Adjusted Body Weight Calculator (Dosing Weight)

Background.

Adjusted body weight is a dosing weight: a compromise between a patient's ideal weight and their actual weight, used because many drugs distribute into lean tissue far more readily than into fat. The standard formula is ideal body weight plus 40 % of everything above it. This calculator returns that figure, the Devine ideal weight it is built on, the excess weight between them, and the same calculation at the two other correction factors that have actually been published. Read all of it as a weight, not a dose — which weight any particular drug should be dosed on is drug-specific and set by local protocol, and that decision belongs to a prescriber or pharmacist, not to a web page.

The reasoning behind it is straightforward pharmacokinetics. If a drug distributes only into lean tissue, dosing on total body weight overdoses a heavier patient, sometimes dangerously; but dosing on ideal body weight underdoses them, because adipose tissue is not inert and some of the drug does go there. Adjusted body weight splits the difference by adding back a fixed fraction of the excess. The classic case is the aminoglycosides — gentamicin, tobramycin, amikacin — where the therapeutic window is narrow enough that getting the weight wrong shows up as either treatment failure or nephrotoxicity.

The 0.4 in the standard formula is not a round number someone invented, and it is worth knowing where it came from because the answer explains why this page shows three of them. In 1980 Bauer and colleagues studied single-dose amikacin in seven morbidly obese patients with a mean total body weight of 166.5 kg, and reported that 'a correction factor (CF) of 0.38 was found to normalize the patients' volume of distribution' to 0.26 litres per kilogram. That 0.38, rounded to 0.4, became the convention. Fifteen years later Traynor, Nafziger and Bertino revisited it in 1,708 patients receiving gentamicin and tobramycin, explicitly naming the incumbent standard as 'ideal body weight (IBW) plus 40 % excess body weight', and concluded that the factor giving equivalent predicted peak concentrations was 0.43 for overweight patients — along with a completely different rule, 1.13 times total body weight, for underweight ones. So the published range runs 0.38 to 0.43 with the convention sitting in the middle on a sample of seven. This calculator leads with 0.4 because that is what protocols and reference works use, and shows the other two beside it because the spread is real: for the worked example on this page they differ by 2.25 kg, which at 5 mg/kg of gentamicin is about 11 mg of drug.

The ideal weight half comes from the Devine equation, published by B. J. Devine in 1974 in a paper about gentamicin therapy — the same clinical problem. It gives 50 kg for a man of five feet and 45.5 kg for a woman, plus 2.3 kg for every inch above that. It was derived empirically rather than from a population study, and it is an extrapolation from a single anchor point, which is why this calculator refuses heights below 130 cm: keep extrapolating downwards and the numbers stop meaning anything. We use exactly the coefficients our ideal weight calculator uses, so the two pages cannot give you different answers for the same patient.

One thing this calculator deliberately does not tell you is when to use the adjusted weight rather than the actual or ideal weight. Thresholds of 'more than 120 % of ideal' and 'more than 130 % of ideal' circulate widely, but we could not trace either to a primary publication — Traynor's paper says only that patients were stratified 'based on both TBW/IBW ratio and body mass index' without publishing the cut-points in the accessible text. Rather than print a threshold we cannot cite, the page shows your weight as a percentage of ideal and stops there. Different drug classes genuinely use different answers anyway: aminoglycosides commonly use adjusted weight, vancomycin loading doses commonly use total body weight, and some agents use lean body weight or body surface area instead.

One last case worth flagging because the arithmetic hides it. If the patient weighs less than their ideal weight, the excess is negative and the formula returns a number above their actual weight — mathematically correct, clinically wrong. Traynor's own answer for that group was a separate equation entirely. The calculator returns the value rather than hiding it, and flags the negative excess where you will see it.

What is adjusted body weight calculator?

Adjusted body weight, sometimes written AdjBW or ABW-adjusted, is a weight used for calculating drug doses in patients whose total body weight substantially exceeds their ideal body weight. It is defined as ideal body weight plus a fixed fraction — conventionally 0.4, or 40 % — of the difference between actual and ideal weight. The concept exists because the volume of distribution of many drugs scales with lean mass rather than total mass: hydrophilic drugs such as the aminoglycosides distribute mainly into body water and lean tissue, so dosing them on total body weight in an obese patient produces higher concentrations than intended, while dosing on ideal body weight alone produces lower ones, because adipose tissue does take up some drug. The correction factor traces to Bauer and colleagues' 1980 study of amikacin in seven morbidly obese patients, which found that adding 0.38 of the excess weight to ideal body weight normalised the observed volume of distribution; that value was rounded to 0.4 in practice. Traynor, Nafziger and Bertino re-derived it in 1995 from 1,708 patients and obtained 0.43 for overweight patients, together with a separate 1.13 × total body weight rule for underweight ones. The ideal body weight component is almost always the Devine equation of 1974, itself published in the context of gentamicin dosing. Adjusted body weight is not a measure of body composition, is not an estimate of lean mass, and carries no independent clinical meaning outside the dosing calculation it feeds.

