Ideal Body Weight Calculator (IBW)
Ideal Body Weight Calculator (IBW)
Ideal body weight by the Devine formula, from sex and height in centimetres or inches. It is worth knowing what this number is: Devine published it in 1974 as a way to size gentamicin doses, not as a statement of what anyone ought to weigh — and it is the weight most often fed into Cockcroft-Gault.
Ideal body weight (Devine)
Sex + height → IBWMan, 178 cm
The Devine equation (1974)
Women: IBW = 45.5 kg + 2.3 kg × (height in inches − 60)
- height
- in inches; centimetres are divided by 2.54 first
- 60
- 5 feet — the equation is written for heights above it
- 2.3 kg
- per inch, the same for both sexes; only the starting weight differs
Worked example
Man, 178 cm
178 ÷ 2.54 = 70.08 inches
70.08 − 60 = 10.08 inches over 5 feet
2.3 × 10.08 = 23.18 kg
50 + 23.18 = 73.2 kg
Four ideal body weight equations, side by side (kg)
| Sex and height | Devine 1974 | Robinson 1983 | Miller 1983 | Hamwi 1964 | Spread |
|---|---|---|---|---|---|
| Man, 5 ft 0 in (152.4 cm) | 50.0 | 52.0 | 56.2 | 48.0 | 8.2 |
| Man, 5 ft 10 in (177.8 cm) | 73.0 | 71.0 | 70.3 | 75.0 | 4.7 |
| Man, 6 ft 4 in (193.0 cm) | 86.8 | 82.4 | 78.8 | 91.2 | 12.4 |
| Woman, 5 ft 0 in (152.4 cm) | 45.5 | 49.0 | 53.1 | 45.5 | 7.6 |
| Woman, 5 ft 4 in (162.6 cm) | 54.7 | 55.8 | 58.5 | 54.3 | 4.2 |
| Woman, 5 ft 10 in (177.8 cm) | 68.5 | 66.0 | 66.7 | 67.5 | 2.5 |
Where Devine stops working
| Situation | What the formula does | What to do |
|---|---|---|
| Height under 5 ft (152.4 cm) | Undefined as written; extrapolates downward | Follow your protocol; no single sourced method |
| Height under about 97 cm (men) or 102 cm (women) | Zero or negative | Not usable. This page refuses to print a value |
| Children | Not designed for them | Use paediatric dosing references |
| Actual weight below IBW | Returns a weight the patient does not have | Most dosing conventions use actual weight — see the adjusted body weight page |
| Amputation | Ignores the missing limb | Reduce by the limb’s share of body weight, per local reference |
A dosing number from a gentamicin paper, not a target weight
Ideal body weight sounds like advice about what someone should weigh. It is not, and the name has caused decades of confusion. The equation almost everyone uses came from a 1974 paper by Ben Devine on gentamicin therapy. Aminoglycosides distribute into water and lean tissue, not fat, so dosing them on total weight in a heavy patient overshoots. Devine needed a weight that tracked height rather than fat, and wrote one down: 50 kg for a man at 5 feet, 45.5 kg for a woman, plus 2.3 kg for every inch above that. Pai and Paloucek, tracing the origin of the equations in 2000, found no population data behind it. It stuck because it was simple, not because it was measured.
That origin explains its behaviour. It uses only sex and height, so it is identical for everyone of the same height. It is written from 5 feet upward, so below 152.4 cm it has no defined value: extended downward at the same slope it reaches zero at about 97 cm in men and 102 cm in women. None of the sources consulted for this page — including the protocols that print the formula — say what to do below 5 feet, and practice varies between extrapolating, using actual weight when that is lower, and using paediatric methods in children. This page shows the extrapolated figure with a warning and refuses to print zero or a negative number.
Where it matters most is Cockcroft-Gault. The creatinine clearance equation takes a weight, and which weight is entered — actual, ideal or adjusted — can move a patient across a dose band. Ideal body weight is the one most often used, and the one most often used without saying so. The Cockcroft-Gault creatinine clearance calculator discusses the choice; the dosing weight selection page sets the published conventions for aminoglycosides, vancomycin, Cockcroft-Gault and heparin side by side, including where they disagree.
