Name the equation version before comparing results
Two BMR calculators can accept the same height, weight and age yet show different resting-energy predictions because they use different equations. Mifflin-St Jeor and Harris-Benedict are families of published prediction methods, not direct measurements of one person's metabolism. In particular, the original Harris-Benedict equations and their later revision have different coefficients. A comparison that names only 'Harris-Benedict' is incomplete.
This guide compares NumUtility's simplified Mifflin-St Jeor implementation with the revised Harris-Benedict coefficients associated with the 1984 reassessment. It does not compare the original 1919 coefficients or a body-composition-based model. Both outputs below use kcal/day and the same hypothetical adult inputs. Keeping that identity visible makes differences reproducible instead of treating every alternative number as a software error.
Write the Mifflin-St Jeor equations
With W in kilograms, H in centimeters and A in years, the simplified male equation is 10W + 6.25H − 5A + 5. The simplified female equation is 10W + 6.25H − 5A − 161. These are the equations implemented by NumUtility. The original study predicted resting energy expenditure from 498 participants aged 19–78, using indirect calorimetry as the measured comparison.
The paper also reports an unsimplified regression expression. Mixing its age or weight coefficients with the simplified constants creates a hybrid formula that is neither published version. Use one complete expression consistently. Historical binary equation terms also do not establish that a menu choice represents every individual's physiology; they describe the prediction model rather than an individualized medical classification.
Write the revised Harris-Benedict equations
The revised male expression used here is 88.362 + 13.397W + 4.799H − 5.677A. The revised female expression is 447.593 + 9.247W + 3.098H − 4.330A. Weight remains kilograms, height centimeters and age years. These expressions appear in the cited research protocol and are distinct from Mifflin-St Jeor, despite using the same categories of input.
Coefficients carry the required unit scaling. Entering height as 1.72 rather than 172 while retaining centimeter coefficients does not create a metric alternative; it creates a unit error. Likewise, pounds cannot be substituted for kilograms without conversion. Write the input units next to the equation rather than relying on the fact that both methods happen to display calories at the end.
Calculate both predictions for matched inputs
For a hypothetical adult aged 40, weighing 75 kg and measuring 172 cm, the Mifflin male equation gives 750 + 1,075 − 200 + 5 = 1,630 kcal/day. Revised Harris-Benedict gives 88.362 + 1,004.775 + 825.428 − 227.08 = 1,691.485 kcal/day. The difference is 61.485 kcal/day, approximately 3.77% relative to the Mifflin result.
Using the female terms with the same hypothetical measurements gives Mifflin 1,464 kcal/day and revised Harris-Benedict 1,500.774 kcal/day. Their difference is 36.774 kcal/day, approximately 2.51% of 1,464. These examples show that the gap is not a universal fixed number or percentage. Changing age, height or weight changes each equation through different coefficients. For a coefficient-only check, adding 10 cm while holding every other male-equation input fixed increases Mifflin by 62.5 kcal/day but revised Harris-Benedict by 47.99. This is sensitivity of the equations, not a claim about changing a real person's height.
Do not mistake agreement for measured accuracy
If two equations produce nearby predictions, that demonstrates numerical agreement between models for those inputs. It does not show either result equals measured resting expenditure for that individual. Conversely, their disagreement does not identify which is correct without an appropriate independent measurement. Averaging the two values is another calculation, not a validated method of removing prediction error.
A study's average prediction performance is not a guaranteed error bound for every reader. Population, measurement procedure and individual characteristics matter. The Mifflin study's reported comparison with historical Harris-Benedict predictions should not be turned into a claim that this guide's revised equation is always wrong by a fixed percentage. Be explicit about which version a research finding actually evaluated.
Keep resting expenditure separate from daily energy use
Neither calculation above includes an activity multiplier. Multiplying a resting prediction by an activity factor creates another estimate of total daily expenditure, not a new measurement of resting metabolism. For example, multiplying 1,630 by 1.55 gives 2,526.5 kcal/day. Comparing that result with the other equation's unmultiplied 1,691.485 would confuse activity assumptions with equation choice.
The name BMR is commonly used on calculator pages, while the Mifflin research concerns resting energy expenditure. Strict measurement protocols distinguish related resting and basal concepts. Regardless of terminology, a predicted resting value is not automatically a minimum safe intake, weight-loss target or personalized food prescription. Those interpretations require information beyond either formula.
Reproduce the implemented result in NumUtility
Open the BMR Calculator, use metric mode and enter age 40, weight 75 kg and height 172 cm. Select the relevant historical equation term to check 1,630 or 1,464. NumUtility currently implements Mifflin-St Jeor only; it does not provide a Harris-Benedict selector. Calculate the revised comparison manually rather than looking for a control that is not present.
If another calculator differs, first check units, equation version, sex term and whether it has already applied activity. Then inspect display rounding. These checks can explain a discrepancy without claiming that one site is more medically accurate. A supported software input range is also not the same as the age range used to develop a study equation.
Use the comparison within adult educational scope
These examples are not designed for children, pregnancy, breastfeeding, acute illness or individualized clinical nutrition. Predictive equations can be inappropriate for a reader even when the interface accepts the numbers. NIDDK's adult planning material has its own scope and assumptions; it is not validation of every other calculator's coefficients or activity categories.
This guide is educational and is not medical advice or a recommendation to eat the predicted number of calories. If the purpose is clinical assessment, a qualified professional can choose appropriate methods and interpret measured or predicted expenditure. Keep the equation name, version, inputs and result together so that a comparison remains a transparent calculation rather than a verdict about metabolism.