Body Mass Index (BMI) is calculated from total body weight divided by height squared. It's a simple formula, and it assumes an intact body — every limb present and accounted for. After amputation, that assumption breaks down.
A meaningful percentage of total body mass is missing after limb loss. A below-knee amputation removes roughly 4.4% of total body weight; an above-knee amputation removes about 9.9%. An above-elbow amputation removes about 3.35%, and a below-elbow amputation about 2.3%. Bilateral amputations roughly double the relevant figure, and mixed presentations (a different level on each side, or amputations affecting more than one limb) stack additively.
Because that mass is simply gone, the number on a scale is lower than it would be with all limbs intact. That pulls standard BMI down — sometimes into a range that reads as "underweight," or falsely "normal," when a person's actual body composition and health risk profile would place them elsewhere on the scale entirely. For someone with bilateral above-knee amputations, this isn't a rounding error — it can be a difference of several BMI points, enough to flip a clinical interpretation entirely.
This matters beyond a number on a chart. BMI (corrected or not) often factors into conversations about nutrition, cardiovascular risk screening, prosthetic fitting and weight-bearing load calculations, medication dosing guidance, and surgical risk assessment. An artificially low BMI can mask a weight-related health risk, or lead to a mismatch between a patient's actual body composition and the assumptions baked into a clinical tool or guideline that wasn't built with amputees in mind.
This calculator uses published body-segment weight tables (the Winter/Dempster biomechanics research referenced in our Research & Method page) to estimate what total body weight would be with the missing limb segment(s) restored, then recalculates BMI from that estimate. The result — corrected BMI — gives patients and clinicians a more accurate starting point for those conversations.
This is an educational and reference tool, not a diagnostic device. It's meant to support — not replace — a conversation with your healthcare provider.