Where the error comes from
- The BMR equation. Equations predict the average for people with your measurements. Two people with identical height, weight, age and sex can have different resting rates because of differences in body composition, organ size, genetics and health.
- The activity factor. One multiplier has to cover digestion, workouts and all everyday movement. Everyday movement alone varies widely between people, and most of us aren’t good at judging our own activity.
- Inputs. Body-fat estimates from home scales or photos can be several points off, which matters for Katch-McArdle.
What “within 10%” looks like
Take a 36-year-old man, 6′0″ and 210 lb, lightly active. The calculator estimates about 2,641 calories a day. If his true resting rate were 10% above or below the equation, with the same activity factor his real TDEE would be roughly 2,377 to 2,905. That range is wider than many people expect, and it is before any error in the activity level.
What the research says
A 2005 systematic review for the American Dietetic Association (Frankenfield, Roth-Yousey and Compher) compared common prediction equations against measured resting metabolic rate in healthy adults with and without obesity. It concluded that Mifflin-St Jeor was the most reliable, estimating within 10% of measured values more often than the other equations, while also noting errors in individuals and gaps in the data for some groups, such as older adults and some ethnic groups.
That is why this site uses Mifflin-St Jeor by default, shows the other equations side by side, and suggests checking the estimate with your own data. Full references are on the methodology page.
Using an imperfect estimate well
An estimate that might be 10% off is still useful: it puts you in the right neighborhood. From there, a few weeks of consistent eating and weekly average weights will tell you whether to nudge it up or down. The steps are in how to find your maintenance calories.
Frequently asked questions
What is the most accurate TDEE formula?
For healthy adults, Mifflin-St Jeor has the best track record among the common equations: a 2005 systematic review found it predicted resting metabolic rate within 10% of measured values more often than the alternatives. If you have a reliable body-fat measurement and an unusual build, Katch-McArdle may suit you better.
Why do different TDEE calculators give different results?
They may use different equations, different activity multipliers or different descriptions of the activity levels, and they round differently. A difference of 100 to 200 calories between calculators is normal and well within the uncertainty of any estimate.
Is a lab test more accurate?
Yes. Indirect calorimetry measures resting energy use directly, and doubly labeled water measures total daily energy use over one to two weeks. Both are more accurate than any equation but cost money and are not widely available.
How can I make my estimate more accurate?
Choose an honest activity level, add a body-fat % only if it is from a reliable method, and then test the estimate against your weight trend for a few weeks. That last step corrects for most of the error.