Why the Formula You Use Matters
BMR (Basal Metabolic Rate) can't be measured casually — the only fully accurate method is indirect calorimetry, done in a lab with specialised equipment while you lie completely still. Since almost nobody has access to that, researchers developed predictive equations that estimate BMR from simple inputs like age, height, weight and gender.
Three formulas dominate — Mifflin-St Jeor, Harris-Benedict, and Katch-McArdle. They don't all agree. Depending on your body composition, the gap between them can be 100-200 calories a day, which is enough to matter if you're planning a cut, a bulk, or just trying to eat at maintenance.
Quick answer: for most people, Mifflin-St Jeor is the best default — it's the formula our BMR calculator uses automatically. Read on to see when the other two make more sense.
1. Mifflin-St Jeor Formula
Published in 1990 by Dr. Mifflin and Dr. St Jeor, this formula replaced Harris-Benedict as the preferred clinical standard after research showed it predicted resting metabolic rate more accurately across a broad, modern population sample.
Strengths
- Most accurate for the general population on average
- Only needs age, gender, height and weight
- Recommended by the Academy of Nutrition and Dietetics
Limitations
- Doesn't account for muscle mass directly
- Less precise for very lean or very muscular individuals
2. Harris-Benedict Formula
The oldest of the three. Originally published in 1919 and revised in 1984, it was the default clinical formula for most of the 20th century. It's still widely used today, though it tends to overestimate BMR slightly compared to Mifflin-St Jeor, particularly in people with higher body fat.
Strengths
- Long clinical track record, well studied
- Simple inputs, same as Mifflin-St Jeor
Limitations
- Tends to overestimate BMR by ~5%
- Based on a smaller, older reference population
3. Katch-McArdle Formula
Unlike the other two, Katch-McArdle calculates BMR from lean body mass instead of total body weight — which makes it more precise for people who are noticeably leaner or more muscular than average, where weight-based formulas lose accuracy.
Strengths
- Most accurate for athletic or very lean individuals
- Accounts for body composition, not just weight
Limitations
- Requires a reasonably accurate body fat % measurement
- Errors in body fat estimate directly skew the result
Side-by-Side Comparison
| Formula | Year | Inputs Needed | Best For |
|---|---|---|---|
| Mifflin-St Jeor | 1990 | Age, gender, height, weight | General population (recommended default) |
| Harris-Benedict | 1919 / 1984 | Age, gender, height, weight | Historical reference, broad estimates |
| Katch-McArdle | 1996 | Weight, body fat % | Lean, muscular or athletic individuals |
Worked Example: Same Person, Three Formulas
Take a 30-year-old male, 175 cm tall, 70 kg, with 18% body fat. Here's how each formula scores him:
All three land within about 100 calories of each other here — a normal spread. The gap widens for people who are significantly leaner, heavier, or more muscular than average.
Which Formula Should You Use?
- Don't know your body fat %? Use Mifflin-St Jeor — it's the safest, most broadly accurate default.
- Know your body fat % and are lean/athletic? Katch-McArdle will likely be more precise for you.
- Comparing against older research or a doctor's chart? Harris-Benedict may match legacy figures more closely, though it typically runs slightly high.
Our BMR Calculator defaults to Mifflin-St Jeor, and automatically switches to Katch-McArdle if you enter a body fat percentage in the advanced field — so you don't have to pick manually.
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