The Body Mass Index (BMI) is a widely used metric to categorize a person's weight relative to their height. It provides a general indication of whether an individual is underweight, normal weight, overweight, or obese. The standard formula for BMI is:
BMI = Weight (kg) / (Height (m))^2
In this reverse calculator, we work backward from a desired BMI value to determine either the required weight for a given height, or the required height for a given weight. This can be useful for setting realistic health goals or understanding the physical changes needed to reach a specific BMI category.
How the Reverse Calculation Works
We rearrange the standard BMI formula to solve for the unknown variable (either weight or height).
To calculate required Weight (kg) for a target BMI and given Height (m):
Weight (kg) = Target BMI * (Height (m))^2
To calculate required Height (m) for a target BMI and given Weight (kg):
Height (m) = sqrt(Weight (kg) / Target BMI)
Note that in this calculator, height is accepted in centimeters (cm) but converted to meters (m) for the calculation, as per the standard BMI formula.
Use Cases for the BMI Reverse Calculator
Goal Setting: Individuals can determine the exact weight they need to be to achieve a healthy BMI, given their current height.
Understanding Proportions: It helps visualize how much weight needs to be gained or lost to reach a specific BMI category.
Health Planning: Useful for fitness professionals and individuals planning weight management strategies.
Height Adjustment (Theoretical): While height is fixed, theoretically, one could determine what height would be needed to maintain a certain BMI with a specific weight.
BMI is a screening tool and does not account for body composition (muscle mass vs. fat mass). Consult with a healthcare professional for personalized health advice.
function calculateReverseBMI() {
var bmiValue = parseFloat(document.getElementById("bmiValue").value);
var calculationType = document.getElementById("calculationType").value;
var resultDiv = document.getElementById("result");
var result = "";
if (isNaN(bmiValue) || bmiValue <= 0) {
resultDiv.innerHTML = "Please enter a valid Target BMI Value.";
return;
}
if (calculationType === "weight") {
var heightCm = parseFloat(document.getElementById("heightForWeight").value);
if (isNaN(heightCm) || heightCm <= 0) {
resultDiv.innerHTML = "Please enter a valid Height in centimeters.";
return;
}
var heightM = heightCm / 100; // Convert cm to meters
var calculatedWeight = bmiValue * (heightM * heightM);
result = "To achieve a BMI of " + bmiValue.toFixed(1) + " with a height of " + heightCm.toFixed(0) + " cm, your weight should be approximately " + calculatedWeight.toFixed(1) + " kg.";
} else { // calculationType === "height"
var weightKg = parseFloat(document.getElementById("weightForHeight").value);
if (isNaN(weightKg) || weightKg <= 0) {
resultDiv.innerHTML = "Please enter a valid Weight in kilograms.";
return;
}
// Ensure BMI is not zero to avoid division by zero
if (bmiValue === 0) {
resultDiv.innerHTML = "Target BMI cannot be zero.";
return;
}
var heightM = Math.sqrt(weightKg / bmiValue);
var heightCm = heightM * 100;
result = "To achieve a BMI of " + bmiValue.toFixed(1) + " with a weight of " + weightKg.toFixed(1) + " kg, your height should be approximately " + heightCm.toFixed(1) + " cm.";
}
resultDiv.innerHTML = result;
}
function toggleInputFields() {
var calculationType = document.getElementById("calculationType").value;
var weightInputGroup = document.getElementById("weightInputGroup");
var heightInputGroup = document.getElementById("heightInputGroup");
if (calculationType === "weight") {
weightInputGroup.style.display = "flex";
heightInputGroup.style.display = "none";
} else { // calculationType === "height"
weightInputGroup.style.display = "none";
heightInputGroup.style.display = "flex";
}
}
// Initial setup and event listener for dropdown change
document.getElementById("calculationType").addEventListener("change", toggleInputFields);
toggleInputFields(); // Call once on load to set initial state