This calculator computes ideal body weight (IBW) with the Devine formula from height and sex. IBW is the dosing weight for the Cockcroft-Gault creatinine clearance equation when actual weight is from ideal up to 130% of ideal.
What Is Ideal Body Weight?
Ideal body weight (IBW) is a height-and-sex-based reference weight used to standardise drug-dosing and renal-function calculations, not a body-composition measurement or a personal weight goal. The Devine formula, introduced in 1974 for aminoglycoside dosing, became the standard weight input for the Cockcroft-Gault creatinine clearance equation. Using IBW rather than total body weight keeps the clearance estimate from being inflated by fat mass, which contributes little to creatinine production or filtration.
The Devine Formula
Devine IBW depends only on height and sex:
- Men: 50 kg + 2.3 kg per inch over 5 feet (60 inches).
- Women: 45.5 kg + 2.3 kg per inch over 5 feet (60 inches).
The 2.3 kg per inch increment is the same for both sexes; only the baseline at 5 feet differs. Below 5 feet the formula is not validated, so clinicians often default to actual weight.
Worked Example
For a man who is 5 feet 10 inches (70 inches), there are 10 inches over 5 feet. IBW = 50 + 2.3 × 10 = 73 kg. For a woman of the same height, IBW = 45.5 + 2.3 × 10 = 68.5 kg. You would enter these IBW values as the weight in the Cockcroft-Gault equation when the patient's actual weight is from IBW up to 130% of IBW. For the man, that range runs from 73 kg up to about 94.9 kg.
IBW by Height
A few reference points show how IBW scales with height:
| Height | Men | Women |
|---|---|---|
| 5′ 2″ (62 in) | 54.6 | 50.1 |
| 5′ 6″ (66 in) | 63.8 | 59.3 |
| 5′ 10″ (70 in) | 73.0 | 68.5 |
| 6′ 2″ (74 in) | 82.2 | 77.7 |
Choosing the Dosing Weight
IBW is one of three weights used in renal dosing, and the choice depends on how actual weight compares with ideal:
| Actual weight | Weight to use |
|---|---|
| Below IBW | actual (total) body weight |
| From IBW up to 130% of IBW | ideal body weight (IBW) |
| More than 130% of IBW | adjusted body weight (AjBW) |
When actual weight is more than 130% of IBW, use adjusted body weight instead, because total body weight overestimates clearance. When actual weight is below IBW, use the actual weight, since IBW would then credit the patient with more weight than they carry. See creatinine clearance in obesity for detail.
This rule follows Winter and colleagues (2012), who compared Cockcroft-Gault estimates with measured 24-hour creatinine clearance in 3,678 patients. Actual body weight gave an unbiased estimate in underweight patients, and IBW gave an unbiased estimate in patients of normal weight. Adjusted body weight with a 0.4 factor was the least biased and most accurate option in overweight, obese, and morbidly obese patients. The FDA 2024 guidance on pharmacokinetics in renal impairment likewise addresses overweight or obese people. It states that using IBW or adjusted body weight in Cockcroft-Gault is likely to estimate kidney function more accurately than actual body weight. Some hospitals switch to adjusted body weight at 120% of IBW rather than 130%. Trial protocols and drug labels can also set their own rule, which then takes priority; SWOG Policy 38, for example, uses the patient's weight but caps it at 140% of IBW.
Limitations and Edge Cases
- Height under 5 feet: the formula is not validated; default to actual weight or clinical judgement.
- Actual weight above 130% of IBW: IBW alone underestimates the dosing weight; switch to adjusted body weight.
- Amputation: height-based IBW will not reflect true body size; adjust clinically.
- Children: the Devine formula is for adults; pediatric dosing uses weight-based and Schwartz-based methods.
- Not a health target: IBW is a dosing reference, not a goal weight or measure of body fat.
Why Ideal Body Weight Matters for Creatinine Clearance
The Cockcroft-Gault equation multiplies clearance by body weight, so the weight you choose directly scales the result. Creatinine is produced mainly by muscle, and adipose tissue contributes comparatively little. Using total body weight in an obese patient therefore credits the kidneys with clearing more than they do, which overestimates creatinine clearance and, in turn, the safe drug dose. Ideal body weight avoids that error in patients of normal weight by tying the weight input to a lean reference rather than to accumulated fat. This is why the Devine formula, originally devised for aminoglycoside dosing, became the conventional weight term for the equation.
Where the Formula Comes From
Devine published the formula in 1974 as a practical rule for dosing gentamicin, an aminoglycoside antibiotic with a narrow safety margin. The values, a baseline at 5 feet plus a fixed increment per inch, were chosen to give a reproducible, height-based reference rather than to model body composition precisely. Decades of use in renal-dosing and pharmacokinetic work have kept it as the default, even though it is an approximation. Because it depends only on height and sex, two people of the same height and sex always share the same IBW regardless of their actual weight, which is what makes it a stable dosing reference.
Worked Comparison: IBW vs Actual Weight
Consider a man who is 5 feet 10 inches tall with an actual weight of 110 kg. His IBW is 73 kg. Feeding 110 kg into Cockcroft-Gault would inflate the estimated clearance relative to feeding 73 kg, because the extra 37 kg is largely fat that does little to clear creatinine. At about 151% of his 73 kg IBW, 110 kg is well above the 130% cut point of about 94.9 kg. The correct input here is therefore adjusted body weight rather than IBW alone, and it adds back 40% of that excess: 73 + 0.4 × 37 = about 87.8 kg. For a patient whose actual weight is from IBW up to 130% of IBW, the IBW value itself is the right input.
Common Questions About IBW
Ideal body weight is often mistaken for a target weight, when it is a dosing and calculation reference. Two people of the same height and sex share the same Devine IBW even if one is lean and the other carries extra weight, because the formula never sees actual weight. This design gives a stable, reproducible number for plugging into the Cockcroft-Gault equation. Another common question is which weight to use when actual weight sits close to IBW. From IBW up to 130% of IBW, IBW itself is the right input; only once actual weight climbs past 130% of IBW does the calculation switch to adjusted body weight. For body-composition questions rather than dosing, lean body weight is the relevant measure.