BMR Calculator

The Mifflin-St Jeor equation

BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + s, where s is +5 for men and −161 for women. That is the entire model: four inputs, four fitted terms, one figure in kcal per day. Entered as 70 kg, 175 cm, age 25, male, it returns 1,673.75 kcal/day, which the calculator above rounds to 1,674 for display. Every figure printed in this narrative is produced by the same tested engine at build time, so the copy and the code cannot drift apart — if they disagreed, the site would fail to build.

The equation was published in 1990 in the American Journal of Clinical Nutrition. It was derived by indirect calorimetry in 498 healthy adults aged roughly 19–78, including both normal-weight and obese participants. Two things follow from that sentence and are worth carrying through the rest of the page. The equation is a regression fitted to those measurements, so what it returns is the value its coefficients predict for a set of inputs, not a measurement of anybody. And the derivation population is the ground it stands on: healthy adults, across a wide but bounded age range.

Reading the coefficients as coefficients

The clearest way to see what the equation does is to move one input at a time and leave everything else exactly where it was. Starting from the reference case above:

• 10 kg heavier, nothing else changed → 1,773.75 kcal/day, a difference of 100.
• 10 cm taller → 1,736.25, a difference of 62.5.
• 10 years older → 1,623.75, lower by 50.
• The same three measurements with the other sex constant → 1,507.75, lower by 166.

Those differences are the coefficients themselves, nothing more: 100 across 10 kg is the 10 per kilogram written into the equation, 62.5 across 10 cm is the 6.25 per centimetre, 50 across 10 years is the 5 per year, and the 166 between the two sex constants is the distance from +5 to −161. This page's build asserts each of those relationships against the engine; if the engine ever stopped reproducing them, the paragraph would fail to compile rather than quietly become false.

It is worth being precise about what that list is and is not. These are terms in a published regression, and the size of a term describes the equation's behaviour — it is not a mechanism inside a person, and this page makes no claim about one. The formula also has no term for body composition and none for training history, so it cannot respond to either: identical weight, height, age, and sex constant produce an identical estimate, however differently two people are built. That is the shape of the published formula rather than a shortcut in this implementation, and it is the first thing to check when an output looks surprising.

The ±10% band around the figure

The estimate arrives with a published expectation attached: roughly a ±10% band around measured resting expenditure for most people. On the reference case that is about 167 kcal/day in either direction — a range of roughly 1,507 to 1,841 rather than the single value the calculator prints. The precision of the display belongs to the arithmetic, not to the estimate: the engine keeps full precision internally and rounds only when it draws the number.

"For most people" is doing real work in that sentence. A band is a statement about a population of estimates, and nothing in the equation reports where inside the band any particular result sits.

Why this equation rather than Harris-Benedict

Mifflin-St Jeor is not the only resting-metabolism equation in circulation, and the one it is usually set against is the older Harris-Benedict formula. For contemporary populations, Mifflin-St Jeor is commonly recommended over it, and this site implements the commonly recommended one. That is a statement about which equation is generally preferred — not a measurement of how far apart the two land on any particular set of inputs, and not a promise about either. A calculator can honestly name the formula it implements and its published error band; ranking formulas against each other is a separate job, and one this page does not attempt.

Where the equation fits least well

Because the derivation population was healthy adults, the equation is least reliable for teenagers, for athletes at body-composition extremes, and for people with conditions that alter metabolism. The middle case is the composition blind spot from two sections above, seen from the other side: there is no composition term in the equation, so there is no correction to apply within it. A second estimate from a different method — the body-fat calculator works from height plus neck and waist girths, and hip for women — is a different number to hold alongside this one, not a fix for this one.

Part of that boundary is encoded in the engine rather than left to prose. It accepts ages from 15 to 100 for BMR and calories, weights from 10 to 500 kg, and heights from 50 to 280 cm, returning an explicit message rather than a wrong answer outside those ranges. Note that the accepted age range is wider at both ends than the derivation population's roughly 19–78: an entry can be inside what the tool accepts and outside what the equation was fitted on, and the calculator will compute it anyway. A number being returned is not a claim that the number applies.

BMR is not maintenance calories

The most common misreading of this page's output is to treat it as a daily calorie figure. It is not one. The equation estimates energy used at complete rest, and a day is not spent at complete rest; the resting estimate is only the first link in the site's chain, and the step that covers everything else is a multiplication.

That step is the activity factor, and the engine offers five rungs, from 1.2 (sedentary) to 1.9 (extra active) — which is why the result panel above lists an activity-adjusted line for each of them. Applied to the reference case, the lowest rung takes the resting figure to 2,009 kcal/day, 335 kcal above it, and the highest rung takes it to 3,180. None of the four measured inputs changed between those two figures; only the multiplier did.

Those multipliers are conventional rather than precise and, unlike the resting equation, they are not measured — one is chosen. Choosing between "lightly" and "moderately" active is a self-assessment most people get wrong in the optimistic direction, and the difference alone shifts the estimate by hundreds of kcal/day. The ±10% band from the resting step is carried through the multiplication as well, reaching about 201 kcal/day at the sedentary maintenance figure above.

The companion guide — calorie needs: BMR, TDEE, and why targets drift — is the longer read on everything downstream of this page: the activity ladder with worked numbers for each rung, the fixed goal offsets, and the safe-intake floors the engine flags rather than hides when a loss target runs out of room. On this site the TDEE Calculator is where the multiplication happens and the Calorie Calculator adds the goal offsets on top — the same arithmetic exposed at three depths, not three different opinions.

The end of that chain is a population formula's output for four inputs, not a prescription. A measured weight trend is a different kind of information — data collected over time rather than a value predicted from four numbers — and where the two disagree the trend is the better one. Re-running the chain on changed inputs is part of how the tool is used rather than a sign the first answer was broken.

Frequently asked questions

Not medical advice: this calculator provides general educational estimates from published population formulas. It cannot account for your individual health context and must not be used to diagnose or treat any condition — consult a qualified healthcare professional. Inputs are processed locally in your browser and never transmitted. See the methodology page.