BMI Calculator

What BMI measures

Body Mass Index (BMI) compares your weight to your height with a single number. It's a quick screening tool that works reasonably well across large populations — health researchers use it to spot, at a glance, whether a person's weight sits in a range linked to higher or lower health risk. It's a useful starting point, not a diagnosis. BMI says nothing about what your weight is made of or how you feel, so treat the result as one clue among many.

How BMI is calculated

The formula is straightforward. In metric units, BMI is your weight in kilograms divided by your height in metres squared:

If you work in pounds and inches, the imperial version divides weight in pounds by height in inches squared and then multiplies by 703 (a conversion factor that lines the result up with the metric scale):

Both formulas give the same number for the same person — they're just different measuring sticks. This calculator does the arithmetic and the unit conversion for you, so you can enter whichever units you're comfortable with and switch between them with the Metric / Imperial buttons.

What each input means

The calculator asks for only two things, because those are the two the formula needs:

• Height. In metric mode you enter height in centimetres; in imperial mode you enter feet and inches separately. Height matters a lot — because it is squared in the formula, a small measuring error changes the result more than the same error in weight would.
• Weight. In metric mode this is kilograms; in imperial mode it's pounds. Use your current, everyday weight rather than a goal weight, and ideally measure at a consistent time of day.
• Units. The Metric / Imperial toggle simply changes which fields you see and how the answer is displayed. It does not change the underlying calculation — the two systems agree.

BMI categories for adults

The World Health Organization (WHO) groups adult BMI into four standard ranges. These are the same for men and women and don't change with age:

These cut-offs are the widely used international standard for adults. Some health bodies use slightly lower thresholds for people of South Asian, Chinese, and some other backgrounds, because the health risks linked to body fat can appear at a lower BMI. The category is a signpost, not a verdict — where you sit within a range, and how your weight is trending over time, both matter.

A worked example

Here's an illustration to show the formula in action. Suppose someone is 1.75 m tall (175 cm) and weighs 70 kg. You square the height (1.75 × 1.75 = 3.0625) and divide the weight by that figure:

Using imperial units for the same person — roughly 5 ft 9 in and 154 lb — you'd compute 154 ÷ 69² × 703, which also lands near 22.9. This is an example only, chosen to show the maths; your own result will depend on your own height and weight.

Limitations of BMI

Keep these limits in mind when you read your number:

• Muscle mass and athletes. Muscle is denser than fat, so very muscular people — weightlifters, rugby players, sprinters — can land in the "overweight" or even "obesity" range while carrying little fat.
• Older adults. With age, people tend to lose muscle and bone. An older person can have a "healthy" BMI while carrying more fat and less muscle than the number suggests.
• Pregnancy. BMI isn't meaningful during pregnancy, when weight gain is expected and normal. Pregnant people should follow guidance from their own care team.
• Ethnicity and body composition. Health risk at a given BMI varies between population groups, which is why some countries adjust the thresholds.
• It's a screening tool, not a diagnosis. BMI can flag that a closer look might be worthwhile, but it can't tell you whether you're healthy on its own.

Pairing BMI with a waist measurement and a few basic health markers gives far more insight than the number alone. For more on interpreting your result, read what does my BMI mean.

BMI in children and teenagers

The adult categories above do not apply to anyone under 18. Children and teens are still growing, and healthy weight changes with age and sex. For them, BMI is compared against age-and-sex percentile charts rather than fixed cut-offs, so this adult calculator shouldn't be used to assess a child. A paediatrician or family doctor can interpret a child's growth properly.

Key terms

• BMI (Body Mass Index). A number from your weight and height that estimates whether your weight sits in a healthier or riskier range for the population.
• Overweight. The BMI range of 25.0 to 29.9 for adults, linked on average to somewhat higher health risk than the healthy range.
• Obesity. A BMI of 30 or above for adults, associated with a higher risk of conditions such as type 2 diabetes and heart disease.
• Screening tool. A quick, low-cost check that flags who might benefit from a closer look — not a test that diagnoses anything by itself.
• Body composition. What your weight is actually made of — muscle, fat, bone, and water — which BMI cannot see.

Common misunderstandings

• "A high BMI always means too much fat." Not necessarily — muscle adds weight too, so athletes can read high without excess fat.
• "BMI is different for men and women." The adult formula and categories are identical for both; only children use sex-specific charts.
• "A healthy BMI means I'm healthy." BMI ignores diet, fitness, blood pressure, blood sugar, and where fat is stored, so a normal number isn't a clean bill of health.
• "Getting to a lower BMI number is the goal." Health is the goal; BMI is only a rough gauge. Rapid or extreme changes to hit a number can do more harm than good.

Please remember

This tool is for general information and isn't medical advice. It isn't appropriate for children, pregnant people, or athletes. If you have any concern about your weight or health, talk with a doctor who can consider your whole situation.

Frequently asked questions

How is BMI calculated?

BMI is your weight in kilograms divided by your height in metres squared (kg/m²), or in imperial units, weight in pounds divided by height in inches squared, times 703. This calculator does both automatically — just enter your height and weight.

What is a healthy BMI range for adults?

A healthy BMI is 18.5 to 24.9. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is the obesity range. These World Health Organization ranges are the same for adult men and women.

Is BMI accurate for everyone?

No. Because BMI only uses height and weight, it can't tell muscle from fat. Muscular athletes may read as overweight while lean, and older adults may have a normal BMI with low muscle. Use it alongside a waist measurement for a fuller picture.

Does BMI differ for men, women, or age?

The formula and categories are the same for adult men and women of any age. Children and teenagers are assessed differently using age-and-sex percentile charts, so this adult calculator shouldn't be used for anyone under 18.

Why is my BMI high even though I'm fit and muscular?

Muscle is denser and heavier than fat, so athletic, muscular people often score in the overweight or obesity range while carrying very little fat. BMI can't tell muscle from fat, so for very muscular bodies it's a poor guide. A body-fat measurement or waist reading tells you more.

What's the difference between BMI and body fat percentage?

BMI estimates weight relative to height and says nothing about what that weight is made of. Body fat percentage measures how much of your weight is fat specifically. Body fat percentage is more precise but needs special equipment, while BMI only needs a scale and a tape measure.

How can I lower my BMI healthily?

Aim for gradual weight loss of about 0.5–1 kg (1–2 lb) per week through a modest calorie deficit and regular activity, not crash dieting. Our calorie calculator can help you plan it. Always check with a doctor first.

This calculator provides estimates for general information only and is not medical advice. It cannot diagnose any condition — always consult a qualified healthcare professional about your weight or health. See our disclaimer.