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Amputation Adjusted BMI Calculator
Calculate conventional BMI and amputation-adjusted BMI with Mellor/Garrison segment weight correction, interactive body map, metric and imperial units.
Segments selected 0
No amputated segments selected
Click the missing limb segments on the body map
Estimated original weight kg
Missing body mass %
The BMI category is unchanged
Formula: adjusted weight = current weight / (1 - sum of missing segment fractions). Adjusted BMI = adjusted weight / height^2.
Segment logic: selecting a proximal level automatically includes the distal missing parts; selecting a distal part off also clears incompatible proximal selections.
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Frequently Asked Questions
Why does standard BMI underestimate BMI after an amputation?
Standard BMI divides the current body weight by height squared. After limb loss, current body weight is lower because a body segment is missing, so the calculation can make a person appear leaner than their pre-amputation equivalent body mass would indicate.
What does adjusted body weight mean in this calculator?
Adjusted body weight is an estimate of the person's theoretical full-body weight before the missing limb mass was removed. It is calculated as current weight divided by one minus the sum of missing segment percentages.
Can this replace a dietitian or rehabilitation assessment?
No. It is a screening and documentation aid. Clinical decisions should also consider edema, fluid shifts, prosthesis use, residual limb changes, body composition, appetite, labs and functional goals.
Can I select multiple amputations?
Yes. Select every missing segment on the body map. The calculator sums the selected segment percentages before estimating original body weight and adjusted BMI.
Amputation adjusted BMI in one screen
Corrects the weight denominator problem: conventional BMI uses the measured post-amputation weight, while adjusted BMI estimates the equivalent full-body weight before limb loss.
Uses Mellor/Garrison segment fractions: upper arm 3.5%, forearm 2.3%, hand 0.8%, thigh 11.6%, lower leg 5.3% and foot 1.8% per side, with distal segments linked automatically.
Shows category movement: the comparison panel displays whether the correction changes the person from normal weight to overweight, or from overweight to an obesity class.
Works in metric and imperial units: inputs can be entered in centimeters and kilograms or in feet/inches and pounds while the calculation remains metric internally.
# Why BMI needs adjustment after limb loss
Body Mass Index is calculated as weight divided by height squared. In an adult without limb loss, weight is intended to represent the whole body. In an amputee, the measured weight is lower because part of the body is absent. If that lower measured weight is used directly, BMI can be artificially low and the person may be placed in a lighter category than their estimated full-body equivalent would suggest.The practical consequence is not cosmetic. BMI categories are used in nutrition screening, rehabilitation documentation, medication dosing discussions, pressure injury risk review, cardiometabolic risk conversations and eligibility criteria for some programs. A transtibial or transfemoral amputation can move the adjusted BMI enough to change the clinical category, especially near thresholds such as 25, 30 or 35 kg/m2.Do not add the missing limb weight as a fixed number
The missing mass is estimated as a percentage of total body weight, not as the same number of kilograms for every person. A missing lower leg represents a different absolute mass in a 50 kg person than in a 110 kg person, so the correction uses a fraction-based formula.
