A worked example
A 30-year-old male at 170 lb and 5'9" gets a Mifflin-St Jeor BMR of about 1,721 calories/day, versus roughly 1,792 under Harris-Benedict — a 4% gap between two widely used formulas for the exact same person.
Frequently asked questions
What exactly does BMR measure?
The energy your body would burn over 24 hours doing absolutely nothing — no movement, no digestion beyond baseline, just keeping your organs and systems running. It's the floor your total calorie burn builds on top of.
Why do the formulas disagree?
Each was developed from a different research population and era — Mifflin-St Jeor (1990) is generally considered the most accurate for most people today; Harris-Benedict (1919, revised 1984) runs slightly higher for many people. Katch-McArdle uses your actual lean mass instead of estimating it from height, which helps if you know your real body fat percentage.
Is BMR the number I should eat at?
No — BMR doesn't include any activity. See the TDEE Calculator for your total maintenance calories including movement, or the Calorie Calculator for a goal-based target.
This calculator provides estimates for general informational purposes only and is not medical advice.