BMI Calculator
Check your BMI and healthy range without overthinking the math
BMI, categories and healthy range FAQ
What exactly is BMI measuring?
Body mass index (BMI) is a weight-for-height calculation. In metric units, it is your weight in kilograms divided by your height in metres squared. In US units, the same idea is used with a constant so the result is still expressed as kg/m². It is a simple way to group adults into broad categories such as underweight, healthy weight, overweight and obesity.
Is BMI the same thing as body fat percentage?
No. BMI does not directly measure body fat. It does not know whether your weight comes from muscle, bone, water or fat. Two people with the same BMI can have very different body-fat levels and health risks. That is why many guidelines say BMI should be a starting point, used along with things like waist measurements, blood tests, fitness and medical history.
What BMI counts as a “healthy” range for most adults?
Many public-health sources describe a general adult “healthy” range as a BMI from about 18.5 to 24.9. Below 18.5 is often called underweight, 25.0–29.9 overweight, and 30 or higher obesity. Different organisations break obesity into classes such as 30–34.9, 35–39.9 and 40+. These cut-offs are based on population risk and are not a verdict on any one person.
Who is BMI less useful or misleading for?
BMI can be misleading for people with very high muscle mass, some athletes, people with certain disabilities, older adults who have lost height, and anyone with big changes in fluid balance. It is also interpreted differently for children and teenagers. If you’re unsure how much weight to give your BMI, that is a good reason to ask a clinician who knows your history.
Can I use this BMI calculator for children or teens?
No. Childhood BMI uses age- and sex-specific percentiles, not just one set of adult cut-offs. This page is aimed at adults roughly 18 and older. For children and teenagers, use a dedicated child/teen BMI chart or calculator that takes age and sex into account, and follow your paediatric team’s guidance.
Does a “healthy weight” BMI mean I don’t have health risks?
Not necessarily. BMI is only one piece of the picture. Someone with a healthy-weight BMI can still have risk factors like high blood pressure, high blood sugar, smoking or family history. Likewise, some people outside the 18.5–24.9 range can have relatively favourable health markers. Blood tests, blood pressure, symptoms and lifestyle usually matter more than the BMI number alone.
What if my BMI is in the overweight or obesity range?
A BMI in the overweight or obesity range can be a sign to check in with a clinician about heart, metabolic and joint health. It does not tell you what to do by itself. Many people focus on small, sustainable changes in eating, movement, sleep and stress, sometimes with medications or other treatments when needed. Stigma is never a treatment; support and individual planning are.
Should I aim exactly for the middle of the healthy BMI range?
Not everyone’s best number is the same. For many people, any BMI in the healthy range, or a move in a helpful direction, is more realistic than chasing a single “perfect” BMI. The best target is personal and should come from a conversation with your doctor or another qualified professional who knows your whole situation.
How to use this BMI calculator as a simple screen
This calculator turns your height and weight into a BMI value, tells you which common category that number lands in, and shows a healthy-weight range for your height based on standard cut-offs. It is meant to support conversations with professionals, not replace them.
1. Choose units that feel natural
Start by picking whether you prefer US units (lb, ft/in) or metric (kg, cm). The calculation behind the scenes is the same. In US mode, you enter weight in pounds and height in feet and inches. In metric mode, you use kilograms and centimetres.
2. Enter current height and weight
Type in your current height and weight, not a goal number. BMI is only designed for adults, so if you add your age, it should be 18 or above. If your height has changed with age or medical issues, use your best current measurement, not the tallest you ever were.
3. Read your BMI and category
When you tap Calculate BMI and healthy range, you’ll see:
- Your BMI to one decimal place.
- The category that BMI falls into for most adults (for example, healthy weight or overweight).
- A healthy-weight range for your height based on BMI 18.5–24.9.
These labels follow widely used public-health cut-offs, but they do not know your muscles, bones, medications or lab results.
4. Copy the summary for your notes or next appointment
Use Copy summary if you want to paste the result into a note, a tracker, or a message to your healthcare team. Having the number, category and range written down can make it easier to talk about a plan, especially if you are monitoring changes over time.
Treat this tool as a conversation starter, not a verdict. Your blood pressure, blood sugar, cholesterol, fitness, mental health, sleep, medications and lived experience all matter more than a single BMI value when it comes to your long-term health.
How the BMI and healthy-weight range math works
The calculator keeps the math simple and transparent: it converts your height and weight into metric units, applies the standard BMI formula, then uses common cut-offs to show your category and the weight range that would land your BMI in the usual “healthy-weight” band.
1. Convert US inputs to metric
If you use US units, your weight in pounds is converted to kilograms and your height in feet and inches is converted to total inches, then to metres. In simplified form:
kg ≈ lb ÷ 2.2046
m ≈ in × 0.0254
Working in metric makes it easy to use the standard BMI formula and cut-offs that most guidelines are based on.
2. Calculate BMI from height and weight
The standard adult BMI formula is:
BMI = weight (kg) ÷ [height (m)]²
The calculator rounds the result to one decimal place so it is easy to read while still matching common BMI charts closely.
3. Classify BMI into common categories
Once BMI is calculated, it is compared to widely used adult categories, such as:
- Underweight: BMI < 18.5
- Healthy weight: 18.5–24.9
- Overweight: 25.0–29.9
- Obesity class I: 30.0–34.9
- Obesity class II: 35.0–39.9
- Obesity class III: ≥ 40.0
These cut-offs are based on population data and are meant for screening, not for making individual diagnoses.
4. Estimate a healthy-weight range for your height
To estimate a healthy-weight band, the tool solves the BMI formula backwards using 18.5 and 24.9 as the lower and upper ends of the range:
Weight (kg) = BMI × [height (m)]²
This gives a lower and upper weight in kilograms, which are then converted back into your chosen unit (lb or kg) and rounded to simple numbers. That range is a generic band, not a personalised target.
As with any screening tool, this math is best used as a starting point. If you and your healthcare team decide that another range or focus is better for your body, that tailored guidance should always come first.
References and further reading on BMI and healthy weight
These resources explain how BMI is calculated, how adult categories are defined and why BMI is only one part of health:
- Centers for Disease Control and Prevention — Adult BMI categories — outlines adult BMI cut-offs for underweight, healthy weight, overweight and obesity and notes that BMI is a screening tool, not a diagnosis.
- National Heart, Lung, and Blood Institute — Calculate your BMI — describes BMI as one measure of body fat based on height and weight and links BMI ranges to heart-health information.
- World Health Organization — Body mass index (BMI) — presents global data on underweight, overweight and obesity in adults using BMI cut-offs such as <18.5, ≥25 and ≥30 kg/m².
Use these as background reading and pair them with individual guidance from your own healthcare professionals, especially if you have existing health conditions or concerns about weight-related risk.