| Situation | Conventional BMI problem | Adjusted BMI purpose |
|---|---|---|
| Recent limb loss | Measured weight drops abruptly | Estimate comparable pre-amputation body weight |
| Long-term amputee follow-up | Weight trends may be hard to compare with standard BMI charts | Use a consistent anthropometric reference |
| Bilateral limb loss | The underestimation can be larger | Sum segment percentages before correction |
| Near a BMI threshold | A small correction may change category | Document category based on adjusted BMI |
# Mellor/Garrison method and segment percentages
The calculator applies the commonly cited Mellor/Garrison approach for body weight adjustment in people with limb loss. Each missing segment is represented by an estimated percentage of total body mass. The selected percentages are summed, then current body weight is divided by the remaining body fraction. This gives an estimated original weight that can be used in the BMI formula.| Segment selected | Fraction of body weight | Clinical interpretation |
|---|---|---|
| Upper arm, shoulder to elbow | 3.5% | Use when the humeral segment is absent or effectively not part of measured body mass |
| Forearm, elbow to wrist | 2.3% | Use for forearm loss in addition to hand loss when both are absent |
| Hand | 0.8% | Use for partial upper extremity loss limited to the hand level |
| Thigh | 11.6% | Represents the largest single selectable segment in this tool |
| Lower leg, below knee | 5.3% | Often relevant for transtibial amputation calculations |
| Foot | 1.8% | Use for foot-level amputation when the lower leg remains present |
Selecting complete limb loss
Select the highest missing level on the limb. The interface automatically includes the distal absent parts: upper arm adds forearm and hand, forearm adds hand, thigh adds lower leg and foot, and lower leg adds foot. Their upper-limb total is 6.6% in this segmented implementation, which approximates the commonly rounded full upper limb value of 6.5%. A full lower extremity totals 18.7%, close to the rounded full lower limb value of 18.5%. 6.5% Rounded whole upper limb estimate
18.5% Rounded whole lower limb estimate
kg/m2 BMI unit after weight correction
# How the formula works step by step
The formula starts with the measured post-amputation weight. Suppose a person weighs 70 kg and has a unilateral lower-leg segment selected at 5.3%. The remaining body fraction is 1 - 0.053 = 0.947. The estimated original weight is 70 / 0.947 = 73.9 kg. If height is 1.70 m, conventional BMI is 24.2 kg/m2 and adjusted BMI is 25.6 kg/m2.Core equation
Estimated original weight = current weight / (1 - missing fraction). Adjusted BMI = estimated original weight / height^2. The missing fraction must be entered as a decimal, so 5.3% becomes 0.053.
- Enter current measured weight, not a guessed pre-amputation weight.
- Select the most proximal missing level on every affected side; the interface will add the distal missing segments.
- Use current adult height unless spinal deformity, contracture or measurement limitations require an alternative clinical height estimate.
- Compare both BMI values when documenting the result, because the difference explains why the correction was applied.
Why adjusted weight is higher than current weight
The adjusted weight is not a target weight and it is not the weight of the prosthesis. It is the theoretical full-body weight that would correspond to the current measured weight if the missing body segment were still present.
# BMI category thresholds used by the calculator
| Category | BMI range | Use in adjusted BMI documentation |
|---|---|---|
| Underweight | < 18.5 | May support nutrition risk review when combined with intake, weight history and clinical signs |
| Normal weight | 18.5 to 24.9 | Can become overweight after adjustment if missing mass is clinically meaningful |
| Overweight | 25.0 to 29.9 | Common threshold affected by unilateral lower-limb correction |
| Obesity class I | 30.0 to 34.9 | May influence cardiometabolic risk documentation and care planning |
| Obesity class II | 35.0 to 39.9 | Often requires more detailed comorbidity and mobility assessment |
| Obesity class III | >= 40.0 | Use cautiously and interpret with full clinical context |
Conventional BMI
Uses measured post-amputation weight directly. It is simple but can underestimate the category because missing limb mass has already been removed from the numerator.
- Best for quick uncorrected comparison
- May be misleading after major limb loss
- Does not show estimated original weight
Adjusted BMI
Uses estimated original body weight derived from missing segment percentages. It is more appropriate for anthropometric classification in people with amputations.
- Better category screening after limb loss
- Requires accurate segment selection
- Still does not measure body composition
Strengths and limitations
Advantages
- Corrects a known mathematical bias in standard BMI after limb loss.
- Can be calculated quickly at bedside or during outpatient review.
- Makes BMI category changes transparent by showing conventional and adjusted values together.
- Supports consistent documentation across nutrition, rehabilitation and medical notes.
Disadvantages
- Segment percentages are population estimates, not a scan of the individual person.
- Edema, fluid retention and residual limb volume changes can distort measured weight.
- BMI still cannot distinguish fat mass from lean mass or muscle atrophy.
- Partial amputations may not map perfectly onto the simplified segment choices.
