What ideal body weight really is, and what it is not
Ideal body weight is a clinical reference number derived from height and sex alone. It is not a goal, not a diagnosis, and not the output of any study of what weight makes people healthiest. Its purpose is narrow and technical: it gives a stable estimate of the size of the body compartment that most drugs distribute into.
That matters because many drugs — aminoglycoside antibiotics, neuromuscular blockers, some chemotherapy agents — partition into lean tissue and water rather than fat. Dose a 140 kg patient on total body weight and you can overdose them badly, because a large share of that mass is adipose tissue the drug never reaches. Dose them on ideal body weight and you may underdose, because obesity does add some lean mass. Hospital protocols therefore specify, drug by drug, which weight to use: total, ideal, or the adjusted body weight this calculator also returns.
The formulas trace back to B. J. Devine, who in 1974 published the 50 kg + 2.3 kg per inch rule for men in a short pharmacy note about gentamicin dosing. It was never validated against body composition data; it was a convention that worked, and it stuck. Robinson and Miller, both in 1983, refitted the same linear shape to actuarial height-weight tables and arrived at flatter slopes. Hamwi's rule, published in 1964 for diabetes diet planning, is older than all of them and is still the fastest to do in your head.
If what you actually want is a healthy weight range, the BMI band this calculator also prints is the better answer. At 1.78 m it spans 18.5 × 3.1684 = 58.6 kg to 24.9 × 3.1684 = 78.9 kg, a 20.3 kg window, against a 5.1 kg spread between the four ideal-weight formulas at that height. The formulas are precise about a quantity that is not what you are looking for.
The four formulas, and why they disagree
Every one of them has the same shape: a base weight for someone exactly five feet tall, plus a fixed number of kilograms for each inch above that. They differ only in the two constants.
Devine uses 50 kg (men) or 45.5 kg (women) plus 2.3 kg per inch. Its slope is the steepest, so it separates from the others as height rises: at 5 ft 10 in it gives a man 73.0 kg, at 6 ft 4 in it gives 86.8 kg.
Robinson uses 52/49 kg plus 1.9/1.7 kg per inch. Higher base, shallower slope — so it exceeds Devine for short people and falls below it for tall ones. The crossover for men is where 52 + 1.9n = 50 + 2.3n, which solves to n = 5, or exactly 5 ft 5 in.
Miller uses 56.2/53.1 kg plus 1.41/1.36 kg per inch, the flattest of the four. It is the highest estimate for short people by a wide margin and the lowest for tall ones.
Hamwi is native to pounds: 106 lb plus 6 lb per inch for men, 100 lb plus 5 lb per inch for women. Six pounds per inch is 2.72 kg, so its slope sits above Devine's while its base sits well below. It is also the only one of the four published with a frame correction — subtract 10% for a small frame, add 10% for a large one.
The disagreement is not error. Three of the four are curve fits to mid-twentieth-century insurance tables from different populations, and the fourth is a dosing convention. There is no underlying physical quantity they are all trying to measure, so there is nothing for them to converge on.
Worked example: a woman 5 ft 6 in tall
Five foot six is 66 inches, so n = 66 − 60 = 6 inches above five feet.
- Devine. 45.5 + 2.3 × 6 = 45.5 + 13.8 = 59.3 kg (130.7 lb).
- Robinson. 49 + 1.7 × 6 = 49 + 10.2 = 59.2 kg.
- Miller. 53.1 + 1.36 × 6 = 53.1 + 8.16 = 61.26 kg.
- Hamwi. 100 + 5 × 6 = 130 lb; 130 × 0.45359 = 58.97 kg.
- Healthy BMI band. 66 in is 167.64 cm, or 1.6764 m; 1.6764² = 2.8103 m². The band runs 18.5 × 2.8103 = 52.0 kg to 24.9 × 2.8103 = 70.0 kg.
The four ideal weights span 58.97 to 61.26 kg — a spread of 2.3 kg, or under 4%. The BMI band spans 18 kg. Both are legitimate answers to different questions: the first says what a dosing protocol will use, the second says what weight range is associated with the lowest population risk.
If this woman weighs 70 kg, her adjusted body weight for dosing is 59.3 + 0.4 × (70 − 59.3) = 59.3 + 4.28 = 63.6 kg. Her BMI is 70 ÷ 2.8103 = 24.9, which is at the top of the normal band, and her ideal weight sits 10.7 kg below her actual weight without either number implying she should lose it.
How to use the number you get
If you are dosing a drug, use the weight your protocol names and nothing else. Aminoglycosides are conventionally dosed on adjusted body weight in obesity, many anaesthetic induction agents on total body weight, and tidal volume in mechanical ventilation on predicted body weight — which is the Devine formula under another name, and is the reason lung-protective ventilation targets 6 mL per kilogram of predicted rather than actual weight. Getting that wrong is a genuine patient-safety event, not a rounding issue.
