Calorie Calculator: Mifflin-St Jeor BMR and What Activity Multipliers Can and Cannot Add

Most calorie calculators on the internet multiply a resting equation by an activity factor. This page does the same arithmetic with the Mifflin-St Jeor equation, prints each multiplier separately, and states what the two sources behind the numbers measured and did not measure. The result is an estimate of total daily energy expenditure, not a target to eat to.

Worked examples

These are produced by the same function your browser just ran, computed when this page was built.

How it is calculated

TDEE = BMR x activity multiplier. BMR (men) = 10 x weight(kg) + 6.25 x height(cm) - 5 x age(y) + 5. BMR (women) = 10 x weight(kg) + 6.25 x height(cm) - 5 x age(y) - 161.

A 34-year-old man weighing 82 kg at 178 cm has BMR 10 x 82 + 6.25 x 178 - 5 x 34 + 5 = 1767.5 kcal/day. If he chooses moderately active, the multiplier 1.55 gives TDEE 2739 kcal/day. The same man at sedentary would be 2121 kcal/day, a spread of 618 kcal from the activity choice alone.

Formula source: BMR constants from Mifflin MD et al., Am J Clin Nutr 1990 Feb;51(2):241-7 (PMID 2305711, doi 10.1093/ajcn/51.2.241). Activity multipliers are the widely used Harris-Benedict revision bands described in Frankenfield DC et al., J Am Diet Assoc 1998 Apr;98(4):439-45 (PMID 9550168).

Reference data on this page

Sources: Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. "A new predictive equation for resting energy expenditure in healthy individuals", Am J Clin Nutr 1990 Feb;51(2):241-7, doi 10.1093/ajcn/51.2.241, PMID 2305711 - the abstract states the equation was derived from data from 498 healthy subjects, including females (n = 247) and males (n = 251), aged 19-78 y; that normal-weight (n = 264) and obese (n = 234) individuals were studied and REE was measured by indirect calorimetry; that multiple-regression analyses gave R2 = 0.71 and REE = 9.99 x weight + 6.25 x height - 4.92 x age + 166 x sex - 161; that the simplified equations are REE (males) = 10 x weight + 6.25 x height - 5 x age + 5 and REE (females) = 10 x weight + 6.25 x height - 5 x age - 161; that fat-free mass was the best single predictor of REE (R2 = 0.64). Frankenfield DC, Muth ER, Rowe WA. "The Harris-Benedict studies of human basal metabolism: history and limitations", J Am Diet Assoc 1998 Apr;98(4):439-45, doi 10.1016/S0002-8223(98)00100-X, PMID 9550168 - the abstract states that the Harris-Benedict equations were developed to establish normal standards for comparing people with disease states, and that the 1990 equations overestimated measured REE by 5% in the Mifflin dataset.

https://pubmed.ncbi.nlm.nih.gov/2305711/ · retrieved 2026-10-03 · citation checked

Limits of this tool

This page estimates resting energy expenditure from four inputs and then multiplies it by a broad activity band. Neither source behind those numbers reports a per-person error band for the multiplied result, and the activity bands are not measured outcomes from the 1990 study. The 498 subjects were healthy adults aged 19 to 78 y measured at rest, so the equation itself is an extrapolation outside that window, and the multipliers add a second layer of assumption. This is an arithmetic tool, not a meal plan or a weight-loss prescription. If the number matters to a health decision, a clinician who can measure actual intake and expenditure is a more reliable source than any form on this page.

What the output is and is not

The page returns a resting figure and a total. The resting figure is the Mifflin-St Jeor prediction for the four inputs you gave it. The total is that figure multiplied by the activity band you selected. The resting figure is the more grounded of the two, because it is the quantity the 1990 study actually measured by indirect calorimetry.

The total is useful as a reference point, and it is also weaker evidence than the resting number. The activity bands were not derived from the same dataset, and they are not personalised: two people who both choose "moderately active" can differ by several hundred kilocalories of true expenditure, because the band covers a range of training frequencies, commute distances, and jobs. Overestimating activity is the most common error in calorie work, and this page cannot tell you whether your selection is right.

