Body & Weight
BMI Calculator
Work out your Body Mass Index in seconds and see where you sit on the standard WHO weight categories.
BMI Calculator Tool
What is BMI?
Body Mass Index, or BMI, is a simple screening number that compares your weight to your height. Doctors, dietitians and public health bodies such as the World Health Organization have used it for decades because it takes seconds to calculate and needs nothing more than a scale and a tape measure. Our BMI calculator does the arithmetic for you: enter your height and weight in either metric or imperial units and you'll instantly see your BMI value along with the category it falls into — underweight, healthy weight, overweight or obesity.
How BMI is calculated
The formula is simply weight in kilograms divided by height in metres squared (kg/m²). If you prefer pounds and inches, the calculator automatically converts your entries behind the scenes, so you never need to remember the imperial version of the equation yourself. Because the formula only uses two inputs, BMI is fast to compute but it cannot see the difference between muscle and fat, which is why it works best as a starting point rather than a final verdict on your health.
Reading your result
A BMI under 18.5 is generally classed as underweight, 18.5–24.9 as a healthy weight, 25–29.9 as overweight, and 30 or above as obesity. These bands are population-level guidelines, not a diagnosis. Athletes with a lot of muscle mass, older adults who have lost muscle, and people of different ethnic backgrounds can all fall outside the 'expected' pattern while still being perfectly healthy, or vice versa. If your number surprises you, it's worth discussing it with a doctor rather than reacting to the label alone.
Why people use a BMI calculator
BMI is often the first metric requested at a medical check-up, before starting a fitness programme, or when comparing progress over time. Because it is quick and free to calculate, it also pairs well with the other tools on LifeViora — once you know your BMI, our Calorie Calculator and Body Fat Calculator can help you build a fuller picture of your overall body composition and daily energy needs.
Frequently Asked Questions
Is BMI accurate for everyone?
BMI is a useful population-level screening tool, but it does not directly measure body fat. It can misclassify very muscular people as overweight, and it may understate risk in people with low muscle mass. Use it alongside measurements like waist circumference or body fat percentage for a fuller picture.
What is a healthy BMI range?
For most adults, a BMI between 18.5 and 24.9 is considered a healthy weight range according to World Health Organization guidelines. Ranges can vary slightly for children, teenagers, older adults and some ethnic groups.
Does BMI apply to children?
No. Children and teenagers need age- and sex-specific growth charts rather than the adult BMI categories, since their bodies are still developing. Speak to a pediatrician for guidance on a child's healthy weight.
Can I use BMI to track fat loss?
BMI can show a general trend if your height stays constant, but it won't tell you whether the weight you lost was fat or muscle. Pairing it with our Body Fat Calculator gives a more complete view of body composition changes.
How often should I check my BMI?
There's no strict rule — many people check every few months or whenever their weight changes meaningfully. BMI changes slowly, so daily checks aren't necessary.