Three other equations — Robinson and Miller in 1983, Hamwi in 1964 — have the same shape with different numbers, and the table above shows how far apart they drift at the extremes of height. None is more correct than the others as a description of a person; they are conventions, and the right one is whichever your protocol names. From ideal body weight, the adjusted body weight calculator gives the weight used for aminoglycosides in obesity, and the lean body weight calculator gives a size descriptor that, unlike IBW, responds to actual weight.
Frequently asked questions
What is the formula for ideal body weight?
The usual one is Devine’s (1974): 50 kg plus 2.3 kg per inch over 5 feet for men, and 45.5 kg plus 2.3 kg per inch over 5 feet for women. In centimetres, convert height to inches by dividing by 2.54 first. A man of 178 cm is 70.08 inches, 10.08 inches over 5 feet, so his IBW is 50 + 2.3 × 10.08 = 73.2 kg. Some institutional protocols use 45 kg rather than 45.5 kg for women, so check which your protocol prints.
Is ideal body weight what I should weigh?
No. Devine devised it as a dosing weight for gentamicin, and there is no population data behind it. It is a drug-dosing convention that approximates lean mass from height, not a health target. For whether a weight is healthy, clinicians use BMI and waist measures, not IBW.
How do I calculate IBW for someone under 5 feet?
The equation as written has no value below 5 feet (152.4 cm). Extending the line downward at 2.3 kg per inch is common and is what this page shows, flagged as extrapolation, but it reaches zero at about 97 cm in men and 102 cm in women. No guideline found for this page sets a single rule; protocols variously extrapolate, use actual weight if that is lower, or use paediatric methods in children. Use your own protocol and record which you used.
What is the difference between the Devine, Robinson, Miller and Hamwi formulas?
They all start at 5 feet and add a fixed amount per inch, with different starting weights and slopes. Near average height they agree within a few kilograms; at the extremes they diverge — by about 12 kg for a 6 ft 4 in man, where Hamwi gives 91.2 kg and Miller 78.8 kg. Devine is the one most drug-dosing protocols name. This calculator computes Devine only, so the answer matches those protocols.
Should I use ideal body weight in Cockcroft-Gault?
It depends which convention your institution follows, and the conventions genuinely disagree in obesity. Cockcroft and Gault derived the equation on actual weight; many protocols use ideal or adjusted weight in obese patients; and one study in morbid obesity found ideal weight underestimated clearance by about 22%. The dosing weight selection page sets the options out with their sources.
Related calculators
References
- Devine BJ. Gentamicin therapy. Drug Intell Clin Pharm. 1974;8:650–655. The source of the Devine equation: men 50 kg + 2.3 kg per inch over 5 feet, women 45.5 kg + 2.3 kg per inch over 5 feet, proposed for estimating gentamicin doses.
- Pai MP, Paloucek FP. The origin of the "ideal" body weight equations. Ann Pharmacother. 2000;34(9):1066–1069. The comparison of the Devine, Robinson, Miller and Hamwi equations used in the table on the ideal body weight page.
- Stanford Health Care. Aminoglycoside dosing guideline (Antimicrobial Stewardship Program). An institutional protocol: Devine for heights over 60 inches with 45 kg for women; total body weight unless obese, then ideal + 0.4 × excess; Cockcroft-Gault on ideal or adjusted weight; extended-interval dosing avoided in pregnancy, gram-positive synergy, ascites, burns over 20% and creatinine clearance below 30 mL/min or rapidly declining renal function.
- Barletta JF. Drug dosing in special populations: obesity and geriatrics. In: CCSAP 2020 Book 2, Issues in Critical Care Practice. Lenexa, KS: American College of Clinical Pharmacy; 2020. Transcribes the Janmahasatian and Devine equations; names Bauer 1983 as the source of the 0.4 factor and reports published correction factors from 0.14 to 0.98; summarises Demirovic 2009 and the heparin literature.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.