weeks. Your weekly average weight will tell you more than arguing over 120 estimated calories.
Sources and method notes
TDEETools articles explain calculator outputs in plain English. They are educational and are not medical advice.