Ideal Weight vs BMI — Not the Same Calculation

Ideal body weight and BMI are often described as the same relationship seen from two sides. They are not. BMI is a ratio you can invert to get a weight range; ideal body weight is a separate straight-line equation fitted to insurance tables in the mid-twentieth century. They were built for different jobs, they give different answers, and the gap between them changes with height.

Two different objects

Inverting BMI is arithmetic on one formula. Take the cutoffs the NHLBI publishes, 18.5 and 24.9, multiply each by your height squared, divide by 703, and you have the healthy weight band in pounds. Nothing new enters — it is the same equation solved for the other variable, and the output is necessarily a range because the input is a range.

Ideal body weight equations do not come from BMI at all. Devine (1974), Robinson (1983), Miller (1983) and Hamwi (1964) are each a base weight at five feet plus a fixed number of pounds per inch above it. Pai and Paloucek traced them in 2000 to Metropolitan Life height–weight tables, with no clinical outcome study underneath. Their output is a single number because dosing math needs one.

What Devine’s number is in BMI terms

This is the test that settles it. If ideal body weight were BMI in disguise, converting it back would land on the same BMI at every height. It does not:

Height	Devine, men	as BMI	Devine, women	as BMI
5 ft 3 in	125 lb	22.2	116 lb	20.5
5 ft 7 in	146 lb	22.8	136 lb	21.3
5 ft 11 in	166 lb	23.2	156 lb	21.8

The formula creeps up the BMI scale as people get taller, and it sits a full BMI point and a half lower for women. Neither drift is a health finding; both are artifacts of fitting a straight line to a table. Peterson and colleagues named the problem in 2016 — the old equations underestimate at short heights and overestimate at tall ones — and published a replacement that stays aligned with BMI by construction.

Which one answers which question

	Ideal body weight	Healthy BMI range
You supply	Height and sex	Height
You get	One number, in kilograms	A weight band, in pounds
Built for	Drug dosing, ventilator settings	Population health screening
Published by US agencies as a target	No	Yes, as a range
Changes with sex	Yes, by about 10 lb	No

If the question is how much medication does this patient get, ideal body weight is the right tool and has been for fifty years. If the question is is my weight in a healthy place, the band is the answer that US guidance actually supports. The ideal weight calculator gives the band; the BMI calculator gives your current number against the same cutoffs.

Neither one sees body fat

Both take height and weight and nothing else, so both are blind in the same way: a lifter and a sedentary person of identical height and weight get identical readings. That is why waist circumference and body fat percentage exist as separate measurements rather than as corrections to these formulas.

To see the two laid out across every common height at once, use the ideal weight table by height and age.

This content is informational and does not replace evaluation by a health professional.