If you are setting a personal weight goal, do not use ideal body weight at all. It ignores frame, muscle mass, age and body composition, and for a trained lifter it will read absurdly low — a 1.83 m man has a Devine IBW of 50 + 2.3 × 12.05 = 77.7 kg, yet may carry 88 kg at 12% body fat and be in excellent health. Use the BMI band as an outer boundary and then a composition measure such as the Navy body fat estimate or lean body mass to decide where inside it you want to sit.
The one situation where the ideal-weight figure is genuinely informative outside pharmacy is as a sanity check on a target that came from somewhere else. If a goal weight sits more than about 10% below every one of the four formulas, it is worth asking where the number came from before working towards it.
Ideal body weight by height and formula
| Sex | Height | Devine | Robinson | Miller | Hamwi |
|---|---|---|---|---|---|
| Male | 5′2″ | 54.6 kg | 55.8 kg | 59.0 kg | 53.5 kg |
| Male | 5′4″ | 59.2 kg | 59.6 kg | 61.8 kg | 59.0 kg |
| Male | 5′6″ | 63.8 kg | 63.4 kg | 64.7 kg | 64.4 kg |
| Male | 5′8″ | 68.4 kg | 67.2 kg | 67.5 kg | 69.9 kg |
| Male | 5′10″ | 73.0 kg | 71.0 kg | 70.3 kg | 75.3 kg |
| Male | 6′0″ | 77.6 kg | 74.8 kg | 73.1 kg | 80.7 kg |
| Male | 6′2″ | 82.2 kg | 78.6 kg | 75.9 kg | 86.2 kg |
| Male | 6′4″ | 86.8 kg | 82.4 kg | 78.8 kg | 91.6 kg |
| Female | 5′0″ | 45.5 kg | 49.0 kg | 53.1 kg | 45.4 kg |
| Female | 5′2″ | 50.1 kg | 52.4 kg | 55.8 kg | 49.9 kg |
| Female | 5′4″ | 54.7 kg | 55.8 kg | 58.5 kg | 54.4 kg |
| Female | 5′6″ | 59.3 kg | 59.2 kg | 61.3 kg | 59.0 kg |
| Female | 5′8″ | 63.9 kg | 62.6 kg | 64.0 kg | 63.5 kg |
| Female | 5′10″ | 68.5 kg | 66.0 kg | 66.7 kg | 68.0 kg |
Read across a row to see the spread. It is widest at the extremes of height — 91.6 − 78.8 = 12.8 kg between Hamwi and Miller at 6′4″ — and narrowest in the middle, where all four were fitted.
Assumptions and limits you should know about
- Height is the only real input. Age, activity, muscle mass and ethnicity do not appear in any of the four formulas. Two people of the same height and sex get the same answer regardless of how differently they are built.
- Nothing below five feet was validated. The formulas were published as a base plus an increment above five feet. This calculator extends the line downwards because the alternative is refusing to answer, but the original papers say nothing about that region and it warns you when it happens.
- The frame correction is Hamwi's alone. Applying ±10% to Devine or Robinson is a common improvisation with no published basis, so this calculator does not do it.
- Adjusted body weight uses a 0.4 correction factor by convention. Some protocols use 0.3 and a few use 0.5, drug by drug. Check the one you are dosing rather than assuming 0.4.
- It says nothing about body composition. A result of 73 kg does not mean 73 kg of any particular tissue. Use a body-fat or lean-mass measure for that.
- It is not a paediatric tool. None of the four formulas applies below adult height, and children's dosing weights come from growth references instead.
Do not treat this as a weight-loss target
Ideal body weight was never intended to describe a healthy weight for an individual, and the four formulas do not agree closely enough to support that use even if it had been. For a personal goal, work from the BMI range printed above, adjust it for your body composition, and set the rate of change with the calorie deficit calculator rather than the endpoint alone.
Related reference weights and when to use each
Predicted body weight (PBW) in critical care is the Devine formula, used to set tidal volume. The ARDSNet protocol's 6 mL/kg target is defined against it precisely because lungs scale with height, not with fat mass.
Lean body mass is the physically real quantity ideal body weight gestures at. It is estimated from weight and body-fat percentage, or from the Boer and James equations, and it is what you should use if you want to know how much of you is not fat.
The healthy BMI range is the right tool for a personal weight target, wide though it is. Run your numbers through the BMI calculator to see where you sit inside it.
Body-fat-based targets are better still for anyone training. Decide the body-fat percentage you want, estimate your current lean mass, and the goal weight falls out of the arithmetic — lean mass ÷ (1 − target fat fraction). That target moves with your training, which is exactly what a height-only formula cannot do. Pair it with the TDEE calculator to set the intake that gets you there.