Why the equation splits by sex

The published regression used a sex indicator coded 1 for males and 0 for females, with coefficient 166. When the authors simplified and split the equation, that 166 became the gap between +5 for men and -161 for women. It is the largest single switch on the form, larger than the effect of ten extra years of age or ten extra centimetres of height.

That constant describes an average difference between the men and women in the 1990 sample once weight, height and age are already accounted for. It is not a claim about body composition, hormone levels, or why the two groups differ. The abstract gives no mechanism, and the page does not invent one.

What the 1990 study actually measured

The abstract states 498 healthy subjects, 247 female and 251 male, aged 19 to 78 years, mean 45 +/- 14. Both normal-weight and obese individuals were included, and REE was measured by indirect calorimetry rather than estimated from survey data. Multiple regression of weight, height and age gave R2 = 0.71.

Two unflattering results appear in the abstract and are left on this page because they are the useful part. Adding relative body weight and body-weight distribution did not significantly improve predictive value. Fat-free mass was the best single predictor at R2 = 0.64, with REE = 19.7 x FFM + 413. Read those figures together: the four-input equation buys an improvement over fat-free mass alone, but the improvement is modest, and the strongest predictor the study measured is not available to a calculator that asks only for weight, height, age and sex.

Why there is no cut or bulk target on this form

Many calorie calculators add a deficit or surplus target and present it as a plan. Neither source on this page publishes intake advice, and the arithmetic to get from a predicted expenditure to a safe deficit depends on clinical judgement about the person, the time horizon, and the nutrient composition of the diet. That judgement is outside the scope of a static form.

A 500 kcal deficit is a widely used heuristic, not a law of energy balance, and applying it to a predicted number multiplies the error in the prediction by the error in the activity choice. The honest reading of a TDEE figure on this page is "this is the resting number multiplied by the band you selected", not "this is the amount you should eat".

How the two published forms of the equation differ

The 1990 abstract gives both the regression form and the simplified sex-split form. The regression form is REE = 9.99 x weight + 6.25 x height - 4.92 x age + 166 x sex - 161, with sex coded 1 for males and 0 for females. The simplified form rounds to integers and separates the constant into +5 and -161. The authors state that the simplification and separation did not affect predictive value.

This page prints both forms on tool pages that use the underlying resting equation, and the difference line is there to make the authors' claim checkable. For most adult inputs the two forms differ by less than two kilocalories, which is about one tenth of a percent of the total. That gap is visible here because the arithmetic is shown; most competing calculators hide it.

Questions

How is TDEE calculated here?

Resting energy expenditure is calculated with the Mifflin-St Jeor equation, then multiplied by the activity band you choose. The page prints BMR, the multiplier, and the resulting TDEE separately, so you can see how much of the total comes from the activity choice.

Is this the same as a calorie needs calculator?

Yes in arithmetic, but the framing is different. This page estimates expenditure, not requirement. Turning an expenditure estimate into an intake target needs a second step that depends on the person's goals, health status and monitoring, and neither source published here supplies that step.

Why is the result different from other calorie calculators?

The resting equation is widely used and should agree with other Mifflin-St Jeor implementations to within rounding. Differences in the total usually come from different activity multipliers or from adding or removing the thermic effect of food, which this page does not include because the cited sources do not publish a value for it.

How accurate is the number for me personally?

The source reports R2 = 0.71 for weight, height and age together, and does not give a per-person error band for the resting prediction. Multiplying by an activity band adds a further unknown. Predictions can be several hundred kilocalories away from a measured value, and only indirect calorimetry can locate where you actually sit.

Should I eat exactly this number to lose weight?

That decision belongs with a clinician or dietitian who can measure things this form cannot. The page prints an estimate of expenditure, not a prescription for intake, and the sources it cites do not support a deficit rule.

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