Ideal Body Weight Calculator

How do you calculate ideal body weight?

Use the Devine formula: IBW = 50 kg + 2.3 kg per inch over 5 feet for men, or 45.5 kg + 2.3 kg per inch for women. A man 5 ft 10 in tall has an IBW of 73 kg, and a woman 5 ft 4 in tall, 54.7 kg. Below 5 feet the formula is not defined.

Three weights for three jobs

The calculator gives three numbers because dosing guides and ventilator protocols ask for different ones, and reaching for the wrong one is the usual error.

• Ideal body weight (IBW) is predicted from height and sex alone and ignores what the person actually weighs. Drug references name it for medicines that distribute poorly into fat, and it is the starting point for the other two.
• Adjusted body weight (AdjBW) counts 40 percent of the weight above IBW, for drugs that reach part of the extra tissue. It is best known from aminoglycoside dosing in obesity.
• Predicted body weight (PBW) is the ARDS Network’s name for the same height and sex line, used to set the tidal volume on a ventilator, because lung size follows height and sex rather than body weight.

Where the Devine formula comes from

Devine published the equation in 1974, in a paper on gentamicin therapy, as a way to estimate a weight to dose by. It was not a measure of healthy weight, and the word ideal is easy to misread: this is a dosing convention, not a weight anyone is advised to reach. Weight status is judged by body mass index instead.

It is one of several straight lines of this kind. Hamwi (1964), Robinson (1983) and Miller (1983) published others with different starting weights and slopes, and they give different numbers for the same person. Devine’s is the one most often meant by IBW, and the one the ARDS Network carried over into ventilation, so it is the one used here.

Adjusted body weight and the 0.4

The formula is AdjBW = IBW + 0.4 × (actual weight − IBW). The 0.4 comes from aminoglycoside pharmacokinetics in obesity, studied by Bauer and colleagues in 1983, and it belongs to that setting. Other drugs are dosed on actual weight, on IBW or with a different correction, so the drug’s own guidance decides which weight applies.

Two things the formula leaves to the protocol. The first is when to switch: protocols move to adjusted weight once actual weight passes a set percentage of IBW, and they do not agree on the percentage, so the calculator shows actual weight as a percentage of IBW and leaves the cut-off to you. The second is a patient at or below IBW. There is no excess weight for the 0.4 to act on, and the formula would only pull the number toward the actual weight, so no adjusted weight is shown.

Tidal volume from predicted body weight

In the ARDS Network trial published in 2000, ventilating adults with acute lung injury at 6 mL/kg of predicted body weight instead of 12 mL/kg lowered mortality from 39.8 to 31.0 percent. The network’s protocol starts at 8 mL/kg and steps down 1 mL/kg at a time to 6, and goes as low as 4 if the plateau pressure stays above 30 cm H₂O. The 2017 ATS, ESICM and SCCM guideline recommends 4 to 8 mL/kg of predicted body weight, with a plateau pressure below 30 cm H₂O, for adults with ARDS.

Lungs do not grow with body fat, so a tidal volume set from actual weight hands the heaviest patients the largest breaths. A 120 kg man 5 ft 10 in tall would receive 720 mL at 6 mL/kg of his actual weight, against 439 mL from his predicted weight.

The trial wrote predicted body weight in cm, 50 or 45.5 + 0.91 × (height − 152.4), and the protocol card in inches, with Devine’s 2.3. Because 0.91 is slightly more than 2.3 divided by 2.54, the two forms differ by 0.14 kg at 6 ft and 0.27 kg at 7 ft. This calculator uses the trial’s form, which is why its PBW sits just above its IBW for the same patient.

Shorter than 5 feet

Devine’s and the ARDS Network’s equations both start at 5 feet and count upward, so a shorter adult has no inch above 5 feet to count. The calculator gives no number there rather than an extrapolated one.

There is no agreed rule for this. Some clinicians extend the line downward, subtracting 2.3 kg for each inch under 5 feet; some use actual body weight when it is the lower figure; others follow a hospital dosing policy or ask a pharmacist. None of these is part of the published formulas, so check which one your institution uses. The formulas were not written for children either, whose doses are worked out by other methods.

Common mistakes

• Setting tidal volume from actual weight. The example above gives 720 mL where 439 mL was meant. Predicted body weight comes from height and sex, so measure the height rather than estimating it.
• Putting cm into the inch formula. A height of 177.8 cm read as inches gives an IBW of about 321 kg. One inch is exactly 2.54 cm, so convert first or choose cm above.
• Entering pounds as kilograms. A man of 5 ft 10 in weighing 220 lb, entered as 220 kg, gets an adjusted body weight about 48 kg too high. One pound is exactly 0.45359237 kg.