Ideal Body Weight Dosing Calculator
Devine ideal body weight and adjusted body weight as used for drug dosing, with the 25%-over rule that FDA labelling uses to decide between them.
Ideal Body Weight Dosing Calculator: with the default inputs, ideal body weight is 73 kg.
Total body weight, as measured.
Devine's equation.
- Adjusted body weight
- 83.7 kgIBW + 0.4 × (total − IBW).
- Actual body weight
- 99.8 kg
- Actual weight above ideal
- 36.7%
- Weight above ideal
- 26.8 kg
- Which descriptor the FDA labelling rule points to
- Actual weight is 36.7% above ideal — at 25% or more, the plazomicin labelling directs dosing on adjusted body weight
- Inches of height over 5 feet
- 10
Assumptions
- This is an estimate for education. It does not diagnose anything and does not replace clinical judgement. It is not a prescribing tool.
- Ideal body weight is Devine's equation as written into FDA-approved labelling: 50 kg (men) or 45.5 kg (women) at exactly 5 feet, plus 2.3 kg per inch above that.
- Adjusted body weight is IBW + 0.4 × (total − IBW), as written into FDA-approved plazomicin labelling.
- The 25%-over trigger for choosing adjusted body weight comes from that same labelling and applies to that drug; other drugs specify other rules.
- Devine's equation is a linear extrapolation below 5 feet and is unreliable there.
- Ideal body weight is a drug-dosing descriptor, not a healthy or target weight.
| Height | Male (kg) | Female (kg) | |
|---|---|---|---|
| 5 ft 0 in (152 cm) | 50 | 45.5 | |
| 5 ft 2 in (157 cm) | 54.6 | 50.1 | |
| 5 ft 4 in (163 cm) | 59.2 | 54.7 | |
| 5 ft 6 in (168 cm) | 63.8 | 59.3 | |
| 5 ft 8 in (173 cm) | 68.4 | 63.9 | |
| 5 ft 10 in (178 cm) | 73 | 68.5 | ← nearest |
| 6 ft 0 in (183 cm) | 77.6 | 73.1 | |
| 6 ft 2 in (188 cm) | 82.2 | 77.7 | |
| 6 ft 4 in (193 cm) | 86.8 | 82.3 |
The equation is a straight line: 2.3 kg per inch for both sexes, anchored at 50 kg for men and 45.5 kg for women at exactly 5 feet. It has no basis below that anchor.
How this is worked out
The formula
Devine ideal body weight (IBW), in kilograms: men: IBW = 50 + 2.3 × (inches of height over 5 feet) women: IBW = 45.5 + 2.3 × (inches of height over 5 feet) Adjusted body weight (ABW), in kilograms: ABW = IBW + 0.4 × (total body weight − IBW) FDA-approved plazomicin labelling directs dosing on adjusted body weight when total body weight exceeds ideal body weight by 25% or more.
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Height
- A number.in in, cm, m, ft · from 36 to 96 · defaults to 70
- Sex
- Choose one of 2 options.Male — base 50 kg · Female — base 45.5 kg
- Actual body weight
- Total body weight, as measured.in lb, kg, st · from 20 to 800 · defaults to 220
What you get back
- Ideal body weightmain answer
- Devine's equation.
- Adjusted body weight
- IBW + 0.4 × (total − IBW).
- Actual body weight
- Actual weight above ideal
- Weight above ideal
- Which descriptor the FDA labelling rule points to
- Inches of height over 5 feet
What this assumes
- This is an estimate for education. It does not diagnose anything and does not replace clinical judgement. It is not a prescribing tool.
- Ideal body weight is Devine's equation as written into FDA-approved labelling: 50 kg (men) or 45.5 kg (women) at exactly 5 feet, plus 2.3 kg per inch above that.
- Adjusted body weight is IBW + 0.4 × (total − IBW), as written into FDA-approved plazomicin labelling.
- The 25%-over trigger for choosing adjusted body weight comes from that same labelling and applies to that drug; other drugs specify other rules.
