Adjusted Body Weight Calculator

Adjusted body weight is a weight parameter used to calculate medication dosages in overweight individuals. As fat tissue has significantly less activity in the body than lean tissue, it falls between ideal and actual weight. If each kilogram of excess weight was treated as equivalent, calculations would be too high. This tool calculates adjusted body weight which can be used for calculating medication dosages, estimating renal function, and nutritional plans. It is determined by finding the ideal weight based on height and gender, then adding a portion of the excess weight. In addition, it displays the body mass index, body surface area, and lean weight.

What is adjusted body weight?

Adjusted body weight (often abbreviated as ABW or AjBW) is a commonly used clinical estimate. When actual weight is significantly above ideal weight, calculations of drug dosages or caloric requirements based on total actual weight may give inaccurate results. Because adipose tissue has less water content and metabolic activity than muscle and organs, many drug dosages and nutritional requirements do not increase in proportion to total weight.

The adjusted body weight corrects this problem by starting with the ideal body weight and adding only a portion of the excess weight in some ratio. This ratio is referred to as the adjustment factor. The resulting reference weight is closer to the amount of metabolically active tissue than either an ideal or actual weight alone.

The formula for calculating adjusted body weight is:

The formula is very simple, it's based on the ideal weight and then adds to that the adjustment factor multiplied by the difference between actual weight and ideal weight.

In the formulas on this page, IBW is ideal body weight, ABW is actual body weight and f is a correction factor. The standard coefficient is 0.4 and accounts for 40% of excess weight. When used in nutritional therapy it is often changed to 0.25. If the actual body weight is not more than the ideal body weight, there is no excess to correct so the corrected body weight will be the same as the actual body weight.

First, calculate your ideal body weight.

Since adjusted body weight is based on ideal body weight, this tool first calculates the ideal body weight. The most common formula used to do so is Devine's formula, which was proposed in 1974 and originally intended for determining dosing of the antibiotic gentamicin.

In words: first take a reference weight for five feet tall, and then add 2.3 kg for each additional inch over five feet. Some other formulas use the same structure but with different constants. This tool allows you to choose between four of the most common ones, so that you can intuitively see how much the results vary depending on which formula is used.

The differences between the formulas are usually small and only a few kilograms, so the Devine formula is still used as standard for calculating medication dosages. The Robinson formula results are typically slightly lower while the Hamwi formula tends to give slightly higher values.

Example calculation:

For example, a man who is 180 centimeters tall and weighs 90 kilograms. The height corresponds to about 70.9 inches, which is 10.9 inches more than five feet. Ideal weight is calculated by adding 52 plus the product of 1.9 times 10.9, which comes out to be approximately 72.65 kilograms.

So for the dose calculation a body weight of about 79.6 kg is used, not the actual weight of 90 kg. The actual weight is about 17.4 kg above ideal weight but with an adjustment rate of 40 percent only about 7 kg of that will be taken into account.

What adjustment factor should be used?

The adjustment factor used depends on the particular application. For most medications and for estimating renal function using the creatinine clearance formula, a standard factor of 0.4 is commonly used. In nutritional support to estimate caloric and protein requirements, many nutritionists use 0.25, which reduces the amount of excess weight that is taken into account. Some plan to make adjustments only when actual weight is 20-30 percent above ideal body weight. If this threshold has not been reached, then actual weight is used. This tool indicates where the current value falls in relation to this threshold.

Applications for adjusted body weight

The concept of adjusted body weight comes from pharmacology. Water-soluble drugs such as aminoglycoside antibiotics are distributed mainly in fat-free tissues and body fluids, so dosing obese patients based on actual weight may increase the risk of side effects. Adjusted body weight serves as a basis for safer administration. The same principle also applies to dosage of some chemotherapy drugs and estimation of creatinine clearance since using actual weight could overestimate renal function.

In patients whose weight is significantly above the healthy range, nutritionists also use adjusted body weight to set goals for energy and protein. This is because a diet based on actual weight can lead to overnutrition. Regardless of when it's used, adjusted body weight is only an initial estimate, and clinical staff must verify this with specific medications, guidelines or nutritional plans.

Cases not applicable:

The adjusted body weight assumes that most of the excess weight is fat. This assumption does not hold for athletes with large amounts of muscle or people who work physically and have a higher than ideal amount of lean mass, so using the adjusted body weight would incorrectly subtract metabolically active tissues.

The adjusted body weight is also not suitable for people whose actual weight is equal to or below the ideal weight. In this case, there is no excess that needs to be adjusted. It should also not be used in pregnant women and children. The corresponding formulas for calculating the ideal weight have been validated for people with a height of more than 60 inches (about 152 centimeters). For shorter heights, the results are an extrapolation and therefore should be interpreted cautiously.

Additional key figures:

In addition to adjusted body weight this tool also displays some related metrics. Body mass index (BMI) compares weight and height and is used to classify weight categories. The body surface area, calculated using the Mosteller formula, is a metric that is used in determining chemotherapy doses as well as adjusting certain cardiovascular measurements for body size. Fat-free body mass, calculated using the Janmahasatian formula validated in 2005, allows for an estimation of mass directly without taking into account fat tissue. For some medications adjusted body weight may be less appropriate.

This tool is for general educational purposes only and does not constitute medical advice. The adjusted body weight is a basis for dose estimation only, and is no substitute for treatment decisions. Medication doses and nutritional goals should be reviewed by a pharmacist, doctor or dietitian. Professionals always take into account the specific medications and the overall clinical context.