How do you calculate adjusted body weight?
Adjusted body weight sits between ideal and actual weight. It takes two steps. First, work out the ideal body weight (IBW) with the Devine formula. Then add back a fixed share of the excess weight.
IBW (kg) = 50 (male) or 45.5 (female) + 2.3 × (height in cm ÷ 2.54 − 60)
Adjusted body weight (kg) = IBW + 0.4 × (actual weight − IBW)
The calculator always uses the 0.4 factor. The ideal body weight calculator shows the first step on its own, with the conversion to inches.
How to use the adjusted body weight calculator
- Enter the Height in cm.
- Choose the Sex: Female or Male.
- Enter the Actual weight in kg, ideally measured on the day.
- Read the Adjusted body weight, with the Ideal body weight (Devine) underneath. Both are rounded to one decimal place.
Worked examples
Example 1 – a man, 176 cm, 110 kg. IBW = 50 + 2.3 × (176 ÷ 2.54 − 60) = 71.37 kg. Adjusted = 71.37 + 0.4 × (110 − 71.37) = 71.37 + 0.4 × 38.63 = 71.37 + 15.45 = 86.82 → 86.8 kg.
Example 2 – a woman, 160 cm, 120 kg. IBW = 45.5 + 2.3 × (160 ÷ 2.54 − 60) = 52.38 kg. Adjusted = 52.38 + 0.4 × (120 − 52.38) = 52.38 + 0.4 × 67.62 = 52.38 + 27.05 = 79.43 → 79.4 kg.
Example 3 – actual weight below ideal. A woman, 170 cm, 60 kg. Her IBW is 61.44 kg, already above her actual weight. The formula would give 61.44 + 0.4 × (60 − 61.44) = 60.86 → 60.9 kg, slightly above what she weighs and below her IBW. The calculator warns that an adjusted weight is not normally used here.
Quick reference
Ideal and adjusted body weight in kg at a few heights and actual weights, rounded as in the calculator.
| Height, sex | IBW | Actual 80 kg | Actual 100 kg | Actual 120 kg | Actual 140 kg |
|---|---|---|---|---|---|
| 160 cm, female | 52.4 | 63.4 | 71.4 | 79.4 | 87.4 |
| 170 cm, female | 61.4 | 68.9 | 76.9 | 84.9 | 92.9 |
| 170 cm, male | 65.9 | 71.6 | 79.6 | 87.6 | 95.6 |
| 180 cm, male | 75 | 77 | 85 | 93 | 101 |
Each extra 20 kg of actual weight adds 0.4 × 20 = 8 kg to the adjusted weight.
Common mistakes
- Applying 0.4 to the whole weight. The factor multiplies only the excess (actual − IBW), not the actual weight.
- Mixing up adjusted, ideal and lean weight. They are three different numbers. It’s best to use the one your protocol names.
- Using it when it does not apply. If the actual weight is at or below IBW, the adjusted weight is not meaningful.
- An estimated or outdated weight. The result is only as good as the actual weight entered.
Where adjusted weight is used
Adjusted weight most often appears in weight-based doses and in renal dose checks. The creatinine clearance calculator accepts whichever weight you enter, so you can run it with the adjusted value when your protocol says so. Our guide to actual, ideal and adjusted body weight compares the three and when each applies.
A quick sense check
When actual weight is above IBW, the adjusted weight lies between the two, closer to the ideal one: 40% of the way from IBW to actual weight. If, in that case, it comes out above the actual weight or below the IBW, something was entered wrongly.
Frequently asked questions
What is the adjusted body weight formula?
Adjusted body weight = IBW + 0.4 × (actual weight − IBW). It starts from the ideal body weight and adds back 40% of the weight above it. A man of 176 cm and 110 kg has an IBW of 71.4 kg and an adjusted body weight of 86.8 kg.
Which ideal body weight does the calculator use?
The Devine formula: 50 kg for men or 45.5 kg for women, plus 2.3 kg per inch of height over 60 inches. The ideal weight is shown as a second result, labelled Ideal body weight (Devine), so you can check it. If your protocol uses a different IBW equation, the adjusted weight will differ too.
Why 0.4?
Because only part of the extra weight, mostly fat, behaves like lean tissue for the distribution of certain drugs. The 0.4 value comes from studies of aminoglycosides in people with obesity. Some protocols use other factors for other drugs. This calculator always uses 0.4.
When is adjusted body weight used?
When a protocol asks for it, typically for some medicines in people whose actual weight is well above their ideal weight. Many protocols set a threshold, such as an actual weight more than 120% or 130% of IBW. Below that threshold, actual or ideal weight is usually used instead.
What if the actual weight is below the ideal weight?
Then an adjusted weight is not normally used. The formula would give a value between the actual and the ideal weight, slightly above what the person weighs, which has no practical meaning. The calculator still shows the number but adds a warning. In this situation, it’s worth checking whether your protocol asks for actual or ideal weight.
Is adjusted body weight the same as lean body weight?
No. Lean body weight is a different estimate, calculated with its own equations from weight, height and sex. The two can differ by several kilograms in the same person. It’s worth checking which one your protocol names before you use a result, because they are not interchangeable.
Sources
- Bauer LA et al. Influence of weight on aminoglycoside pharmacokinetics in normal weight and morbidly obese patients. Eur J Clin Pharmacol 1983
- Pai MP, Paloucek FP. The origin of the ideal body weight equations. Ann Pharmacother 2000
- Devine BJ. Gentamicin therapy. Drug Intell Clin Pharm 1974;8:650–655