The Body Mass Index (BMI) is a simple and widely used tool to assess an individual's weight status relative to their height. It helps categorize whether a person is underweight, a healthy weight, overweight, or obese. Developed by Adolphe Quetelet in the 19th century, it's a screening tool that provides an indication of health, rather than a definitive diagnosis.
In the UK, the National Health Service (NHS) uses BMI as a primary indicator for public health and as a starting point for discussions about weight-related health risks. While BMI doesn't measure body fat directly or distinguish between muscle and fat, it correlates well with body fat percentage for most individuals and is a useful general measure.
This calculator uses the second formula, converting your input in kilograms and centimetres directly into a BMI value.
BMI Categories (According to NHS Guidelines)
Underweight: Below 18.5
Healthy weight: 18.5 to 24.9
Overweight: 25 to 29.9
Obese: 30 and above
It's important to note that these categories are general guidelines. For example, individuals with a high muscle mass might have a BMI in the overweight or obese range but still have low body fat. Conversely, some older adults or those with lower muscle mass might have a BMI in the healthy range but still have excess body fat.
Why is BMI Important?
Maintaining a healthy weight is crucial for overall well-being. Being in the overweight or obese categories can increase the risk of several serious health conditions, including:
Type 2 diabetes
Heart disease
High blood pressure
Certain types of cancer
Stroke
Osteoarthritis
Conversely, being significantly underweight can also lead to health problems such as nutritional deficiencies, weakened immune system, and osteoporosis.
This BMI calculator is a tool to help you understand your current weight status. If you have concerns about your weight or health, it is always recommended to consult with a healthcare professional. They can provide personalized advice and consider other health factors beyond your BMI.
function calculateBMI() {
var weightInput = document.getElementById("weight");
var heightInput = document.getElementById("height");
var resultDiv = document.getElementById("result");
var bmiCategoryDiv = document.getElementById("bmi-category");
var weight = parseFloat(weightInput.value);
var height = parseFloat(heightInput.value);
resultDiv.innerHTML = "; // Clear previous result
bmiCategoryDiv.innerHTML = "; // Clear previous category
bmiCategoryDiv.className = "; // Reset class
if (isNaN(weight) || isNaN(height) || weight <= 0 || height <= 0) {
resultDiv.innerHTML = 'Please enter valid positive numbers for weight and height.';
return;
}
// BMI calculation: (weight in kg / (height in cm * height in cm)) * 10000
var bmi = (weight / (height * height)) * 10000;
// Round BMI to one decimal place
var bmiRounded = bmi.toFixed(1);
resultDiv.innerHTML = 'Your BMI is: ' + bmiRounded + '';
var category = "";
var categoryClass = "";
if (bmi = 18.5 && bmi = 25 && bmi = 30
category = "Obese";
categoryClass = "category-obese";
}
bmiCategoryDiv.innerHTML = 'Category: ' + category;
bmiCategoryDiv.className = categoryClass;
}