BMI Calculator

What is a BMI calculator?

A BMI calculator is a free online tool that works out your body mass index from your weight and height. BMI is a number that doctors, dietitians and health websites use as a quick screening measure, and it helps you see whether your weight is in the underweight, normal, overweight or obesity range for an adult of your height.

Our BMI calculator gives you your result in a few seconds. You can enter your measurements in kilograms and centimeters, or in pounds, feet and inches. It shows your BMI, your category on a color scale, and the healthy weight range for your height. Everything is calculated in your browser, so your weight and height are never uploaded or stored.

Features of this body mass index calculator

• Metric and imperial units: Enter kilograms and centimeters, or pounds, feet and inches. Switch between them with one click.
• Instant result: Your BMI is shown to one decimal place as soon as you enter both your weight and your height.
• Color BMI chart: A color scale with a marker shows where your result falls between underweight, normal weight, overweight and obesity.
• Healthy weight range: See the weight range that gives an adult of your height a BMI between 18.5 and 24.9.
• Value check: The calculator warns you when numbers look unrealistic, so a typing mistake such as 1750 cm does not give a false result.
• Private and free: There is no signup, and nothing you type is saved or sent to a server.

BMI formula used by this calculator

The calculator uses the standard BMI formula that is accepted worldwide. Only the units are different.

When you choose Imperial units, the calculator converts your entries to metric internally. The result is rounded to one decimal place, and your category is chosen from that rounded number, so what you see always matches the scale.

• Metric formula: BMI equals weight in kilograms divided by height in meters squared. For example, 82 kg and 180 cm (1.80 m) gives 82 divided by 3.24, which is 25.3.
• Imperial formula: BMI equals 703 multiplied by weight in pounds, divided by height in inches squared. For example, 180 lb and 5 ft 10 in (70 inches) gives 703 x 180 divided by 4,900, which is 25.8.

BMI categories and what they mean

The calculator uses the World Health Organization categories for adults. Here is what each range means in simple terms:

• Underweight (below 18.5): Your weight is low for your height. This can have many causes, so it is worth speaking to a doctor if it is unexpected or comes with tiredness or other symptoms.
• Normal weight (18.5 to 24.9): This range is linked with the lowest average risk of weight-related health problems, although BMI alone cannot guarantee good health.
• Overweight (25 to 29.9): Your weight is above the normal range for your height. Waist size, activity level and overall health matter as much as the number.
• Obesity (30 and above): Higher BMI values are linked with a greater risk of conditions such as type 2 diabetes, high blood pressure and heart disease. A healthcare professional can advise you on the next steps. The range is split into class I (30 to 34.9), class II (35 to 39.9) and class III (40 or more).

How to find your healthy weight range

Besides your BMI, the calculator shows the weight range that would give an adult of your height a BMI between 18.5 and 24.9. It multiplies your height in meters squared by 18.5 for the lower end and by 24.9 for the upper end.

For a height of 1.80 m, the healthy weight range is about 59.9 kg to 80.7 kg, or roughly 132.1 lb to 177.9 lb. Use the range as a general guide rather than a strict target, because the best weight for you also depends on your age, muscle mass and health.

Example BMI results

These examples all use a height of 1.65 m, so you can see how BMI and the category change with weight:

• 50 kg: BMI 18.4, which falls in the underweight range.
• 65 kg: BMI 23.9, which falls in the normal weight range.
• 85 kg: BMI 31.2, which falls in the obesity range (class I).

When BMI can be misleading

BMI is a quick screening number, not a diagnosis, and it works best for the average adult. Doctors often look at waist size, blood pressure, blood sugar and cholesterol as well. If you are worried about your weight or health, speak to a doctor or another healthcare professional. Be careful with the result in these situations:

• Athletes and very muscular people: Muscle weighs more than fat, so BMI can be high even when body fat is low.
• Older adults: Muscle is often lost with age, so the same BMI can hide more body fat than it would in a younger person.
• Children and teenagers: Their BMI is compared with growth charts for their age and sex, so this adult calculator is not the right tool.
• Pregnancy: Weight changes during pregnancy are normal, and BMI is not the right measure.
• Different ethnic backgrounds: Health risks can start at a lower BMI for some groups, for example people of South Asian and East Asian background.
• Body fat distribution: BMI does not show where fat is stored. Fat around the waist carries higher health risks than fat elsewhere.

Tips for accurate measurements

A BMI calculator is only as accurate as the numbers you enter. These tips help:

• Weigh yourself consistently: Use the same scale at the same time of day, for example in the morning before eating, wearing light clothing.
• Measure your height properly: Stand straight without shoes against a wall and measure to the top of your head.
• Enter feet and inches separately: Type 5 in the feet field and 9 in the inches field. 5.9 feet is not the same as 5 feet 9 inches.
• Pick the right unit: Choose Metric or Imperial before typing, so kilograms are not entered as pounds.
• Re-check unusual results: If the calculator warns about unrealistic values, look for a typing mistake such as an extra zero.