Ideal Body Weight Calculator
Compare major ideal-body-weight formulas and estimate a practical body-weight range for height-based clinical and weight-planning use.
Formula
Devine IBW (male) = 50 + 2.3 x (inches over 60)
The Devine formula calculates IBW as 50 kg + 2.3 kg per inch over 5 feet for males, and 45.5 kg + 2.3 kg per inch over 5 feet for females. Robinson, Miller, and Hamwi formulas use different baseline weights and increments. All formulas are valid for adults 5 feet (60 inches) or taller.
Worked Examples
Example 1: IBW for Ventilator Settings (Male Patient)
Problem: Calculate ideal body weight for a 6-foot-tall male patient weighing 240 lbs who requires mechanical ventilation.
Solution: Height: 6 feet 0 inches = 72 inches total\nInches over 5 feet: 72 - 60 = 12 inches\nDevine IBW (male): 50 + 2.3(12) = 50 + 27.6 = 77.6 kg\nTidal volume range (6-8 mL/kg IBW): 466 - 621 mL\nActual weight: 240 lbs = 108.9 kg\nAdjusted BW: 77.6 + 0.4(108.9 - 77.6) = 77.6 + 12.5 = 90.1 kg\nPatient is 40.3% over IBW
Result: Devine IBW: 77.6 kg (171 lbs) | Tidal Volume: 466-621 mL | AdjBW: 90.1 kg
Example 2: Comparing All Four Formulas (Female Patient)
Problem: A 5-foot-4-inch female patient needs IBW calculated for aminoglycoside dosing.
Solution: Height: 5 feet 4 inches = 64 inches\nInches over 5 feet: 64 - 60 = 4 inches\nDevine: 45.5 + 2.3(4) = 54.7 kg\nRobinson: 49 + 1.7(4) = 55.8 kg\nMiller: 53.1 + 1.36(4) = 58.5 kg\nHamwi: 45.5 + 2.2(4) = 54.3 kg\nAverage: (54.7 + 55.8 + 58.5 + 54.3) / 4 = 55.8 kg\nRange: 54.3 - 58.5 kg
Result: Devine: 54.7 kg | Robinson: 55.8 kg | Miller: 58.5 kg | Hamwi: 54.3 kg | Avg: 55.8 kg
Frequently Asked Questions
What is ideal body weight and why is it important in medicine?
Ideal body weight (IBW) is a clinically estimated weight based on a person's height and sex, originally developed for medication dosing and ventilator settings rather than aesthetic goals. IBW is critical in pulmonology for calculating lung-protective tidal volumes during mechanical ventilation, since lung size correlates with height rather than actual body weight. It is also used in pharmacology for dosing medications with narrow therapeutic windows, such as aminoglycosides and chemotherapy agents. Using actual body weight in obese patients can lead to overdosing, while using IBW helps standardize treatment across different body compositions.
When should I use adjusted body weight instead of ideal body weight?
Adjusted body weight (AdjBW) is used when a patient's actual body weight significantly exceeds their ideal body weight, typically by more than 20-30%. The formula is AdjBW = IBW + 0.4 times (Actual Weight minus IBW). This correction factor acknowledges that obese patients have some additional lean tissue mass beyond their IBW that contributes to drug distribution. AdjBW is commonly used for dosing aminoglycosides, vancomycin, heparin, and some chemotherapy regimens in obese patients. Using IBW alone in severely obese patients may result in subtherapeutic dosing, while using actual body weight may cause toxicity.
How is ideal body weight used for ventilator tidal volume settings?
In mechanical ventilation, lung-protective ventilation strategy uses tidal volumes of 6 to 8 mL per kilogram of ideal body weight (IBW), not actual body weight. This is because lung size is determined by height, not by how much someone weighs. An obese patient who is 5 feet 6 inches tall has the same lung capacity as a lean patient of the same height. Setting tidal volumes based on actual weight in an obese patient would deliver excessive volumes, causing barotrauma, volutrauma, and ventilator-induced lung injury. The ARDSNet protocol specifically mandates using predicted (ideal) body weight for all tidal volume calculations.
Is ideal body weight the same as a healthy weight?
No, ideal body weight as calculated by these formulas is a clinical tool, not a health recommendation or aesthetic target. The formulas produce single-point estimates that do not account for individual variation in body composition, muscle mass, bone density, or ethnicity. A muscular athlete may weigh significantly more than their IBW while being perfectly healthy. Health professionals now recognize that a range of weights can be healthy for any given height, which is why BMI ranges (18.5 to 24.9) are preferred for general health assessment. IBW formulas remain valuable primarily for their original purpose of standardizing clinical calculations.
How does sex affect ideal body weight calculations?
All four IBW formulas produce lower estimates for females than males at the same height, reflecting average differences in body composition between sexes. Males typically have more lean muscle mass and larger bone structures, resulting in a higher baseline weight. The Devine formula starts males at 50 kg and females at 45.5 kg at 5 feet, a difference of 4.5 kg that is maintained at every height since both sexes use the same per-inch increment of 2.3 kg. In clinical practice, using the correct sex-based formula is essential for accurate drug dosing and ventilator settings, as using the wrong formula could result in a 10% dosing error.
Can ideal body weight be used for nutritional assessment?
IBW is sometimes used in nutritional assessment to determine a patient's percentage of ideal body weight, which helps classify nutritional status. A patient at less than 80% of IBW may be considered severely malnourished, 80-90% moderately malnourished, and 90-110% within normal range. However, this approach has significant limitations. Modern nutritional assessment relies more on BMI, body composition analysis, serum albumin and prealbumin levels, muscle function tests, and clinical assessment tools like the Subjective Global Assessment. IBW-based nutritional assessment should be used alongside other indicators rather than as a standalone metric.