This calculator computes ideal body weight (IBW) with the Devine formula from height and sex. IBW is the default dosing weight for the Cockcroft–Gault creatinine clearance equation in normal-to-lean patients.
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. These IBW values are what you would enter as the weight in the Cockcroft–Gault equation for a normal-to-lean patient.
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:
| Situation | Weight to use |
|---|---|
| Actual weight below IBW | actual (total) body weight |
| Actual roughly normal-to-lean | ideal body weight (IBW) |
| Obese (actual > ~1.2× IBW) | adjusted body weight (AjBW) |
When a patient is obese, use adjusted body weight instead, because total body weight overestimates clearance; when actual weight is below IBW, use the actual weight so the estimate is not inflated. See creatinine clearance in obesity for detail.
Limitations and Edge Cases
- Height under 5 feet: the formula is not validated; default to actual weight or clinical judgement.
- Obesity: 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, so using total body weight in an obese patient credits the kidneys with clearing more than they actually do — overestimating creatinine clearance and, in turn, the safe drug dose. Ideal body weight sidesteps that error in normal-to-lean patients by tying the weight input to a lean reference rather than to accumulated fat. This is exactly 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. Since 110 kg is more than 1.2× his 73 kg IBW, the correct choice here is not IBW alone but adjusted body weight, which adds back 40% of that excess. For a normal-weight patient near IBW, the IBW value itself is the right input.
Common Questions About IBW
A frequent point of confusion is that ideal body weight looks like a target weight, when it is really 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. That is a feature, not a flaw: it 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 — in that normal-to-lean range, IBW itself is the right input, and only once actual weight climbs past roughly 1.2× IBW does the calculation switch to adjusted body weight. For body-composition questions rather than dosing, lean body weight is the relevant measure.