Ideal body weight (IBW) is calculated using height-based formulas that produce a single target number in pounds or kilograms. The most widely used formula, the Devine formula, starts with a base weight at 5 feet tall and adds a fixed amount for every inch above that. Several variations exist, each giving slightly different results, and none of them account for muscle mass, bone structure, or ethnicity.
The Devine Formula
The Devine formula is the most common IBW equation, originally developed in the 1970s and still used in hospitals today for drug dosing and clinical assessments. It works the same way for both sexes but starts from different base weights:
- Men: 110 pounds for the first 5 feet of height, plus 5.1 pounds for each additional inch
- Women: 100 pounds for the first 5 feet of height, plus 5.1 pounds for each additional inch
So a man who is 5’10” would calculate: 110 + (10 × 5.1) = 161 pounds. A woman who is 5’6″ would calculate: 100 + (6 × 5.1) = 130.6 pounds. The formula only works for people 5 feet tall or taller. In metric terms, the equation gives men a base of 50 kg and women a base of 45.5 kg, adding 2.3 kg per inch above 60 inches.
The Hamwi Method
The Hamwi method is another popular formula, often taught in nutrition courses and used by dietitians. It follows the same structure as Devine but with slightly different numbers:
- Men: 106 pounds for the first 5 feet, plus 6 pounds per additional inch
- Women: 100 pounds for the first 5 feet, plus 5 pounds per additional inch
Using Hamwi, that same 5’10” man gets 106 + (10 × 6) = 166 pounds, and the 5’6″ woman gets 100 + (6 × 5) = 130 pounds. The results are close to Devine but not identical. Neither formula is more “correct” than the other. They were both derived from insurance company mortality tables from the early-to-mid 20th century, not from large-scale health studies.
Adjusting for Body Frame Size
Some versions of these formulas include a 10% adjustment based on body frame. If you have a small frame, you subtract 10% from the result. If you have a large frame, you add 10%. A medium frame uses the number as-is.
You can estimate your frame size by measuring your wrist circumference with a tape measure. For women 5’2″ to 5’5″, a wrist under 6 inches indicates a small frame, 6 to 6.25 inches is medium, and over 6.25 inches is large. For women over 5’5″, those thresholds shift to under 6.25, 6.25 to 6.5, and over 6.5 inches. For men over 5’5″, a wrist of 5.5 to 6.5 inches is small, 6.5 to 7.5 is medium, and over 7.5 is large.
Going back to the Hamwi example: if that 5’10” man has a wrist measuring 8 inches, he’d have a large frame, and his adjusted IBW would be 166 + 16.6 = about 183 pounds.
Why These Formulas Exist
IBW formulas weren’t designed as personal health goals. Their primary use today is clinical: hospitals rely on them to calculate medication doses for patients, particularly for sedatives, pain medications, blood thinners, and drugs used to maintain blood pressure. Many of these drugs distribute mainly into lean tissue rather than fat, so dosing based on actual weight in a heavier patient could lead to dangerously high levels. IBW gives clinicians a quick estimate of lean body mass based on height alone.
This is also why the formulas are so simple. A pharmacist calculating a dose in a time-sensitive situation needs a number they can get in seconds, not a measurement that requires body composition testing.
IBW Calculations for Children
There is no single agreed-upon method for calculating ideal body weight in children. The CDC recommends using growth charts instead of adult formulas. The most common approach takes the BMI at the 50th percentile for the child’s age and plugs it into a simple equation: multiply that BMI value by the child’s height in meters squared. This gives a weight that corresponds to a statistically typical BMI for a child of that age and height, which is more meaningful than applying a fixed formula designed for fully grown adults.
Why IBW Formulas Can Be Misleading
Every standard IBW formula shares the same core limitation: height is the only input. Two people who are 5’8″ get the same ideal body weight regardless of whether one is a competitive swimmer with dense muscle and the other is sedentary with a very different body composition. The formulas also ignore age, ethnicity, and genetic variation in bone density and fat distribution.
This isn’t a minor quibble. The original data behind these equations came from studies conducted primarily on white men in the early 1900s. The BMI scale, which IBW formulas are closely related to, traces back to a 19th-century Belgian astronomer working with a sample of high-income, mostly white men for a population census in the Netherlands. Neither the original IBW studies nor the BMI framework was designed to reflect the health of a diverse population.
Research in the American Journal of Clinical Nutrition has noted that BMI is actually a better metric than IBW in several respects: adult weight increases proportionally to the square of height (not linearly, as IBW formulas assume), BMI correlates reasonably well with fat mass in population studies, and the relationship between BMI and mortality follows a U-shaped curve that can be adjusted for age, ethnicity, and chronic conditions. IBW formulas produce a single number with no such flexibility.
What IBW Can and Can’t Tell You
If you’re calculating your ideal body weight out of curiosity or as a rough reference point, the Devine or Hamwi formulas will give you a ballpark number in about ten seconds. That number represents a weight statistically associated with lower mortality risk in mid-20th-century insurance data. It’s a starting point, not a verdict on your health.
A more useful personal assessment combines multiple factors. Your BMI (weight in kilograms divided by height in meters squared), waist circumference, blood pressure, blood sugar levels, cholesterol, fitness level, and family history all contribute more to your actual health picture than a single formula based on height. A person whose IBW formula says they’re 20 pounds “over” but who exercises regularly, has healthy metabolic markers, and carries that weight as muscle is in a very different situation from someone at their calculated IBW who is sedentary with poor cardiovascular fitness.
Health is influenced by a complex mix of behaviors, genetics, lean mass, fitness, and environmental factors. IBW formulas were never intended to capture that complexity. They remain useful in clinical settings where a quick, standardized estimate is needed, but for personal health decisions, they’re one data point among many.