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Your TDEE can read lower than the calculator because the calculator gives an average, not a reading of your body. In a systematic review of adults with obesity, the best equation landed within 10 percent of measured resting burn about 75 percent of the time. Resting burn only, not total daily burn.
A lower number is not a verdict on your body
The calculator did not lie, and your body did not fail. The calculator answered the question "what do people with your stats burn, on average?" Your own log answers a different one.
Errors run in both directions. For a given person, a calculator can be too high or too low. A number worked out from your log can also read low when the log misses food. This post cannot tell you which applies to you, and anyone who claims to know from a number alone is guessing.
What a calculator actually estimates
A calculator makes two steps. First it estimates your resting burn from a formula that uses age, sex, height and weight. Then it multiplies that by an activity guess you pick from a short list.
Our view: the second step is the loosest. Nobody measured your steps when you chose "lightly active", and the formula behind the first step cannot see how you live. No equation is exact, and a burn is a range.
How big the miss is
The review pooled studies of equations that predict resting burn in healthy adults with BMI 25 and above. It covered 21 studies of resting burn, with searches run to March 2014. The best equations, in adults with BMI 30 and above, put about 75 percent of predictions within 10 percent of measured resting burn. About a quarter fell outside it.
Three limits matter. The best-looking equations had been tested on small groups, between 81 and 182 people in the BMI 30-plus subgroups. Figures were lower in some other groups. And results for people outside these groups may differ.
For total burn, the number a calculator really hands you, the same review found four studies. It found that no equation predicted total burn accurately or precisely, and those four studies did not report how often a prediction landed within 10 percent. So this review offers no error rate for the full number. We would not trust a page that quotes one without a source.
Why your real burn can sit off the number
Some reasons are ordinary physiology. Some are about the record you keep.
- Body size. A model of adult metabolism by Hall and colleagues in The Lancet includes burn falling as weight falls. It is a mathematical model checked against published data, not a measurement of you, but it explains why the number from a calculator you used at your starting weight drifts as you lose.
- Body composition. The equations in the review use simple inputs such as age, sex, height and weight. Two people with the same inputs can differ in how much of their weight is lean tissue, and the formula cannot see that.
- Activity you did not count. Many people find the daily movement that is not exercise is the part they guess worst, in either direction.
- After weight loss. The National Academies' energy chapter reports that after weight loss, resting burn falls more than the smaller body alone predicts: about 220 fewer calories a day, with results that varied widely between studies. It draws that figure from systematic reviews. This is a sized, normal effect, not damage, and your own gap could be smaller or larger.
- A log that misses food. A 1992 New England Journal of Medicine study looked at 10 people, 9 women and 1 man. Researchers chose them from 224 adults with obesity referred to a weight clinic, because the 10 said they were not losing weight on low intakes. Over 14 days, researchers measured their actual intake from energy expenditure and body changes. The 10 reported about half of what they ate, and their measured burn was close to what had been predicted.
That last study is small and self-selected. It does not describe the typical person who logs food. It does show that a log can miss a lot, and that a low estimate from a log sometimes says more about the log than about the body.
How to read your own number
Use your weigh-ins and a rough log across several weeks. Then hold the result as a range, not a point.
Here is an illustrative example, with invented numbers. Your calculator says 2,400. Over four weeks your weight trend is flat and your log averages 2,000. Simple arithmetic puts your burn near 2,000, a gap of 400. Now suppose the log misses about 10 percent of what you ate. Real intake was closer to 2,200, so the gap is about 200. Your burn is somewhere between those two, and the weigh-in trend adds its own noise.
That is the idea behind our position: a calculator estimates your burn from a formula, and your weigh-ins and food log are the better check, because they come from your own life. This is our reasoning, not a finding from a study. It depends on the log, and a log can miss food.
It has a cost. A range from your own data takes weeks, and it needs you to weigh in and log. If you will not do either, a calculator number held loosely is a fine place to start, and you lose little.
What a very low number means
A very low result is a prompt to check the inputs, not a prompt to eat less. Look at whether the log is complete, whether the weigh-in trend covers whole weeks, and whether your first estimate used an activity level that fits.
Whatever the estimate says, a calorie target should sit no lower than your resting burn. If a result looks extreme, ask a doctor or registered dietitian. People under 18, who are pregnant or breastfeeding, who manage a medical condition, or who have a history of disordered eating should set calorie targets with a doctor or registered dietitian.
Questions people ask
Does a lower TDEE than the calculator mean my metabolism is broken?
No. A calculator gives an average, and individual burn can sit above or below it. Your own data can also read low when a log misses food. Neither result says something is wrong with your body.
Can the calculator be too low instead of too high?
Yes. The review counted predictions that landed under or over 10 percent of measured resting burn, so errors run in both directions. A calculator can be too high for one person and too low for another.
How many weeks of data do I need to trust my own number?
There is no single cutoff. Several weeks of weigh-ins and a rough log give a wider range than most people want, and more weeks narrow it. Hold the number loosely until the range stops moving.
What should I do if my result looks extremely low?
Do not eat to it. A target never goes below your resting burn, whatever an estimate says. If the result looks extreme, or you are under 18, pregnant, breastfeeding or managing a medical condition, ask a doctor or registered dietitian.
Keep your burn as a range
BurnCurve is the tool for keeping your burn as a range: it shows your burn as a range that narrows, from Guess, to Learning, to Measured. It is built for people who are tired of restarting from a calculator. BurnCurve is coming soon to the App Store.