BMI vs. Ideal Body Weight: Why These Two Numbers Don't Always Agree
October 8, 2026
BMI defines a broad 'normal' weight range (18.5-24.9) for any given height. Ideal body weight formulas (like Devine) instead compute a single specific number from height alone. These are fundamentally different kinds of answers — a range versus a point estimate — which is why comparing them directly can be confusing.
Skip the math and get your answer instantly:
Open the Ideal Weight Calculator →Worked Example: 5'10" Male
For a height of 5'10" (1.78 m):
- BMI 'normal' range: 18.5 × (1.78)² to 24.9 × (1.78)² ≈ 58.6 kg to 78.9 kg (about 129 lb to 174 lb)
- Devine ideal body weight: 50 kg + 2.3 kg × (70 in − 60 in) = 50 + 23 = 73 kg (about 161 lb)
They're Not Actually in Conflict
The Devine formula's 161 lb falls comfortably within the BMI 'normal' range of 129-174 lb — it's simply one specific point inside that broader range, not a contradictory answer. BMI tells you a wide zone considered healthy; ideal weight formulas pick one number within (or sometimes near) that zone as a reference target, usually for medical dosing purposes rather than general wellness.
Why Neither Number Is a Strict Target
Both methods share the same core limitation: neither accounts for muscle mass, bone structure, or body composition, only height (and weight, for BMI). A muscular person can reasonably sit above both the BMI 'normal' range and any ideal-weight formula's number while being perfectly healthy, since muscle weighs more than fat for the same volume.
Which Number Should You Actually Use?
Treat both as rough reference points, not prescriptive goals — BMI for a quick population-level screening check, and ideal weight formulas mainly as a historical medical-dosing reference rather than a personal fitness target. Body composition measures (body fat percentage, waist circumference) give a more individualized and actionable picture than either number alone.
Frequently Asked Questions
Why does BMI give a range but ideal weight give one number?
They were built for different purposes — BMI is a population-level screening tool that defines a broad healthy zone based on height, while ideal body weight formulas were originally developed for medical dosing calculations that needed one specific reference number.
If my ideal weight formula result is different from my current weight, should I worry?
Not necessarily — both BMI and ideal weight formulas have significant limitations, especially for muscular or atypically built individuals, so a difference from either number alone isn't a reliable health signal on its own.
Can someone be in the BMI 'normal' range but above their 'ideal weight' number?
Yes, commonly — the BMI normal range for most heights spans a wide 40-50 lb window, and an ideal weight formula's single number is just one specific point within (or sometimes outside) that range.
Which is more accurate, BMI or ideal body weight formulas?
Neither is more 'accurate' in a meaningful sense for personal health assessment — both use only height (and weight, for BMI) and ignore body composition, so body fat percentage or waist circumference generally provide more individualized and useful information.
Why do doctors still use these simple formulas if they're limited?
They're fast, free, and require no special equipment, making them useful first-pass screening tools or dosing references — more precise methods (body composition scans, waist-to-hip ratio) are typically used as follow-ups when more detail is needed.