Ideal body weight calculator

Devine's formula gives IBW in kilograms as 50 + 2.3 × (height in inches over 5 feet) for men, and 45.5 + 2.3 × (height in inches over 5 feet) for women. It was introduced by Dr B. J. Devine in 1974 to help estimate drug clearance in obese patients, and — despite being an empiric approximation rather than a formula derived from systematic population data — it remains the equation most commonly used at the bedside and in pharmacokinetic dosing guidance today. Because it is defined relative to 5 feet, it isn't meaningful below that height; this calculator flags heights under 152 cm (5 ft 0 in) rather than showing a figure the formula was never intended to produce.

Adjusted body weight (AdjBW) is calculated as IBW + 0.4 × (actual weight − IBW), and is only shown here when actual weight exceeds IBW by more than 20%. It's a compromise figure used for dosing certain renally-cleared or lean-tissue-distributed drugs in patients who are significantly above their ideal weight — dosing on actual weight alone can overshoot for these drugs, while dosing on IBW alone can undershoot, since some of the extra body mass does still contribute to drug distribution. The 0.4 correction factor is a commonly used clinical convention rather than a fixed physiological constant, and different drugs and local protocols may specify their own adjustment factor.

IBW gives a single target figure derived purely from height and sex, built specifically for drug-dosing calculations. BMI, by contrast, compares actual weight against height to classify a person into a healthy-weight band, and doesn't attempt to name one "ideal" number. The two serve different purposes: BMI is a general population health screening tool, while IBW (and AdjBW) exist specifically to standardise medicine doses across patients of different sizes. Neither is a measure of body composition — a very muscular person can have an actual weight well above their calculated IBW without carrying excess fat.

The Devine formula was not derived from systematic regression analysis or population data — it is a longstanding clinical approximation, not a scientifically validated model of "ideal" weight, and it does not account for frame size, muscle mass, or body composition. It is undefined below 152 cm (5 ft) and becomes less reliable at extremes of height. This tool is for reference only and is not a substitute for clinical judgement — drug-dosing decisions, including which weight descriptor to use for a given medicine, remain with the prescribing clinician and local pharmacy guidance.