The short answer
Online calculators can only estimate your metabolism from population averages—they cannot measure your actual metabolic rate, lean mass or individual adaptations, so for any one person they can be off by hundreds of calories a day.
Two people with the same age, height and weight can have meaningfully different resting metabolic rates. Equations such as Harris-Benedict and Mifflin-St Jeor were built because direct measurement used to be expensive.
Why a 10% error matters
If a formula is off by just 10% for someone burning 2,000 calories a day, that is 200 calories daily—enough to add up to roughly 20 pounds over a year if you plan your diet around the wrong number. A measured RMR test removes the guesswork. RMR testing at Body Fat USA
Related questions
Body Fat USA offers $85 DEXA body composition scans and $100 RMR metabolic tests in Denver (both for $175), with reports emailed in about 5 minutes. See all FAQs