How to use this calculator.

1. 01Select the patient's sex — the Devine equation uses a different base weight for men and women.
2. 02Enter actual measured body weight and pick kilograms or pounds. Use a measured weight, not a stated or estimated one; dosing weights inherit every error in the scale reading.
3. 03Enter height and pick centimetres or inches. Height drives the ideal-weight half of the calculation entirely, at 2.3 kg per inch.
4. 04Read the adjusted body weight in kilograms. This is a weight to feed into a dosing calculation, not a dose.
5. 05Check the excess body weight tile. If it is negative, the patient is below ideal weight and this equation does not apply to them.
6. 06Compare the three correction factors. If the 0.38 and 0.43 figures differ enough to change the dose you would give, that difference is real published uncertainty, not a rounding artefact.
7. 07Confirm the drug you are dosing actually uses adjusted body weight. Many do not: some use total body weight, some ideal body weight, some lean body weight, some body surface area. Check the local protocol or product information.

The formula.

Two steps. First, ideal body weight by the Devine (1974) equation: convert height to inches above 5 feet as (height in cm − 152.4) ÷ 2.54, then IBW = 50 + 2.3 × that for men, or 45.5 + 2.3 × that for women. Second, adjusted body weight = IBW + 0.4 × (actual body weight − IBW). The quantity in brackets is the excess body weight. Working the shipped example: a man 180 cm tall is (180 − 152.4) ÷ 2.54 = 10.8661417323 inches above five feet, so his ideal body weight is 50 + 2.3 × 10.8661417323 = 74.9921259843 kg. At an actual weight of 120 kg his excess is 120 − 74.9921259843 = 45.0078740157 kg, and 40 % of that is 18.0031496063 kg, giving an adjusted body weight of 92.9952755906 kg. The same calculation at Bauer's originally derived 0.38 gives 92.0951181102 kg and at Traynor's 0.43 gives 94.3455118110 kg — a spread of 2.2503937008 kg across the published range. His actual weight is 160.0167996640 % of his ideal weight. ORDERING, which is a useful sanity check: whenever actual weight exceeds ideal, the results must satisfy IBW < 0.38-figure < 0.40-figure < 0.43-figure < actual weight, and they do. ROUNDING STAGE: 152.4 cm and 2.54 cm per inch are exact decimals, so every intermediate value is exact; rounding to ten decimal places happens once, at the return. UNDERWEIGHT BEHAVIOUR: if actual weight is below ideal, the excess is negative, the adjusted weight lands between actual and ideal but ABOVE the actual weight, and the factor ordering inverts. That is arithmetically correct and clinically the wrong equation to use — the published answer for underweight patients is a different rule (1.13 × total body weight). DOMAIN: height is restricted to 130–230 cm and weight to 20–400 kg.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Bauer LA, Blouin RA, Griffen WO Jr, Record KE, Bell RM. "Amikacin pharmacokinetics in morbidly obese patients." Am J Hosp Pharm. 1980 Apr;37(4):519–522. PMID 7377215. The origin of the correction factor: seven morbidly obese patients with a mean total body weight of 166.5 kg, in whom "a correction factor (CF) of 0.38 was found to normalize the patients' volume of distribution" to 0.26 litres/kg. Abstract open access; full text paywalled.
2. [2]Traynor AM, Nafziger AN, Bertino JS Jr. "Aminoglycoside dosing weight correction factors for patients of various body sizes." Antimicrob Agents Chemother. 1995;39(2):545–548. PMID 7726530, PMC162577. The independent second authority consulted for this build. Names the incumbent standard as "a dosing weight correction factor of ideal body weight (IBW) plus 40% excess body weight", and derives from 1,708 patients that the factors "are 1.13 times the TBW for underweight patients and 0.43 times the EBW plus IBW for overweight patients". Open access.
3. [3]Devine BJ. "Gentamicin therapy." Drug Intell Clin Pharm. 1974;8(11):650–655. Source of the ideal body weight equation used here — 50.0 kg (men) or 45.5 kg (women) plus 2.3 kg per inch over 5 feet. PRINT / BIBLIOGRAPHIC ONLY: this 1974 journal is not available online. The same coefficients are used by Quanta's ideal weight calculator, so the two pages cannot disagree.
4. [4]National Institute of Standards and Technology — "Guide for the Use of the International System of Units (SI)," NIST Special Publication 811, 2008 edition, Appendix B.9. Source of the exact conversions 1 lb = 0.45359237 kg and 1 in = 2.54 cm, and of the exact 5 ft = 152.4 cm used as the Devine anchor.