Why Calorie Calculators Are Wrong for You
You entered your height, weight, age and sex. The calculator returned a number. You subtracted 500 from it and called that your fat loss target.
That number has a known error rate, it has been measured, and the size of the error is large enough to change how your next twelve weeks go.
How far off are they, actually
The best available predictive equation is Mifflin St Jeor. A systematic review comparing the four most commonly used clinical equations found it predicted resting metabolic rate within 10% of the measured value in more people, obese and non-obese, than any other equation, and had the narrowest error range (Frankenfield, Roth-Yousey and Compher, Journal of the American Dietetic Association).
A later validation study measured 337 community living adults by indirect calorimetry and tested seven equations against the result (Frankenfield, Clinical Nutrition).
Equation | Within 10% of measured | Direction of bias |
Mifflin St Jeor | 82% | Unbiased, 95% CI -26 to +8 kcal/day |
Livingston | 79% | Underestimates, 95% CI -63 to -25 kcal/day |
Harris Benedict | Not reported individually | Overestimates |
Muller | Not reported individually | Overestimates |
Vander Weg | Not reported individually | Overestimates |
WHO | Not reported individually | Overestimates |
Oxford variation of WHO | Not reported individually | Overestimates |
Two findings deserve emphasis.
Mifflin St Jeor was the only unbiased equation. Every other one tested tended to overestimate resting metabolic rate. If your calculator uses Harris Benedict, which many still do, the systematic direction of its error is to tell you that you burn more than you do.
Accuracy fell in obese participants for every equation. Mifflin St Jeor was accurate in 87% of non-obese participants and 75% of obese participants. One in four people carrying excess body fat received an estimate that was out by more than 10%.
That is the awkward finding. The population most likely to be using a calorie calculator to plan a deficit is the population the calculator serves worst.
Why an equation cannot see you
The inputs are weight, height, age and sex. The dominant physiological determinant of resting metabolic rate is fat free mass.
Two people can weigh 80 kg, be the same height, the same age and the same sex, and have a 15 kg difference in lean tissue between them. The equation gives them identical answers. Their actual resting rates are not identical, because muscle, organs and bone are metabolically active in a way that stored fat is not.
The equation also cannot see:
Your dieting history, and any residual suppression of resting rate from a previous weight loss phase.
Your thyroid function.
Your current energy availability, which can suppress resting rate independently of body composition.
Genuine individual variation in resting rate that persists even after accounting for body composition, age and sex.
None of this makes the equations useless. They are a sensible starting point when you have nothing. They are a poor foundation for a twelve week plan you intend to hold yourself to.
What a 10% miss costs a fat loss phase
Time to do the arithmetic, because the abstract percentage does not land until you convert it into weeks.
Take someone whose true measured RMR is 1,500 kcal per day. Their calculator, using an equation that overestimates, tells them 1,650. A 10% error, which sits well inside the observed range.
They apply a moderate activity multiplier of 1.4 to the calculator figure and get an estimated TDEE of 2,310. Their actual TDEE, from the true RMR, is 2,100. The estimate is 210 kcal per day too high.
They then set a 500 kcal deficit off the wrong number and eat 1,810 per day.
Intended | Actual | |
Assumed TDEE | 2,310 kcal | 2,100 kcal |
Intake | 1,810 kcal | 1,810 kcal |
Daily deficit | 500 kcal | 290 kcal |
Weekly deficit | 3,500 kcal | 2,030 kcal |
Expected weekly fat loss | 0.45 kg | 0.26 kg |
Over 12 weeks | 5.5 kg | 3.2 kg |
Fat loss figures use the conventional approximation of roughly 7,700 kcal per kilogram of fat tissue, which is a simplification, and real world results vary. The point is the ratio rather than the precise kilograms.
They planned for 5.5 kg and got 3.2 kg. They were compliant the entire time. Nothing went wrong with their adherence, their training or their willpower. The starting number was wrong.
Now consider what usually happens next. They conclude their metabolism is broken, or that they must be tracking badly, and they cut another 300 kcal. That takes them to an unnecessarily aggressive deficit for the result they are getting, which increases the risk of losing lean mass and of suppressing resting rate further.
The error compounds into a worse decision.
The other direction is a problem too
If the equation underestimates your resting rate, you set a deficit that is deeper than you intended. That sounds efficient. It is not.
An unnecessarily large deficit increases lean tissue loss, reduces training quality, and raises the risk of the resting rate suppression that makes maintaining the loss harder later. You would have got the same fat loss from a smaller deficit with less collateral damage.
Being wrong in either direction costs you something.
What to do instead
Measure your RMR. Indirect calorimetry, 15 minutes, 4 hour fast. This removes the largest removable error.
Measure your body composition. A DEXA scan gives you fat mass and lean mass separately, which is what you are actually trying to change.
Calibrate your activity multiplier by outcome. Set intake at a conservative estimate for two to three weeks and track weekly average weight. Stable weight means you have found your real TDEE.
Set the deficit from the verified number, not the calculated one.
Retest at the end of the phase, so you know whether your resting rate has changed and whether the weight you lost was fat or lean.
Common questions
Which calculator is least bad? Any that uses Mifflin St Jeor. Avoid ones based on Harris Benedict, since it tends to overestimate.
If I know my body fat percentage, is the estimate better? Equations that use fat free mass directly tend to perform better than those using total body weight, because they are using the actual determinant. It is an improvement, not a measurement.
Does this mean calorie counting does not work? No. Energy balance works. The issue is the accuracy of the number you are balancing against.
How often would I need to retest? Before and after a significant fat loss phase. An RMR retest is $99.
The short version
The best predictive equation is within 10% of your measured resting rate about 82% of the time, and about 75% of the time if you carry excess body fat. Most other equations systematically overestimate. A 10% error at the RMR stage, carried through an activity multiplier and into a deficit, can cut your actual rate of fat loss by roughly 40% without you doing anything wrong.
An RMR test at Precision Body Lab is $149, or $279 with a DEXA scan as a Metabolic Baseline. Book online or call 1300 910 163.
Related reading
Sources
Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association, 2005. https://pubmed.ncbi.nlm.nih.gov/15883556/
Frankenfield DC. Bias and accuracy of resting metabolic rate equations in non-obese and obese adults. Clinical Nutrition, 2013. https://pubmed.ncbi.nlm.nih.gov/23631843/
Compher C, Frankenfield D, Keim N, Roth-Yousey L. Best practice methods to apply to measurement of resting metabolic rate in adults: a systematic review. Journal of the American Dietetic Association, 2006. https://pubmed.ncbi.nlm.nih.gov/16720129/
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