- Devine's equation is a linear extrapolation below 5 feet and is unreliable there.
- Ideal body weight is a drug-dosing descriptor, not a healthy or target weight.
About this calculator
This is an estimate for education. It does not diagnose anything and does not replace clinical judgement. It is not a prescribing tool.
What these numbers are for
Some drugs distribute mainly into lean tissue. Dose them on total body weight in a patient carrying a large fat mass and the exposure is too high; dose them on ideal body weight and it may be too low, because fat tissue does contribute something. Two descriptors exist to handle that, and both are dosing conventions rather than statements about anybody's body.
Ideal body weight comes from Devine's equation, devised in the 1970s to scale aminoglycoside doses. It is a straight line: 50 kg for a man of exactly 5 feet, 45.5 kg for a woman, plus 2.3 kg for every inch above that. The equation is written into FDA-approved labelling — the iron dextran label states it in exactly those words.
Adjusted body weight splits the difference. It takes the ideal weight and adds 40% of the excess over it. The plazomicin (ZEMDRI) label gives the equation verbatim: "Adjusted body weight = IBW + 0.4 × [TBW – IBW]", and directs its use "for patients with TBW greater than IBW by 25% or more". That 25% trigger is the rule this calculator applies.
The name is the problem
"Ideal body weight" is one of the worst-named quantities in medicine. Nothing about Devine's equation describes a healthy weight, a target weight, or a weight anyone should aim for. It has no evidence base as a health goal, it takes no account of build, muscle or age, and it was reverse-engineered from mid-century height-weight insurance tables for an entirely different purpose. It is a number used to size a dose. Treat it as nothing else.
Which descriptor to use
That is a property of the drug, not of the patient, and it is stated in the drug's own labelling. Some drugs dose on total body weight throughout. Some use ideal body weight. Some use adjusted body weight above a threshold. Some switch depending on renal function. There is no general rule to carry from one drug to the next, and inventing one is a good way to get a dose wrong.
One place the choice is well documented is renal function estimation. FDA guidance on studies in patients with impaired renal function notes that in overweight or obese individuals, "use of alternative body weight metrics such as ideal body weight (IBW) or adjusted body weight (ABW) when calculating eCLcr using the C-G equation is likely to provide a more accurate estimate of renal function than using actual body weight."
Where the equation runs out
Devine's line is anchored at 5 feet and only defined upward from there. Below about 5 feet it is extrapolating, and it becomes unreliable quickly — at sufficiently short stature it returns values that are plainly wrong. The calculator will still show a figure below that anchor but says so plainly. If actual weight is below ideal, adjusted body weight is not meaningful either, and labelling that uses this scheme generally falls back to the actual weight.
The 0.4 factor deserves the same scepticism. It came out of aminoglycoside pharmacokinetics and is an empirical compromise, not a physiological constant. Other drug classes use other factors, and some use none.
Frequently asked questions
▸What is the Devine formula for ideal body weight?
50 kg plus 2.3 kg for each inch over 5 feet for men, and 45.5 kg plus 2.3 kg per inch for women. FDA-approved labelling states it in exactly those terms.
▸How do I calculate adjusted body weight?
Adjusted body weight = ideal body weight + 0.4 × (total body weight − ideal body weight). The plazomicin label gives that equation verbatim and applies it when total weight exceeds ideal by 25% or more.
▸Is ideal body weight a healthy weight to aim for?
No. Devine's equation was built to size aminoglycoside doses, not to describe health. It has no evidence base as a weight target and takes no account of build, muscle or age.
▸When should I use adjusted rather than ideal body weight?
When the drug's own labelling says to. It is a property of the drug, not the patient — some drugs use total weight, some ideal, some adjusted, and some switch by renal function.
▸Does the formula work for short people?
Poorly. It is anchored at exactly 5 feet and only defined upward from there. Below that it extrapolates along a straight line and becomes unreliable, eventually returning values that are obviously wrong.
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