Adjusted Body Weight Calculator
Devine ideal weight and the adjusted weight used for dosing in obesity.
What it does
Adjusted body weight is a dosing weight. It splits the difference between ideal body weight, which ignores the extra tissue entirely, and actual body weight, which pretends every kilogram behaves the same. Drug dosing studies for patients with obesity use it because fat tissue takes up less of certain drugs than lean tissue does.
The tool follows the Devine formula for ideal body weight: 50 kg for a man or 45.5 kg for a woman at 5 feet, plus 2.3 kg for every inch above that. The adjusted weight then adds 40 percent of whatever the actual weight sits above the ideal. A patient below or at the ideal line simply keeps the actual weight.
The 0.4 factor comes from aminoglycoside pharmacokinetics. These antibiotics distribute into fat less completely than into lean tissue, so dosing on total body weight overshoots and dosing on ideal body weight undershoots. The adjusted figure lands close to the volume of distribution measured in patients with obesity, which is why hospital dosing protocols lean on it.
Both unit systems appear in practice, so the page carries both. Metric is the default because most clinical guidance publishes centimeters and kilograms, and the imperial entries mirror the same defaults converted.
This is a dosing aid, not a body image measure. Which weight a given drug should use is a decision each protocol makes: some drugs dose on actual weight, some on ideal, and some on adjusted. Follow the protocol your clinician or pharmacist works from.
How to use
- Pick metric or imperial to match your measurements.
- Select sex, which shifts the ideal weight line.
- Enter the height and the actual weight.
- Read the ideal body weight first as the reference line.
- Read the adjusted weight next, which blends ideal and actual.
- Use the excess figure to see how far above ideal the patient sits.
Formula
- IBW
- Ideal body weight from the Devine formula, in kg.
- ABW
- Adjusted body weight used for dosing, in kg.
- 0.4
- Share of the excess weight added back, from distribution data in obesity.
The height enters in inches, with 60 inches as the 5 foot baseline. When the actual weight is at or below the ideal weight, the excess is zero and the adjusted weight stays at the actual weight. Height inputs are limited to 152 to 244 cm (60 to 96 in), the range where the Devine formula is used in practice.
Worked example
Example: man, 170 cm and 90 kg
Male, 170 cm tall, actual weight 90 kg.
- Convert the height: 170 / 2.54 = 66.9 inches.
- Devine IBW: 50 + 2.3 x (66.9 - 60) = 65.9 kg.
- Excess weight: 90 - 65.9 = 24.1 kg.
- Adjusted weight: 65.9 + 0.4 x 24.1 = 75.6 kg.
- Dosing protocols for that drug would use 75.6 kg, not the full 90 kg.
Reference table
| Height | Men | Women |
|---|---|---|
| 152 cm (60 in) | 50.0 kg | 45.5 kg |
| 165 cm (65 in) | 61.5 kg | 57.0 kg |
| 170 cm (67 in) | 66.1 kg | 61.6 kg |
| 175 cm (69 in) | 70.7 kg | 66.2 kg |
| 180 cm (71 in) | 75.3 kg | 70.8 kg |
Limits
- Covers the Devine formula only; Robinson, Miller, and Hamwi give different ideal weights.
- The 0.4 factor fits aminoglycoside style distribution, not every drug class.
- Blood levels and clinical response still guide final dosing.
- Not for children, whose dosing uses their own growth based rules.
Common mistakes
- Dosing every drug on adjusted weight instead of the one the protocol names.
- Applying the 40 percent factor to the full weight instead of the excess.
- Mixing units, such as a height in inches with a weight in kilograms.
- Missing that underweight patients keep their actual weight, since excess cannot be negative.
Try it yourself
Quick checks. Answers sit tucked under each question.
A woman is 160 cm tall and weighs 80 kg. What are her ideal and adjusted weights?
IBW 52.4 kg and adjusted 63.4 kg.
A man stands 180 cm and weighs 70 kg. What is his adjusted weight?
70 kg. His ideal weight is about 75 kg, so he sits below the line and no adjustment applies.
Imperial: 66.9 in and 198.4 lb for a man. What is the adjusted weight in pounds?
About 166.5 lb, the same value as the metric example at 75.6 kg.
Questions
What is adjusted body weight?
Ideal body weight plus 40 percent of the weight above it, giving a middle figure between ideal and actual for dosing.
Why 0.4?
Distribution studies in patients with obesity show about 40 percent of the weight above ideal behaves like the lean compartment, which the factor approximates.
Which drugs use adjusted body weight?
Aminoglycosides top the list, and some other drugs with limited fat distribution follow. Each protocol names its weight basis.
What if the patient weighs less than ideal?
The excess is zero, so the adjusted weight equals the actual weight. The tool reports that case directly.
Is the Devine formula metric?
It is usually written in kilograms and inches. This page converts metric input so both systems give the same answer.
Can I use this for chemotherapy dosing?
Only if the protocol says so. Cancer drugs often dose on actual or capped body surface area, which is a separate calculation.
Sources
- Hong Kong Hospital Authority IMPACT: adjusted body weight calculator
- NCBI: pharmacokinetic changes and dosing modification of aminoglycosides in obesity