Every BMR calculator on the internet uses one of two equations: Mifflin-St Jeor (1990) or Harris-Benedict (originally 1919, revised 1984). They look almost identical. They produce different numbers. This guide explains where the differences come from and which one to trust for your plan.
Quick Answer
- Mifflin-St Jeor is the modern default. More accurate for today's populations, ~5% closer to measured RMR on average.
- Harris-Benedict (revised) is the historical standard. Tends to over-estimate BMR by 50–100 kcal/day in sedentary adults.
- For most people, the two agree within ~3%. Either is a fine starting point; the 2–3 week trial closes the gap.
Both are shown side by side in our BMR Calculator.
The Two Formulas
Mifflin-St Jeor (1990)
- Men: BMR = 10·W + 6.25·H − 5·A + 5
- Women: BMR = 10·W + 6.25·H − 5·A − 161
Harris-Benedict, Revised (Roza & Shizgal, 1984)
- Men: 88.362 + 13.397·W + 4.799·H − 5.677·A
- Women: 447.593 + 9.247·W + 3.098·H − 4.330·A
Where W = weight (kg), H = height (cm), A = age (years).
The original 1919 Harris-Benedict was revised in 1984; that revision is what most calculators use today.
Side-by-Side Worked Example
A 30-year-old man, 175 cm, 70 kg:
| Method | Calculation | BMR | |---|---|---| | Mifflin-St Jeor | 10·70 + 6.25·175 − 5·30 + 5 | 1,649 | | Harris-Benedict | 88.4 + 13.397·70 + 4.799·175 − 5.677·30 | 1,696 |
Difference: ~47 kcal/day (~3%).
A 30-year-old woman, 165 cm, 60 kg:
| Method | Calculation | BMR | |---|---|---| | Mifflin-St Jeor | 10·60 + 6.25·165 − 5·30 − 161 | 1,320 | | Harris-Benedict | 447.6 + 9.247·60 + 3.098·165 − 4.330·30 | 1,381 |
Difference: ~61 kcal/day (~4.5%).
Harris-Benedict consistently runs a touch higher.
Why the Difference
Mifflin-St Jeor was derived from a more recent, broader cross-section of adults (1990) than the original Harris-Benedict subjects (Boston, ~1900). Body composition averages and activity patterns have shifted over a century, and modern populations carry slightly different BMRs at the same height and weight.
Research comparing the two formulas against measured resting metabolic rate (via indirect calorimetry) generally finds Mifflin-St Jeor within ~10% of measured RMR for ~80% of adults, slightly better than Harris-Benedict.
When Each Wins
Mifflin-St Jeor is better for…
- Most healthy adults today, including office workers and recreational athletes
- Adults with BMI 18.5–35
- Modern clinical and dietitian practice (most professional guidelines default to Mifflin-St Jeor)
Harris-Benedict is still useful for…
- Quick comparison alongside Mifflin-St Jeor — flagging large disagreements
- Older population-comparison studies that used the original formula
- Software that hasn't been updated (a surprising amount)
Neither is reliable at the extremes — very lean athletes (BMI under 18.5) or severely obese (BMI over 35) need measured RMR or specialised equations.
What These Formulas Cannot See
Both equations rely on the same four inputs: sex, weight, height, age. Anything beyond those four is invisible to the math — but very real for your actual metabolism:
- Lean body mass. Two people at 70 kg can differ by 5 kg of muscle. That's 100+ kcal/day of unmeasured BMR difference.
- Thyroid status. A mildly under-active thyroid can suppress BMR by 100–300 kcal/day; the formulas don't know.
- Dieting history. Extended deficits leave a measurable BMR drop (often called "adaptive thermogenesis") that lingers for months after the diet ends.
- Sleep and stress. Chronically short sleep reduces measured RMR by 5–8% in controlled studies.
- Ambient temperature. Cold-exposed adults run higher BMRs; people who live in heated/cooled homes year-round run a touch lower.
- Menstrual cycle (women). RMR rises 50–150 kcal/day in the luteal phase vs. the follicular phase. Neither formula models this.
The formulas estimate the average person with your stats. You are not the average — but the average is the right starting point. Reality refines it.
