Not every BMR calculator uses the same formula, and the one behind a tool can shift your result by 100 kcal or more. The two most common are the Mifflin–St Jeor equation and the older Harris–Benedict equation. Here’s how they differ and why most modern tools, including our own BMR Calculator, have moved to Mifflin–St Jeor.
The Harris-Benedict Equation
Developed in 1919 and revised in 1984, Harris–Benedict was for decades the standard BMR formula:
- Men: 88.362 + (13.397 × weight in kg) + (4.799 × height in cm) − (5.677 × age)
- Women: 447.593 + (9.247 × weight in kg) + (3.098 × height in cm) − (4.330 × age)
It was groundbreaking for its time, but it was built on a study population that doesn’t reflect the general population today, and later research found it tends to overestimate BMR for many people, particularly those who are overweight.
The Mifflin-St Jeor Equation
Published in 1990, Mifflin–St Jeor was developed specifically to address that overestimation issue, using a more contemporary and representative study sample:
- Men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
- Women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
Multiple validation studies since have found Mifflin–St Jeor to be more accurate across a broader range of body types than Harris–Benedict, which is why it’s now the formula generally recommended in clinical and sports-nutrition settings, and the one used throughout Calorie Check – as outlined in our editorial methodology.
Side-by-Side Comparison
| Harris-Benedict | Mifflin-St Jeor | |
|---|---|---|
| Published | 1919 (revised 1984) | 1990 |
| General tendency | Tends to overestimate BMR | Generally more accurate across body types |
| Current clinical status | Largely superseded | Widely recommended |
How Much Difference Does It Actually Make?
For an average adult, the two formulas can differ by roughly 50–150 kcal, which sounds small but compounds once it’s carried through into a full TDEE calculation and a calorie deficit target – potentially explaining several hundred calories of difference in a final daily target, enough to affect the pace of weight loss.
Which Should You Use?
If you’re comparing tools or calculators online and getting different BMR numbers for the same stats, check which formula is being used before assuming one is wrong. Based on the current body of validation research, Mifflin–St Jeor is the more defensible default for most adults, which is why it’s the formula behind our BMR Calculator and TDEE Calculator.
A Direct Numerical Comparison
To see the practical difference, take a 45-year-old man, 178cm, 95kg:
| Formula | Calculation | Result |
|---|---|---|
| Harris-Benedict | 88.362 + (13.397 × 95) + (4.799 × 178) − (5.677 × 45) | 2,004 kcal |
| Mifflin-St Jeor | (10 × 95) + (6.25 × 178) − (5 × 45) + 5 | 1,868 kcal |
A gap of over 130 kcal for the same person – enough, once carried through into a full TDEE and deficit calculation, to noticeably change the pace of weight loss the plan is built to deliver.
Other Formulas You Might Encounter
Beyond these two, a handful of other BMR equations exist in specialist or clinical contexts:
- Katch-McArdle – uses lean body mass directly rather than total weight, which can be more precise for people who know their body fat percentage accurately, but is impractical for most people without specialist measurement.
- Cunningham equation – also based on lean mass, sometimes used in athletic and clinical settings.
- Owen equation – an older, simpler formula, generally considered less accurate across diverse body types than Mifflin–St Jeor.
For the vast majority of people without access to a body composition scan, Mifflin–St Jeor using weight, height, age and sex remains the most practical, well-validated option.
Why Formula Choice Matters Most for These Groups
The gap between formulas isn’t uniform across everyone – it tends to matter most for:
- People who are overweight, where Harris–Benedict’s overestimation tends to be more pronounced
- People tracking progress closely, where a consistent 100–150 kcal difference compounds into a noticeably different weekly or monthly calorie budget
- People comparing tools, where seeing different numbers from different calculators without knowing why can be genuinely confusing and undermine trust in the process
If you’re just after a rough, one-off estimate of your metabolism out of curiosity, the formula matters less. If you’re building a structured weight loss or muscle-gain plan around the number, it’s worth using a calculator that’s transparent about which formula it applies – and sticking with the same one throughout your plan, rather than switching between tools that may be calculating slightly different things.
This is precisely the reasoning behind our own editorial approach at Calorie Check: rather than building a proprietary, unpublished formula, every calculator uses a named, published equation, so you always know exactly what’s behind the number you’re given. You can read more about this approach in our editorial methodology.
How to Check Which Formula a Calculator Is Using
Many online BMR and TDEE calculators don’t clearly state which formula they use, which makes it hard to understand why results might differ from one tool to the next. A few practical ways to check:
- Look for an “about,” “methodology,” or “how we calculate” page, which reputable calculators should provide
- Compare the calculator’s result for your own stats against a manually calculated Mifflin–St Jeor figure – a close match suggests that’s the formula in use, while a gap of 100+ kcal suggests an older formula like Harris–Benedict
- Be cautious of tools that don’t disclose their formula at all, since this makes it impossible to know how much confidence to place in the result, or how it compares to other estimates you may have seen
Transparency about methodology is one of the simplest ways to judge whether a health calculator is being built with genuine accuracy in mind, rather than presenting an unverifiable number as fact. It’s a small check that takes moments to do, but it can save considerable confusion later if you ever compare your results across different tools or sources and find they don’t quite line up.
Ultimately, the choice of formula matters less than using one, consistently, and pairing it with honest tracking and periodic recalculation. Even the more accurate of the two equations is still an estimate, and the real test of whether it’s working for you is how your actual results compare to what it predicted over several weeks – not which name is attached to the maths behind it.
Frequently Asked Questions
Is there a formula more accurate than Mifflin-St Jeor?
The Katch–McArdle formula, which factors in lean body mass directly, can be more accurate for people who know their body fat percentage precisely, but it requires that additional data point which most people don’t have readily available.
Why do different calculators online give me different BMR numbers?
Different sites use different formulas (Harris–Benedict, Mifflin–St Jeor, or others), and even small differences in rounding or unit conversion can produce slightly different results.
Does the formula matter if I’m just estimating roughly?
For a rough sense of your metabolism, either formula gets you in the right range. It matters more once you’re using the number precisely to set a calorie deficit or surplus target.
Which formula does Calorie Check use?
All of our calculators use the Mifflin–St Jeor equation, as detailed in our editorial methodology.
Should I switch calculators if mine uses Harris-Benedict?
It’s not essential, but being aware that your result may run slightly higher than a Mifflin–St Jeor-based estimate is useful context, particularly if you’re comparing your number against advice or targets calculated elsewhere using a different formula.
Does the formula difference matter more for some body types than others?
Research suggests Harris–Benedict’s overestimation tends to be more pronounced for people who are overweight, which is part of why Mifflin–St Jeor is generally considered more broadly accurate across a wider range of body types.