Ideal body weight (IBW) is calculated using a simple formula based on your height and sex. The most widely used version, the Devine formula, starts with a base weight at 5 feet tall and adds a fixed amount for every inch above that. For men, the base is 50 kg (110 lb) at 5 feet, plus 2.3 kg (about 5 lb) per additional inch. For women, it’s 45.5 kg (100 lb) at 5 feet, plus 2.3 kg per additional inch.
The Devine Formula Step by Step
The Devine formula, published in 1974, is the standard IBW equation used in most clinical settings. Here’s how it works:
- Men: IBW (kg) = 50 + 2.3 × (height in inches − 60)
- Women: IBW (kg) = 45.5 + 2.3 × (height in inches − 60)
The number 60 represents 5 feet converted to inches. So if you’re a 5’8″ man, you’d calculate: 50 + 2.3 × (68 − 60) = 50 + 18.4 = 68.4 kg, or about 151 lb. A 5’4″ woman would calculate: 45.5 + 2.3 × (64 − 60) = 45.5 + 9.2 = 54.7 kg, or about 121 lb.
To convert kilograms to pounds, multiply by 2.2.
Other IBW Formulas and How They Compare
The Devine formula isn’t the only option. Three other equations are commonly referenced, and they each produce slightly different results because they use different base weights and per-inch increments.
- Hamwi (1964): Men = 106 lb + 6 lb per inch over 5 ft. Women = 100 lb + 5 lb per inch over 5 ft.
- Robinson (1983): Men = 52 kg + 1.9 kg per inch over 5 ft. Women = 49 kg + 1.7 kg per inch over 5 ft.
- Miller (1983): Men = 56.2 kg + 1.41 kg per inch over 5 ft. Women = 53.1 kg + 1.36 kg per inch over 5 ft.
These formulas can give noticeably different results, especially at the extremes of height. Research published in The American Journal of Clinical Nutrition found that most IBW equations underestimate weight for shorter people and overestimate it for taller people. The Robinson formula aligned best with BMI values overall, though it mapped to different BMI targets for men (22.5) and women (21.0). The Devine and Hamwi formulas both produced alignment errors of 2.1% or more compared to BMI-based predictions.
In practice, the Devine formula remains the default in most hospitals and pharmacies. If you see “IBW” without further specification, it almost always means Devine.
Why IBW Matters in Medicine
IBW isn’t primarily a fitness or nutrition tool. Its most important use is medication dosing, particularly for patients whose actual weight is significantly above or below their ideal range. Certain drugs distribute mainly into lean tissue rather than fat, so dosing by actual weight in a very heavy person can lead to overdosing.
This is especially relevant in surgical settings. Studies on muscle relaxants used during intubation found that dosing based on IBW rather than total body weight produced better outcomes in morbidly obese patients. Patients regained muscle strength within an hour without needing a reversal agent, while those dosed by actual weight experienced prolonged effects. Similar findings applied to other anesthesia-related drugs, where IBW-based dosing shortened drug duration without compromising effectiveness.
Adjusted Body Weight for Higher BMIs
When someone weighs significantly more than their IBW, clinicians sometimes use an adjusted body weight (AjBW) instead. This is common when a person weighs at least 25% more than their calculated IBW. The formula accounts for the fact that excess weight does contribute some metabolically active tissue, just not as much as lean mass.
The adjusted body weight formula is:
AjBW = IBW + 0.25 × (actual weight − IBW)
So if your IBW is 70 kg and you actually weigh 110 kg, the calculation would be: 70 + 0.25 × (110 − 70) = 70 + 10 = 80 kg. This adjusted figure splits the difference, giving a more realistic estimate for dosing and nutritional calculations.
Factoring In Body Frame Size
Some older IBW charts adjust the result based on whether you have a small, medium, or large frame. You can estimate your frame size by measuring the circumference of your wrist, then comparing that number against your height. MedlinePlus provides these reference ranges:
For women under 5’2″, a wrist smaller than 5.5 inches indicates a small frame, 5.5 to 5.75 inches is medium, and over 5.75 inches is large. For women 5’2″ to 5’5″, the cutoffs shift to under 6 inches (small), 6 to 6.25 inches (medium), and over 6.25 inches (large). For women over 5’5″, small is under 6.25 inches, medium is 6.25 to 6.5 inches, and large is over 6.5 inches.
For men over 5’5″, a wrist of 5.5 to 6.5 inches suggests a small frame, 6.5 to 7.5 inches is medium, and over 7.5 inches is large. A common rule of thumb adds or subtracts 10% from IBW for large or small frames, respectively. This adjustment is less commonly used in clinical practice today but can be helpful for personal reference.
IBW Calculation for Children
The adult formulas don’t apply to children and adolescents. Pediatric IBW is calculated differently because kids are still growing and their proportions change rapidly.
The McLaren method, developed in 1972, uses growth charts. You find the child’s height on a height-for-age chart, then draw a vertical line to the 50th percentile weight for that height. The point where they intersect is the child’s IBW. This method was originally designed to classify malnutrition.
For a quicker calculation without charts, the Traub equation uses only the child’s height in centimeters: IBW (kg) = (height² × 1.65) ÷ 1,000. For a child who is 120 cm tall, that works out to (14,400 × 1.65) ÷ 1,000 = 23.8 kg.
Where These Formulas Fall Short
All IBW formulas share a fundamental limitation: they assume weight increases as a straight line relative to height. In reality, weight also depends on body width, trunk length, and muscle mass. A 6’0″ endurance runner and a 6’0″ rugby player will have very different healthy weights, but the formula produces the same number for both.
These formulas also ignore body composition entirely. They can’t distinguish between someone who is heavy because of muscle and someone who is heavy because of excess fat. They don’t account for age-related changes in lean mass, genetic variation, or medical conditions. Research has highlighted that IBW equations can’t capture the “metabolically healthy obese” category, where people with above-normal BMI show no metabolic complications, or situations where slightly higher weight appears protective in certain diseases.
IBW is best understood as a quick reference point, not a personal health target. It was never designed to tell any individual what they should weigh. Its real strength is as a clinical tool for standardizing drug doses, ventilator settings, and nutritional estimates across large populations.