BMI Calculator

What Is BMI and Why Does It Matter?

Body Mass Index, or BMI, is a simple number that compares your weight with your height. Doctors, dietitians and public health agencies use it as a quick screening tool because it needs no special equipment and gives a consistent starting point. A BMI result does not diagnose any condition, but it can show whether your weight sits in a range that is linked with lower or higher health risks.

Our calculator works with metric (kilograms and centimetres) or imperial (pounds and inches) units. Enter your height and weight, press the button, and you will see your BMI, your weight category and a healthy weight range for your height.

How BMI Is Calculated

The formula divides your weight in kilograms by your height in metres squared. For example, a person who weighs 70 kg and is 1.75 m tall has a BMI of 70 divided by 3.06, which is about 22.9. If you prefer imperial units, multiply your weight in pounds by 703 and divide by your height in inches squared. The calculator does all of this for you, so you only need to type in your numbers.

Adult BMI Categories

The World Health Organization uses the following ranges for adults aged 20 and over:

• Underweight: below 18.5
• Healthy weight: 18.5 to 24.9
• Overweight: 25.0 to 29.9
• Obesity: 30.0 and above

These cut-offs are the same for men and women. Some health organizations suggest lower thresholds for people of South Asian, East Asian and some other ethnic backgrounds, because health risks can rise at a lower BMI in these groups. If you belong to one of them, ask your doctor which range is right for you.

What BMI Can and Cannot Tell You

BMI is useful for studying large populations and for spotting broad trends. For one person, it has real limits. It cannot tell muscle from fat, so a strong athlete with a lot of muscle can land in the overweight range while having low body fat. It also does not show where fat is stored. Fat around the waist carries more risk than fat on the hips or thighs, so waist size is a helpful second measurement. A waist above about 102 cm (40 in) for men or 88 cm (35 in) for women is commonly linked with higher risk.

Age matters too. Older adults naturally lose muscle and height, and a slightly higher BMI can be protective in later life. Pregnant women and people under 20 should not use the adult categories. For children and teens, doctors compare BMI with growth charts for their age and sex.

Who Should Be Careful With BMI?

• Athletes and people who lift weights, whose muscle adds weight
• Adults over 65, who may lose muscle without losing weight
• Pregnant or breastfeeding women
• Children and teenagers, who need age-based percentile charts
• People with swelling or fluid retention caused by a medical condition

Using Your Result Wisely

Think of your BMI as one data point, not a verdict. Combine it with other signs of health: blood pressure, blood sugar, cholesterol, fitness, sleep, energy and how your clothes fit. If your BMI is in the healthy range, keep up the habits that got you there. If it is above or below the range, a small, steady change usually works better than a dramatic one. Losing even 5 to 10 percent of body weight can improve blood pressure and blood sugar for many people who are overweight.

Practical Steps to Move Toward a Healthy Range

• Know your energy needs. Use our Daily Caloric Needs Calculator to see how many calories you need to maintain, lose or gain weight.
• Eat enough protein. It helps you feel full and protects muscle. Check your target with the Protein Requirements Calculator.
• Drink water through the day. Find your goal with the Daily Fluid Intake Calculator.
• Watch added sugar. Sugary drinks add calories quickly, and the Daily Sugar Intake Calculator shows your daily limit.
• Move more. Aim for at least 150 minutes of moderate activity each week, plus two days of strength training.
• Sleep 7 to 9 hours. Poor sleep is linked with weight gain and stronger hunger.

If Your BMI Is Below 18.5

Being underweight can reflect low food intake, illness, stress or a high metabolism. It can lead to tiredness, weak bones and a lower resistance to infection. Focus on regular meals, nutrient-dense foods such as nuts, dairy, eggs, whole grains and healthy oils, and speak with a doctor to rule out an underlying cause.

When to Talk to a Doctor

See a healthcare professional if your BMI is above 30 or below 18.5, if your weight has changed quickly without a clear reason, or if you have a family history of diabetes, heart disease or high blood pressure. A doctor can check blood tests, measure body composition and build a plan that fits your age, health and goals.

Frequently Asked Questions

Is BMI accurate for everyone?

No. It is a screening tool and works best for the average adult. Muscle mass, age, sex and ethnic background all change what a given BMI means.

What is a good BMI for my age?

The adult range of 18.5 to 24.9 applies from age 20 upward, though some experts accept a slightly higher number for adults over 65.

How often should I check my BMI?

Once every few months is enough. Weight naturally moves a little each day, so look at the trend and not a single reading.

This calculator is for general education and is not medical advice. Always consult a qualified professional about your health.

Key Takeaways About BMI

• BMI is a quick screening number, not a full picture of your health.
• A healthy adult range is 18.5 to 24.9, with adjustments for ethnicity, age and muscle mass.
• Pair BMI with waist size, blood pressure, blood sugar and how you feel.
• Small, steady habits such as walking daily, sleeping well and eating whole foods beat crash diets.
• Recheck every few months and focus on the trend over time.

Bookmark this page and come back after a few weeks of new habits to see how your numbers move. If you are planning changes, a short chat with your doctor can confirm a safe target weight for your body and lifestyle, and a registered dietitian can turn it into a simple weekly meal plan.