# Clinical uses in nutrition and rehabilitation
Clinical dietitians often need an anthropometric frame for deciding whether weight loss, weight maintenance or weight gain is appropriate. In an amputee, a low conventional BMI can hide excess adiposity, while a high adjusted BMI may alter the conversation about energy intake, protein targets, wound healing, mobility and cardiometabolic risk.Rehabilitation teams may use adjusted BMI alongside gait tolerance, prosthetic fit, residual limb skin integrity and functional goals. A person with transfemoral limb loss may have reduced activity expenditure during early rehabilitation, but also needs adequate protein and energy for tissue maintenance. The adjusted BMI is one piece of that larger assessment.Documentation wording
A practical note can read: "Measured weight 70.0 kg, height 170 cm, unilateral transtibial segment correction 5.3%, estimated original weight 73.9 kg, adjusted BMI 25.6 kg/m2." This keeps the method auditable.
Important clinical caution
Use adjusted BMI as a screening calculation, not as a standalone diagnosis. In people with limb loss, body composition, muscle atrophy, edema, medications, pressure injury risk, appetite, prosthesis use and cardiometabolic markers may matter more than BMI alone.
# Common measurement mistakes
Including prosthesis weight without a protocol
If the measured weight includes a prosthesis, the result may overestimate body weight unless prosthesis mass is subtracted or your institution uses a specific protocol. For consistent follow-up, record whether the prosthesis was worn during weighing.
- Trying to represent a forearm-level absence with the hand alone instead of selecting the forearm level.
- Trying to represent a whole leg conceptually without marking the thigh level, which automatically includes lower leg and foot.
- Using pounds in a metric field or kilograms in an imperial field when copying values from a chart.
- Comparing adjusted BMI from one visit with conventional BMI from another visit without noting the method.
- Ignoring acute postoperative swelling or fluid shifts that make measured weight unstable.
- Current weight
- The person's measured body weight at the time of assessment, after limb loss.
- Missing fraction
- The summed estimated percentage of total body mass represented by the selected absent segments.
- Estimated original weight
- The theoretical full-body weight calculated by dividing current weight by the remaining body fraction.
- Adjusted BMI
- BMI calculated with estimated original weight instead of current measured weight.
- Residual limb
- The remaining part of a limb after amputation, which can change volume during healing and prosthetic adaptation.
# Worked examples by amputation level
| Example | Selected segments | Missing fraction | Likely impact |
|---|---|---|---|
| Unilateral hand amputation | Hand | 0.8% | Small BMI increase, usually category change only near a threshold |
| Unilateral below-knee pattern | Lower leg + foot | 7.1% | Moderate increase; category changes are common near BMI 25 or 30 |
| Unilateral above-knee pattern | Thigh + lower leg + foot | 18.7% | Large increase; conventional BMI may substantially underestimate classification |
| Bilateral lower-leg pattern | Both lower legs + both feet | 14.2% | Large enough to require careful method documentation |
How to handle borderline cases
When adjusted BMI is within about 0.5 kg/m2 of a category threshold, document the exact inputs and avoid overinterpreting the category label. Measurement error in weight, height or segment choice may be larger than the apparent difference from the threshold.Best practice checklist
Measure weight consistently, noting prosthesis use and fluid status.
Select the highest absent level on the affected side or sides and let the dependent distal segments follow.
Report conventional BMI and adjusted BMI together when the correction affects interpretation.
Use the adjusted BMI category as a screening classification, then confirm clinical decisions with full assessment.
Bibliographic References
- [1] Mellor GE. Body weight adjustments for amputees. Journal of the American Dietetic Association, 1984
https://pubmed.ncbi.nlm.nih.gov/6736384/
- [2] Garrison DM. Calculation of body mass index in patients with limb loss. Archives of Physical Medicine and Rehabilitation, 1996
https://pubmed.ncbi.nlm.nih.gov/8607766/
- [3] World Health Organization: Obesity and overweight fact sheet
https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight
- [4] CDC: About adult BMI
https://www.cdc.gov/bmi/adult-calculator/index.html
- [5] Wikipedia: Human body weight
https://en.wikipedia.org/wiki/Human_body_weight
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