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Adjusted Body Weight Calculator

Find adjusted body weight.

How it is calculated

You enter two weights in kilograms: ideal body weight (IBW) and actual weight (ABWactual). The tool computes adjusted = IBW + 0.4 × (actual − IBW) and shows it in kg to one decimal place. The factor 0.4 means 40% of the excess over ideal weight is added to the ideal weight. The tool does not calculate IBW for you, so you must supply it from a formula you trust (for example one based on height and sex). It also applies the formula whatever the inputs are: if actual weight is below ideal, the result falls below IBW too, even though adjusted weight is normally used only when actual weight is well above ideal. It is a dosing aid for professionals, not medical advice; the choice of weight and factor depends on the drug.

Worked example

A patient has an ideal body weight of 70 kg and an actual weight of 110 kg. The excess over ideal is 110 − 70 = 40 kg. Forty percent of that is 0.4 × 40 = 16 kg. Adding it to the ideal weight gives 70 + 16 = 86. The tool shows 86.0 kg, which sits between ideal and actual weight.

Try this example →

Common mistakes

  • Entering the actual weight in both fields. The first field is the ideal weight, not the measured weight.
  • Mixing units, for example pounds for one value and kilograms for the other. Both must be in kg.
  • Using the tool for a patient at or below ideal weight. The formula still runs and returns a figure below IBW, which is rarely what the dosing protocol intends; use actual or ideal weight as the protocol says.
  • Assuming 0.4 is correct for every drug. Some drugs use a different factor or total body weight, so check the reference for that medicine.
  • Treating the result as the dose. It is a weight to put into a dosing formula, in mg/kg for example.

Sources

About this calculator

This calculator finds adjusted body weight using ABW = Ideal Body Weight + 0.4 × (Actual Weight − Ideal Body Weight), a correction used when a patient's actual weight is significantly above their ideal weight.

Pharmacists, nurses, and physicians use adjusted body weight instead of actual or ideal weight alone when calculating doses for certain medications in obese patients, since dosing on total body weight can overshoot and dosing on ideal weight alone can undershoot for some drugs. It's a standard reference calculation taught in pharmacy and nursing programs.

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Comments (1)

  • Editorial team

    The 0.4 factor is fixed in this tool, so if your protocol or drug reference specifies a different fraction of the excess weight, work that one out by hand. A quick sanity check: when actual weight is above ideal, the answer must land between the two. If it does not, one of the entries is probably swapped. Which ideal body weight formula do you pair with this, and what would make that step easier for you?

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