Standard Body Mass Index (BMI) calculations are fundamentally inaccurate for individuals with limb loss. BMI is a calculation based on mass relative to height. When a limb is missing, the total body mass (weight) is reduced, but the person's height typically remains the same (or is measured as if the limb were present). This results in a significantly lower BMI score that may incorrectly classify an overweight or healthy individual as underweight.
How This Calculator Works
To obtain an accurate BMI for an amputee, we must estimate what the individual would weigh if their limb were present. This is done using established body segment weight proportions (Osterkamp & generic medical standards). This calculator uses the following formula:
Estimated Body Weight = Current Weight / (1 – (Total Missing Percentage / 100))
Once the estimated total body weight is calculated, the standard BMI formula is applied:
BMI = Estimated Weight (kg) / Height (m)²
Body Segment Weight Percentages
This tool uses the following standard percentages of total body weight for calculations:
Body Segment
% of Total Body Weight
Hand
0.7%
Forearm & Hand (Below Elbow)
2.3%
Entire Arm
5.0%
Foot
1.5%
Lower Leg & Foot (Below Knee)
5.9%
Entire Leg (Above Knee)
16.0%
Interpreting Your Results
The resulting BMI is categorized according to World Health Organization (WHO) standards:
Underweight: BMI less than 18.5
Normal Weight: BMI between 18.5 and 24.9
Overweight: BMI between 25 and 29.9
Obesity: BMI of 30 or greater
Note: This calculator provides an estimate for general health tracking. Muscle density, bone structure, and specific amputation nuances can affect individual results. Always consult a healthcare professional for precise medical assessments.
var currentUnit = 'imperial';
function setUnit(unit) {
currentUnit = unit;
var imperialDiv = document.getElementById('imperial-inputs');
var metricDiv = document.getElementById('metric-inputs');
var btnImperial = document.getElementById('btn-imperial');
var btnMetric = document.getElementById('btn-metric');
if (unit === 'imperial') {
imperialDiv.classList.remove('hidden');
metricDiv.classList.add('hidden');
btnImperial.classList.add('active');
btnMetric.classList.remove('active');
} else {
imperialDiv.classList.add('hidden');
metricDiv.classList.remove('hidden');
btnImperial.classList.remove('active');
btnMetric.classList.add('active');
}
// Hide results when switching to avoid confusion
document.getElementById('result-box').style.display = 'none';
}
function calculateAmputeeBMI() {
// 1. Get Weight and Height and convert to Metric (kg / m)
var weightKg = 0;
var heightM = 0;
if (currentUnit === 'imperial') {
var lbs = parseFloat(document.getElementById('weight_lbs').value);
var ft = parseFloat(document.getElementById('height_ft').value);
var inc = parseFloat(document.getElementById('height_in').value);
if (isNaN(lbs) || isNaN(ft)) {
alert("Please enter valid weight and height values.");
return;
}
if (isNaN(inc)) inc = 0;
weightKg = lbs / 2.20462;
var totalInches = (ft * 12) + inc;
heightM = totalInches * 0.0254;
} else {
var kg = parseFloat(document.getElementById('weight_kg').value);
var cm = parseFloat(document.getElementById('height_cm').value);
if (isNaN(kg) || isNaN(cm)) {
alert("Please enter valid weight and height values.");
return;
}
weightKg = kg;
heightM = cm / 100;
}
if (heightM <= 0 || weightKg <= 0) {
alert("Please enter values greater than zero.");
return;
}
// 2. Calculate Missing Percentage
var missingPct = 0;
missingPct += parseFloat(document.getElementById('amp_upper_left').value);
missingPct += parseFloat(document.getElementById('amp_upper_right').value);
missingPct += parseFloat(document.getElementById('amp_lower_left').value);
missingPct += parseFloat(document.getElementById('amp_lower_right').value);
// 3. Calculate Estimated Total Body Weight
// Formula: Estimated = Current / (1 – %missing)
var fractionPresent = 1 – (missingPct / 100);
// Safety check for extreme edge case (100% missing)
if (fractionPresent <= 0.1) fractionPresent = 0.1;
var estimatedTotalWeightKg = weightKg / fractionPresent;
// 4. Calculate BMI
var bmi = estimatedTotalWeightKg / (heightM * heightM);
bmi = Math.round(bmi * 10) / 10; // Round to 1 decimal
// 5. Determine Category
var category = "";
var catClass = "";
if (bmi = 18.5 && bmi = 25 && bmi <= 29.9) {
category = "Overweight";
catClass = "cat-overweight";
} else {
category = "Obese";
catClass = "cat-obese";
}
// 6. Display Results
var displayWeight = "";
if (currentUnit === 'imperial') {
var estLbs = estimatedTotalWeightKg * 2.20462;
displayWeight = Math.round(estLbs) + " lbs";
} else {
displayWeight = Math.round(estimatedTotalWeightKg * 10) / 10 + " kg";
}
document.getElementById('res_bmi').innerText = bmi;
document.getElementById('res_est_weight').innerText = displayWeight;
var catDiv = document.getElementById('res_category');
catDiv.innerText = category;
catDiv.className = "bmi-category " + catClass;
document.getElementById('result-box').style.display = 'block';
}