How to Calculate BMI and Read the Result Sensibly
Calculate your body mass index in metric or imperial units, see the healthy weight range for your height and learn what BMI can and cannot tell you.
Body mass index, or BMI, is one of the quickest health checks there is: a single number from your height and weight. It is widely used by doctors as a screening tool, and you can work it out yourself in seconds with the free BMI Calculator. It is only a starting point, so this guide also explains what it does not show.
This article is general information and not medical advice.
Quick answer
Enter your height and weight in metric (cm and kg) or imperial (feet, inches and pounds). You see your BMI and the healthy weight range for your height.
The formula
BMI = weight in kg / (height in metres x height in metres)
For example, a person who is 1.70 m tall and weighs 68 kg has a BMI of 68 / (1.70 x 1.70) = 68 / 2.89 = 23.5, as in the calculator above.
The standard groups
The World Health Organization groups for adults are:
| BMI | Group |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25 to 29.9 | Overweight |
| 30 or more | Obese |
Some health bodies use lower cut-offs for people of South Asian background, because health risks can rise at a lower BMI. If that applies to you, discuss the right range with a health professional.
What BMI does not tell you
BMI is a simple screening number. It does not measure body fat directly, and it does not know:
- Muscle versus fat. A very muscular person can have a high BMI and be healthy.
- Where fat is stored. Fat around the waist carries more risk than fat elsewhere.
- Age, sex and body frame. These affect what is healthy for you.
- Your overall health, such as blood pressure, blood sugar and fitness.
Used alone it can mislead, so treat it as one clue among several.
How to use the calculator, step by step
- Open the BMI Calculator.
- Choose Metric or Imperial.
- Enter your height and weight.
- Read your BMI and the healthy weight range for your height.
What to do with the number
- If you are in the healthy range, keep up balanced eating and regular movement.
- If you are above or below it, take it as a prompt for a conversation with a doctor or dietitian, not as a diagnosis.
- Look at trends, not a single reading. Gradual changes over months matter more.
Related health tools
- Calorie Calculator for daily energy needs.
- Water Intake Calculator for hydration.
- Sleep Calculator for better rest.
Limitations of BMI, in more detail
BMI was designed to describe populations, not to diagnose individuals. Several situations make it less reliable:
- Athletes and people with high muscle mass can fall in the "overweight" range while having low body fat.
- Older adults may have a normal BMI but low muscle mass, which carries its own risks.
- Children and teenagers need age-specific growth charts.
- Pregnancy changes weight for healthy reasons.
- Different ethnic groups may carry health risks at different BMI values, and some guidelines use lower cut-offs for South Asian adults.
Because of this, doctors often combine BMI with waist circumference, blood tests and a review of lifestyle and family history.
Useful measurements besides BMI
- Waist circumference: a larger waist indicates more fat around the organs. Many guidelines use about 90 cm for men and 80 cm for women as a cautionary level for South Asians, but ask your doctor.
- Waist-to-height ratio: keeping your waist below half your height is a simple target.
- Blood pressure, blood sugar and cholesterol: these show what is happening inside.
- Fitness: how far you can walk or how many stairs you can climb without getting breathless.
Healthy habits that matter more than a number
- Eat mostly whole foods, with plenty of vegetables, fruit, pulses and protein.
- Move daily, with at least 150 minutes of moderate activity a week for most adults.
- Sleep enough. See the Sleep Calculator.
- Drink water through the day. Try the Water Intake Calculator.
- Limit sugary drinks and heavily processed snacks.
- Manage stress with rest, friends and activity you enjoy.
When to speak to a professional
See a doctor or dietitian if your BMI is far outside the healthy range, if your weight changes quickly without a reason, or if you have symptoms such as constant tiredness, breathlessness or swelling. This guide offers general information, not medical advice.
Quick checklist
- Measure height without shoes and weight in light clothing.
- Use the same scale each time.
- Look at the range, not only a single number.
- Combine with waist measurement and how you feel.
A worked example: two people, same BMI
Two men are both 175 cm and 80 kg, so both have a BMI of about 26.1, in the "overweight" range. One lifts weights four times a week, has a waist of 84 cm and a resting heart rate of 58. The other is mostly sedentary, has a waist of 100 cm and has been told that his blood pressure is high. The same number hides two different health pictures. The first needs no action beyond keeping his habits; the second should talk to a doctor and start moving more.
This is why BMI is best used as a prompt for a wider look, not as a verdict. Waist size, fitness, blood tests and how you feel add context that a single number cannot.
How to use BMI well
- Track it over time, not once.
- Pair it with waist measurement.
- Focus on habits: food, movement, sleep and stress.
- Seek professional advice for medical decisions.
Frequently asked questions
What is a healthy BMI? For most adults, 18.5 to 24.9.
Is BMI accurate for athletes? Not always, because muscle weighs more than fat.
Can I use BMI for children? No. Children use age-specific growth charts.
Does BMI diagnose a condition? No. It is a screening number only.
Is the calculator free? Yes, and your numbers stay in your browser.
Is a BMI of 25 bad? It is at the edge of the overweight range. Health depends on many other factors too.
Does BMI work for older people? It can be misleading, since muscle loss may hide excess fat. A doctor can interpret it with other measures.
How often should I check BMI? Once every few months is enough; weight naturally fluctuates daily.
How accurate is a home scale? Good enough for trends. Weigh at the same time of day for comparison.
Should I worry if my BMI is slightly high? A small difference is not a diagnosis. Look at the whole picture or ask a professional.
Next step
Open the BMI Calculator below, check your number and use it as a gentle starting point.