Mifflin–St Jeor vs. Harris–Benedict vs. Katch–McArdle: Which BMR Formula to Use

Mifflin–St Jeor is the most accurate BMR formula for the general population and is the standard used by our free calorie calculator. Harris–Benedict is older and tends to overestimate BMR by 100–200 calories. Katch–McArdle factors in body fat percentage and is more accurate for very lean or very muscular people — but only if you actually know your body fat. For most people, all three produce numbers within 5–10% of each other. Here's the detailed breakdown.

The Three Main BMR Formulas

Mifflin–St Jeor (1990) — the current gold standard:

• Men: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5
• Women: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161

Harris–Benedict (revised 1984) — the previous standard, still widely used:

• Men: BMR = 88.362 + (13.397 × weight kg) + (4.799 × height cm) − (5.677 × age)
• Women: BMR = 447.593 + (9.247 × weight kg) + (3.098 × height cm) − (4.330 × age)

Katch–McArdle — uses lean body mass:

• Both genders: BMR = 370 + (21.6 × lean body mass kg)

Lean body mass = weight × (1 − body fat %). Requires knowing body fat percentage.

Which Formula Is Most Accurate?

The 2005 American Dietetic Association evidence review compared multiple formulas against measured BMR (via indirect calorimetry) in a 2005 study. Mifflin–St Jeor was the most accurate for both normal-weight and obese adults, underestimating measured BMR by about 5%.

Harris–Benedict consistently overestimated BMR by 5–15%, especially in older adults and people with obesity. This matters because an inflated BMR leads to an inflated TDEE and calorie target — a weight-loss diet built on Harris–Benedict numbers may silently under-deliver.

Katch–McArdle can be more accurate than Mifflin–St Jeor for very lean athletes or people with much higher-than-average muscle mass, because it uses lean body mass directly. But that advantage only materializes if your body fat measurement is accurate — and most home methods (scales, tape) have error bars wider than the formula difference.

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Real Numbers Compared

Same person (35-year-old male, 5'10", 180 lb, 18% body fat) calculated three ways:

Formula	BMR	Sedentary TDEE	Moderately active TDEE
Mifflin–St Jeor	1,752	2,102	2,716
Harris–Benedict	1,835	2,202	2,844
Katch–McArdle	1,782	2,138	2,762

Range: about 128 calories at sedentary, 128 at moderate. That's within the noise of daily eating — one handful of nuts, one extra banana. The "which formula" question matters less than people fear.

When Each Formula Wins

Use Mifflin–St Jeor (default) if:

• You don't know your body fat percentage
• You're of average body composition (not extremely lean or extremely muscular)
• You want one formula that works for almost everyone — this is the 2026 standard

Use Katch–McArdle if:

• You have a reliable body fat measurement (DEXA scan, BodPod, or consistent calipers)
• You're very lean (men <10% bf, women <18% bf) or very muscular
• The Mifflin–St Jeor number has been clearly off for you (verified by real-world tracking)

Avoid Harris–Benedict unless:

• A specific program or coach you trust uses it (consistency matters)
• You're replicating an older study or protocol

Our calorie calculator uses Mifflin–St Jeor by default because that matches current evidence-based practice. For body-fat-adjusted calculations, see our body fat percentage guide.

The Real Accuracy Question Isn't the Formula

Obsessing over formula choice misses the bigger accuracy issues:

• Activity multiplier error. Choosing "Moderately Active" when you're "Lightly Active" inflates TDEE by 12%. That's 5x the formula differences above.
• Food tracking error. Portion estimation alone introduces 15–25% error for most people. Hand-measuring pasta makes formula choice look like a rounding issue.
• Individual metabolic variation. Two people with identical Mifflin–St Jeor numbers can have real BMRs that differ by 200+ calories due to genetics, gut microbiome, and stress levels.

The formula gets you a starting number. Real-world tracking — weight change over 2–4 weeks at a set intake — tells you what your actual TDEE is. Adjust from there. The starting formula is much less important than the willingness to adjust based on evidence.

Frequently Asked Questions

Is Mifflin–St Jeor always better than Harris–Benedict?

For the general population, yes — it's more accurate across age groups and body types. Harris–Benedict's main remaining use is continuity with older studies or coaching programs that built their math around it.

Do I need my body fat percentage to use Katch–McArdle?

Yes, and it needs to be accurate. Home scales give rough estimates that often introduce more error than you'd save by using the formula. If you haven't had a DEXA or BodPod, stick with Mifflin–St Jeor.

Why do different websites give me different TDEE numbers?

Different sites use different formulas, different activity multipliers, and sometimes different rounding. A 150-calorie spread between calculators is normal and reflects noise, not one site being right.

Does Mifflin–St Jeor work for muscular people?

Reasonably well, but it can slightly underestimate for very muscular individuals because it uses total body weight, not lean mass. If you're under 12% body fat (men) or 20% (women), Katch–McArdle may be more accurate.

How do I know which formula is right for me?

Pick Mifflin–St Jeor as your starting point. Track weight and intake for 2–4 weeks. If you're gaining or losing faster or slower than the math predicts, the formula isn't the problem — adjust intake, not formula.