Ideal Weight (BMI-Based)

About Ideal Weight (BMI-Based)

The BMI-based ideal weight approach computes the body weight that corresponds to a chosen target BMI for a given height. Unlike the Devine, Robinson, Miller, and Hamwi formulas, which use empirically derived coefficients, the BMI-based approach is transparent and flexible: it simply inverts the BMI formula (weight = BMI x height^2) for a target BMI selected by the user. The default target of 22 kg/m^2 sits near the center of the normal BMI range (18.5 to 24.9) and corresponds approximately to the BMI associated with the lowest all-cause mortality in large pooled analyses.

How the formula works

The formula is weight (kg) = target BMI x height (m)^2. For a 175 cm adult targeting a BMI of 22, the ideal weight is 22 x (1.75)^2 = 22 x 3.0625 = 67.4 kg. The user can choose any target BMI between 18.5 and 27; common choices are 22 (center of normal range, lowest mortality), 21 (leaner target), 23 or 24 (more permissive target), and 25 (upper limit of normal). Some clinicians compute a range by calculating weights at BMI 18.5 and 24.9 to give the full healthy weight range for the height.

Evidence and rationale

The BMI-based approach is grounded in the large epidemiological literature linking BMI to health outcomes. The Prospective Studies Collaboration (Lancet 2009), pooling 900,000 adults in 57 prospective studies, found that all-cause mortality was lowest at BMI 22.5 to 25 and rose progressively above and below that range. The Global BMI Mortality Collaboration (Lancet 2016), with nearly 4 million participants, confirmed the lowest mortality in the 18.5 to 24.9 range, with the nadir near BMI 22. Targeting a BMI of 22 therefore aligns ideal weight with the empirically observed minimum-mortality weight for the height.

Advantages over coefficient-based formulas

The BMI-based approach has several advantages over the Devine, Robinson, Miller, and Hamwi formulas. It is transparent: the user can see exactly which BMI the target weight corresponds to. It is flexible: the target BMI can be customized to the individual's goals and risk profile. It is consistent with the BMI framework that underlies contemporary weight classification. And it avoids the population-specific empiricism of the coefficient-based formulas, which were derived from mid-20th-century insurance data and may not generalize to modern populations. The main disadvantage is that it shares BMI's limitation of not distinguishing fat from lean mass.

Interpretation

The BMI-based ideal weight gives a single target for a chosen BMI. The full healthy weight range can be computed as the weights at BMI 18.5 and 24.9; for a 175 cm adult, this range is 56.7 to 76.3 kg. The target can be adjusted for frame size (higher target BMI for large frames, lower for small) and for muscle mass (higher target for muscular individuals). For most adults, a target BMI of 22 gives a reasonable single-value ideal weight that corresponds to the lowest-mortality BMI.

Limitations

The BMI-based approach inherits all of BMI's limitations: it cannot distinguish fat from muscle, it does not account for fat distribution, and it is biased at the extremes of stature. A BMI of 22 corresponds to very different body compositions in a sedentary adult and a muscular athlete. The approach also requires the user to choose a target BMI, which introduces subjectivity. For individualized assessment, the BMI-based ideal weight should be paired with waist circumference, body fat measurement, and frame size assessment. Despite these limitations, the transparency and flexibility of the BMI-based approach make it a useful complement to the coefficient-based IBW formulas.