IBW Formula

IBW full form and where the idea came from

IBW is short for Ideal Body Weight. The concept did not start as a fitness metric. It was developed in the 1960s and 1970s so that doctors and pharmacists could calculate safe medication doses for patients whose actual body weight might not reflect a "typical" adult of their height, particularly patients who were significantly overweight or underweight. A drug dose calculated per kilogram of actual body weight can be dangerously high or low in those cases, so clinicians needed a quick estimate of what a "standard" body weight would be for someone of a given height and gender, without weighing every organ or measuring body composition directly.

That medical origin explains why every IBW formula has the same basic shape: a fixed base weight for a height of exactly 5 feet (152.4 cm), plus a set amount added for every inch taller than that. It is a simple, fast, paper-and-pencil calculation, which mattered a great deal before computers were standard equipment on a hospital ward. The formula spread well beyond hospital dosing over the following decades into general fitness and weight-management use, which is the context most people searching for "IBW full form" or an IBW calculator are in today.

The four formulas, in full

All four formulas below use the same variable: height in inches above 5 feet (60 inches). If your height is under 60 inches, the same formula still applies, just as a subtraction instead of an addition, though all four formulas were originally built around adult heights at or above 5 feet, so results for shorter heights should be read as a directional estimate rather than a precise clinical figure.

• Hamwi (1964): Male = 48.0 kg + 2.7 kg per inch over 5 ft. Female = 45.5 kg + 2.2 kg per inch over 5 ft. The oldest of the four, originally built for quick bedside dosing estimates.
• Devine (1974): Male = 50.0 kg + 2.3 kg per inch over 5 ft. Female = 45.5 kg + 2.3 kg per inch over 5 ft. The most widely cited today, still used in some drug-dosing formulas (such as certain calculations for medications dosed by lean body weight).
• Robinson (1983): Male = 52 kg + 1.9 kg per inch over 5 ft. Female = 49 kg + 1.7 kg per inch over 5 ft. A refinement of Devine's formula using updated population data.
• Miller (1983): Male = 56.2 kg + 1.41 kg per inch over 5 ft. Female = 53.1 kg + 1.36 kg per inch over 5 ft. Published the same year as Robinson, also a refinement of Devine, with a higher base weight and a smaller per-inch increment.

Worked example: 170 cm (5'7") male, step by step

170 cm converts to 66.93 inches (170 / 2.54). That is 6.93 inches over the 60-inch (5 ft) baseline. Plugging that into each formula:

1. Hamwi: 48.0 + 2.7 x 6.93 = 48.0 + 18.7 = 66.7 kg.
2. Devine: 50.0 + 2.3 x 6.93 = 50.0 + 15.9 = 65.9 kg.
3. Robinson: 52.0 + 1.9 x 6.93 = 52.0 + 13.2 = 65.2 kg.
4. Miller: 56.2 + 1.41 x 6.93 = 56.2 + 9.8 = 66.0 kg.
5. Average of all four (using the unrounded values, not the rounded figures above): 65.9 kg, roughly 145.4 lb.

Worked example: 5'4" (163 cm) female, step by step

5 feet 4 inches is 64 inches total, 4 inches over the 60-inch baseline.

1. Hamwi: 45.5 + 2.2 x 4 = 45.5 + 8.8 = 54.3 kg.
2. Devine: 45.5 + 2.3 x 4 = 45.5 + 9.2 = 54.7 kg.
3. Robinson: 49.0 + 1.7 x 4 = 49.0 + 6.8 = 55.8 kg.
4. Miller: 53.1 + 1.36 x 4 = 53.1 + 5.4 = 58.5 kg.
5. Average of all four: (54.3 + 54.7 + 55.8 + 58.5) / 4 = 55.8 kg, roughly 123.1 lb.

Why the formulas were never meant to be exact for everyone

Each formula's constants came from measuring a specific group of people (often hospital patients or military recruits from the era the formula was published) and fitting a straight line through height versus a "typical" weight for that group. That is a reasonable approach for the formula's original purpose, a fast dosing estimate, but it means none of the four formulas account for frame size, muscle mass, age, ethnicity, or the simple fact that human body shapes have shifted since the 1960s and 1980s. A very muscular person, an older adult who has naturally lost some muscle mass, and a person with a naturally large or small bone frame can all have a "real" healthy weight that sits meaningfully outside what any of these formulas predict.

This is exactly why the weight calculator on this site shows all four formulas side by side, plus their average, plus a separate WHO healthy-BMI-based range, rather than presenting a single number as a definitive target. Four independent estimates that broadly agree with each other is a much more useful signal than any one of them in isolation, and a range is more honest than a single cutoff.

How this differs from a BMI calculation

IBW and BMI use the same two inputs (height and weight) but answer different questions. BMI takes your actual current weight and tells you which category it falls into. IBW takes only your height and gender and estimates what a healthy target weight would look like, before you even enter a current weight at all. The weight calculator on this site combines both ideas: it computes IBW from the four formulas above, and separately shows the WHO's 18.5-25 healthy BMI range converted into an actual weight range for your height, so you get an IBW-style target and a BMI-style range from the same two inputs. All of this runs locally in your browser; nothing you enter is sent to a server, see the privacy page for the full explanation.