Summary: The ideal body weight (IBW) formula is a math equation originally created for drug dosing — not for defining "healthy." This guide breaks down the Devine, Robinson, and Hamwi formulas with real numbers, explains why a range beats a single number, and gives you a 3-step process that actually works.

What Is the Ideal Body Weight Formula?

The ideal body weight formula — often called IBW — is a calculation that gives you a weight target based on height and gender. But here is the critical thing most people do not know: it was never meant to define a "healthy" weight. Doctors invented it in the 1970s to calculate safe drug doses for hospitalized patients. That is it.

Here are the three formulas you will see most often.

The Three Main Formulas (With Examples)

Devine (1974) – most widely used
Men: 50 kg + 2.3 kg per inch over 5 feet
Women: 45.5 kg + 2.3 kg per inch over 5 feet

Example: A 5'9" man → 50 + (9 x 2.3) = 70.7 kg (156 lbs).
A 5'5" woman → 45.5 + (5 x 2.3) = 57 kg (125.5 lbs).

Robinson (1983) – slightly different
Men: 52 kg + 1.9 kg per inch over 5 feet
Women: 49 kg + 1.7 kg per inch over 5 feet

Same 5'9" man → 52 + (9 x 1.9) = 69.1 kg (152 lbs).

Hamwi (1964) – older but still referenced
Men: 48 kg + 2.7 kg per inch over 5 feet
Women: 45.5 kg + 2.2 kg per inch over 5 feet

5'9" man → 48 + (9 x 2.7) = 72.3 kg (159 lbs).

See the problem? Same person, three different "ideal" weights (152–159 lbs). That is a 7-lb swing. Now imagine being 6'3" — the difference gets even bigger.

Ideal body weight formula guide: calculation examples and healthy weight range for adults

Why Your "Ideal" Is Actually a Wide Range

Forget the single number. The NIH says a healthy BMI (18.5–24.9) for a 5'9" adult is roughly 125–168 lbs — a 43-pound window. That is enough room for completely different body types. So instead of obsessing over 156 lbs, ask yourself three questions:

  • Is my BMI between 18.5 and 24.9? (Check with an adult BMI calculator)

  • Is my waist under 40 inches (men) or 35 inches (women)?

  • Do I feel strong and energetic most days?

Who Should Ignore the IBW Formula?

  • Athletes: Muscle is dense. A 200-lb athlete with 10% body fat is healthier than a 160-lb sedentary person. The formula cannot see muscle.

  • Adults over 65: A 2014 meta-analysis in the American Journal of Clinical Nutrition found that older adults with BMI 23–27 had lower all-cause mortality compared to those with BMI 18.5–24.9. Do not chase a lower IBW — focus on strength training and protein.

  • Asian adults: WHO recommends a lower overweight threshold (BMI ≥23). If your BMI hits 24, ask for a fasting glucose test.

  • Pregnancy: IBW is meaningless here. Follow your OB/GYN's guidance.

For a fuller picture, pair BMI with a body fat calculator. It is way more revealing than any formula from the 1970s.

Frequently Asked Questions About Ideal Body Weight Formula

What is the most accurate ideal body weight formula?

None is "accurate" for health. The Devine formula is most common, but it underestimates for tall people and overestimates for short frames. Use a healthy BMI range instead.

How do I calculate ideal body weight in kilograms?

Devine formula: For men, 50 kg + 2.3 kg per inch over 5 feet. For women, 45.5 kg + 2.3 kg per inch over 5 feet. Example: 5'7" man → 50 + (7 x 2.3) = 66.1 kg.

What is my ideal body weight for a 5'6" female?

Devine says 45.5 + (6 x 2.3) = 59.3 kg (130.7 lbs). But a healthy BMI range for 5'6" is 115–154 lbs — so anything inside is fine.

Can I be healthy if I weigh more than the formula says?

Yes. If your waist is healthy and your blood work is good, the extra weight may just be muscle or a larger frame.

Is ideal body weight the same as BMI?

No. BMI gives a category (normal, overweight). IBW gives a single target number. BMI is more useful for general health screening.

Why do different formulas give different numbers?

Because they were created from different patient groups. Devine used a small sample of surgical patients; Robinson used a larger set. All are rough estimates.

Is the ideal body weight formula accurate for people over 50?

No. As you age, muscle mass declines and body fat increases. For adults over 50, a BMI of 23–27 is associated with better health outcomes than the standard IBW results. Focus on waist circumference and strength instead.

Content Integrity Review: This article has been reviewed for alignment with CDC adult BMI classification standards, NIH healthy weight guidelines, WHO global BMI standards, and peer-reviewed literature on ideal body weight formulas. Individual health decisions should always be made with a qualified healthcare provider.

Developed using: CDC adult BMI screening standards, NIH healthy weight guidelines, and peer-reviewed literature on IBW formula origins and limitations.

References & Sources

  1. Devine BJ. Drug Intell Clin Pharm. 1974;8:650-5. DOI: 10.1177/106002807400801203.

  2. Robinson JD, et al. Am J Hosp Pharm. 1983;40(4):613-7. PMID: 6864232.

  3. National Institutes of Health. Healthy weight guidelines.

  4. World Health Organization. Obesity and overweight fact sheet.

  5. Winter JE, et al. Am J Clin Nutr. 2014;100(3):789-96. DOI: 10.3945/ajcn.113.068122.

BMI Calculator Blog. This content is provided for educational and informational purposes only. We encourage sharing with proper attribution to our site. Unauthorized commercial use is prohibited. Medical Disclaimer: This content does not replace professional medical advice, diagnosis, or treatment. Always seek the advice of a qualified physician or other licensed health provider with any questions regarding your body composition, weight status, or health goals.