If you have ever used a BMR formula to estimate your resting calorie burn, you might have noticed different calculators give different numbers. That is because there are two main equations: the Mifflin‑St Jeor and the older Harris‑Benedict. Understanding the difference helps you trust the result from our BMR calculator.
Why Does the BMR Formula Matter?
Your Basal Metabolic Rate (BMR) is the foundation for weight loss, muscle gain, and maintenance. If the formula overestimates your BMR, you will eat too much and not lose weight. If it underestimates, you will eat too little and feel terrible.
Choosing the right BMR formula is not just academic – it directly affects your calorie targets.
Formula 1: Harris‑Benedict (Original from 1919)
The Harris‑Benedict equation was developed in 1919 based on a small sample of 239 people. It was groundbreaking at the time but has known limitations:
- For men: BMR = 66.5 + (13.75 × weight in kg) + (5.003 × height in cm) – (6.755 × age in years)
- For women: BMR = 655.1 + (9.563 × weight in kg) + (1.850 × height in cm) – (4.676 × age in years)
The problem: The original study subjects were young, lean, and mostly male. It tends to overestimate BMR for older adults and underestimate for very muscular people. Also, modern lifestyles and body compositions have changed since 1919.
Formula 2: Mifflin‑St Jeor (Revised in 1990)
The Mifflin‑St Jeor equation was developed in 1990 by a team of researchers who studied a more diverse group of 498 adults aged 19‑78. It is now considered the most accurate BMR formula for the general population.
- For men: BMR = (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) + 5
- For women: BMR = (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) – 161
Why it is better: The Mifflin‑St Jeor equation was validated on a wider range of ages, weights, and body types. Studies consistently show it is within 10‑15% of measured BMR for most people – more accurate than Harris‑Benedict.
Head‑to‑Head Comparison
| Feature | Harris‑Benedict (1919) | Mifflin‑St Jeor (1990) |
|---|---|---|
| Sample size | 239 people | 498 people |
| Age range | Mostly young adults | 19‑78 years |
| Body types | Lean, limited | More diverse |
| Accuracy | Overestimates by 5‑15% typically | Within ±10% for most |
| Best for | Historical reference | Daily calorie planning |
For a typical 40‑year‑old female (70 kg, 165 cm):
- Harris‑Benedict gives: ~1,447 calories/day
- Mifflin‑St Jeor gives: ~1,389 calories/day
The difference is about 58 calories – small but meaningful over weeks.
Which BMR Formula Should You Use?
For almost everyone, use Mifflin‑St Jeor. It is the recommendation of the Academy of Nutrition and Dietetics and most modern research.
When might Harris‑Benedict be useful? Only if you have medical guidance or a known metabolic condition that the standard formulas do not handle well. Even then, Mifflin‑St Jeor remains the better starting point.
Our BMR calculator uses Mifflin‑St Jeor exclusively because it is the most reliable for the general public.
Beyond Formulas: Real‑World Adjustment
No BMR formula is perfect. They are estimates based on population averages. Your actual BMR depends on muscle mass, genetics, hormones, and more.
How to adjust:
- Calculate your BMR using our calculator.
- Calculate your TDEE using our TDEE calculator.
- Eat at that TDEE for two weeks and track your weight.
- If your weight is stable, the estimate is accurate. If you gain, reduce by 100‑200 calories. If you lose, increase by 100‑200 calories.
This real‑world feedback loop corrects for any formula limitations. For a full guide on finding your true maintenance calories, see our maintenance calories article.
Why Do Some Calculators Use Harris‑Benedict?
Legacy. Many older websites and fitness apps still use Harris‑Benedict because that was the standard before 1990. They never updated.
If you see a BMR result that feels too high or too low, check which BMR formula they use. Switching to Mifflin‑St Jeor often fixes the problem.
A Practical Example
Take a 50‑year‑old male, 90 kg, 180 cm.
- Mifflin‑St Jeor: (10×90) + (6.25×180) – (5×50) + 5 = 900 + 1,125 – 250 + 5 = 1,780 calories/day
- Harris‑Benedict: 66.5 + (13.75×90) + (5.003×180) – (6.755×50) = 66.5 + 1,237.5 + 900.5 – 337.75 = 1,867 calories/day
The difference is 87 calories. Over a month, that is 2,600 calories – almost a full day of eating. That could stall weight loss or cause unintended gain.
What About the Katch‑McArdle Formula?
There is a third BMR formula called Katch‑McArdle, which requires knowing your lean body mass (body fat percentage). It is very accurate if you have that data, but most people do not. For general use, Mifflin‑St Jeor remains the best choice.
If you are an athlete or very lean, consider getting a body fat measurement and then using our BMR vs. RMR guide to fine‑tune.
Frequently Asked Questions
Is the Mifflin‑St Jeor formula accurate for older adults?
Yes – it was validated on people up to age 78. However, BMR naturally declines with age due to muscle loss. Strength training and adequate protein can offset this. Learn how to increase metabolism naturally for practical tips.
Which formula do doctors use?
Most clinicians and registered dietitians prefer Mifflin‑St Jeor. Some hospitals still use Harris‑Benedict out of habit, but the research supports Mifflin‑St Jeor.
Can I calculate BMR without a formula?
You can get a measured BMR via indirect calorimetry (breathing test) at a sports medicine lab. But for daily use, a good formula is sufficient.
How does the BMR formula affect weight loss?
If the formula overestimates your BMR, your TDEE will also be overestimated. You might eat too many calories and not lose weight. Using the accurate formula prevents that frustration. See our calories to eat to lose weight guide for the full process.
Does intermittent fasting change which formula to use?
No. The formulas estimate your physiological BMR, which does not change based on meal timing. Read our intermittent fasting beginner’s guide to understand how it interacts with calorie targets.