How to Choose For Your Plan
Pragmatic workflow:
- Start with Mifflin-St Jeor. Our BMR Calculator shows it as the primary number.
- Glance at Harris-Benedict as a sanity check. Agreement within 5% means your inputs are sensible.
- Multiply by activity factor for TDEE → TDEE Calculator.
- Validate over 2–3 weeks by tracking weight at the predicted maintenance. If reality is off, adjust 10%.
The 50–100 kcal difference between the two formulas is smaller than the natural error of any BMR formula vs. your true measured RMR. Don't agonise over it.
What If My Two Numbers Disagree A Lot?
If Mifflin and Harris-Benedict differ by more than 10% for you, two checks:
- Units — confirm kg and cm (not lb and in).
- Age/height/weight typos — even a 10 kg or 10 cm typo can swing both formulas dramatically.
If the inputs check out, that's just normal variance at the edges. Trust Mifflin-St Jeor as the default.
How Accurate Is "Accurate"?
When researchers compare formula-based BMR to indirect calorimetry (the gold standard, measuring oxygen and CO₂ exchange under controlled conditions), the spread is wider than most calculator output suggests.
- Mifflin-St Jeor predicts measured RMR within ±10% for roughly 80% of healthy adults. The other 20% scatter ±15% or more.
- Harris-Benedict sits at roughly 75% within ±10%, with a small systematic over-estimate in sedentary adults.
- For trained athletes, both formulas typically under-estimate by 100–250 kcal because they don't capture the muscle mass premium.
- For adults with obesity, both over-estimate somewhat because fat tissue has lower per-kg metabolic cost than lean tissue, and the formulas treat weight uniformly.
The takeaway: a formula gives you a plausible range, not a precise number. Plan in that range, then let the scale teach you where you actually sit.
Other BMR Formulas Worth Knowing
- Katch-McArdle — uses lean body mass directly: BMR = 370 + 21.6·LBM(kg). Most accurate for very lean or athletic individuals (you need a body fat estimate, e.g. from our Body Fat Calculator).
- Cunningham — similar to Katch-McArdle, slightly higher coefficient. Used in research.
- Oxford / Schofield — population-specific equations; common in European nutrition guidelines.
For most fitness-app users, Mifflin-St Jeor remains the practical default.
Common Mistakes
- Switching formulas mid-plan. Pick one and stay with it; switching changes the reference number without changing the truth.
- Treating the formula as exact. Both are estimates. Your weight trend is the source of truth.
- Comparing across calculators with different formulas. A "Mifflin" calc and a "Harris-Benedict" calc will disagree by 50–100 kcal. That's the formula, not the calculator.
- Using Harris-Benedict because it gives a "higher" number. Bigger calorie budget feels nicer but isn't your real maintenance.
Frequently Asked Questions
Which formula does my fitness app use?
Most modern apps (MyFitnessPal, MacroFactor, Cronometer in recent updates) use Mifflin-St Jeor. Older or simpler tools sometimes default to Harris-Benedict. Check the help section.
Is one safer for cutting?
No. Both estimate the same biological number (BMR). What matters is starting at the calculator's estimate and adjusting from the scale.
Can I average the two?
You can, but it doesn't gain you much. The honest move is start with Mifflin-St Jeor, then let real-world data refine.
Does Mifflin-St Jeor work for athletes?
For athletes near "normal" BMI, yes. For very lean or very muscular individuals, Katch-McArdle (using lean mass) is more accurate.
Why is the women's formula different?
Both formulas account for different body composition averages between sexes — primarily different lean body mass at the same height and weight. The constants (+5 / −161 in Mifflin) capture that.
Bottom Line
- Default to Mifflin-St Jeor for any modern fitness plan.
- Use Harris-Benedict as a quick comparison — both shown in our BMR Calculator.
- Either way, validate with the scale over 2–3 weeks; that closes the formula error.
- For lean / athletic individuals, Katch-McArdle with measured body fat is the more accurate alternative.
The math is honest. So is the conclusion: stop arguing about formulas, start eating to the number, and let your weight trend tell you whether the number was right.
Health and fitness calculations are estimates only and not medical advice. Adults at the extremes of body composition or with metabolic conditions should consult a healthcare provider for measured RMR.