BMI Calculator

What is BMI?

Body Mass Index, or BMI, is a simple number that relates your weight to your height. It was first described in the nineteenth century by Adolphe Quetelet and has since become one of the most widely used screening tools in public health. To calculate it, you divide your weight in kilograms by the square of your height in metres. The resulting figure places you into a broad category that gives a quick sense of whether your weight is in a healthy range for your height.

This BMI calculator makes that calculation instant. Enter your weight and height, and it returns your BMI value along with the corresponding weight category. It also estimates an ideal weight range for your height so you have a practical target to aim for, whether your goal is to gain, lose or maintain weight.

You can switch between Metric (kg, cm) and US Units (ft + in, lb) tabs at the top of the calculator — the result is identical either way, because the values are converted automatically. Age and gender fields are included for completeness, but adult BMI uses only height and weight.

How is BMI calculated?

The formula is straightforward:

BMI = weight (kg) / height (m)²

For example, a person who weighs 70 kilograms and is 1.7 metres tall has a BMI of 70 / (1.7 × 1.7), which equals approximately 24.2. That places them at the upper end of the normal range. If you prefer pounds and inches, the same ratio can be computed using imperial units with a conversion factor, but metric inputs give the cleanest results.

Understanding the BMI categories

The World Health Organization defines standard adult categories that the calculator uses:

• Below 18.5: Underweight
• 18.5 – 24.9: Normal or healthy weight
• 25 – 29.9: Overweight
• 30 and above: Obese

These thresholds are guidelines for the general population. A value in the normal range is associated with the lowest risk of many weight-related conditions, while values at the extremes raise the risk of issues such as heart disease, type 2 diabetes, joint problems and certain cancers.

BMI formula in US customary units (USC)

The metric formula works on kilograms and metres, but when you measure height and weight in feet, inches and pounds you need a conversion factor. The US customary version of the BMI formula is:

BMI = 703 × mass (lb) / height (in)²

The factor 703 converts pounds and square inches into the same kg/m² scale. For example, a person who is 5'10" (70 inches) tall and weighs 160 pounds has a BMI of 703 × 160 ÷ 70² = 703 × 160 ÷ 4900 ≈ 23.0 kg/m² — exactly the same result as the metric calculation. To use it yourself, multiply your weight in pounds by 703 and then divide by your height in inches squared.

BMI Prime

BMI Prime is the ratio of your BMI to the upper limit of the normal range, which the WHO and the CDC define as 25 kg/m². The formula is:

BMI Prime = BMI ÷ 25

Because it is the ratio of two BMI values, BMI Prime is dimensionless — it has no units. A value below 0.74 indicates underweight, 0.74 to 1.00 is normal, 1.00 to 1.20 is overweight, and anything above 1.20 is classified as obese. The main advantage of BMI Prime is that it makes the result instantly readable: a value of 0.92, for example, tells you immediately that your BMI sits at 92% of the upper healthy limit and that you have about 8% of room before you cross into overweight. The classification by both BMI and BMI Prime is shown in the table below:

Ponderal Index

The Ponderal Index (PI) is a close cousin of BMI. It measures the same thing — how heavy a person is relative to their height — but it cubes the height instead of squaring it:

Ponderal Index = mass (kg) / height (m)³

Because height is cubed, the Ponderal Index is more weight-sensitive for body shapes and is generally considered more reliable for very tall or very short individuals, where BMI tends to skew high or low. In US customary units it is expressed the other way around, as height divided by the cube root of mass:

PI (USC) = height (in) / ∛mass (lb)

Using the same example, a person who is 72.57 kg and 1.778 m tall has a Ponderal Index of 72.57 ÷ 1.778³ ≈ 12.9 kg/m³. For most adults, a Ponderal Index between roughly 11 and 14 kg/m³ is considered typical. Because the pair of measures differ slightly even for the same person, the calculator shows your Ponderal Index alongside your BMI so you can weigh both perspectives.

BMI for children and teens (age 2–20)

Adult BMI categories must never be applied to children and teenagers. Because children's bodies are still growing, their BMI is compared against age- and sex-specific growth charts. The Centers for Disease Control and Prevention (CDC) recommends percentile categories rather than fixed numbers, calculated against a reference population of the same age and sex:

In other words, a child whose BMI sits at the 90th percentile is heavier than 90% of boys or girls of the same age and is considered at risk of overweight even though the raw BMI number might look normal by adult standards. Because the classification depends on knowing the child's exact age and sex, the standard adult calculator on this page is not suitable for children — a healthcare professional should use BMI-for-age percentile charts and consider growth, development and family history alongside the number. This is also why the recommended BMI thresholds vary between population groups: many Asian health organisations use a lower overweight cut-off of 23 kg/m² because people of Asian descent tend to accumulate abdominal fat at lower BMIs.

Why BMI is useful

BMI is cheap, quick and requires no special equipment, which is why doctors, insurers and health programmes use it everywhere. It works well as a first-line screening tool for large populations and for tracking trends in your own weight over time. If you are starting a fitness programme, checking your BMI monthly gives you an objective, numbers-based way to see progress alongside the scale.

Why BMI is not perfect

BMI does not distinguish between muscle, fat, bone and water. Because muscle is denser than fat, a very muscular athlete can have a BMI in the overweight range while having very low body fat and excellent health. The reverse is also true: a person with a normal BMI may still carry excess fat around the abdomen, which is a known risk factor. BMI also does not account for age, sex, ethnicity or where fat is distributed on the body, and it is not suitable for children, teenagers, pregnant women or people with significant muscle mass. For those groups, other measures such as body fat percentage, waist circumference or BMI-for-age percentile charts give a more meaningful picture.

What is your ideal weight?

A commonly used target is a BMI in the middle of the healthy range, roughly 22 kg/m². The calculator estimates the weight that corresponds to that BMI for your height. If your height is 1.75 metres, an ideal weight around 67 kilograms keeps your BMI close to 22. This is a general benchmark; the healthiest weight for you personally depends on your frame, muscle mass and overall health, so treat the figure as guidance rather than a strict requirement.

How to use your BMI to improve your health

If your BMI is in the normal range, maintaining your current habits and staying active is usually enough to keep it there. If it is in the overweight or obese range, even a modest reduction of five to ten percent of your body weight can meaningfully lower blood pressure, improve blood sugar control and reduce strain on your joints. The most sustainable approach combines a balanced diet, regular physical activity and enough sleep — crash diets rarely produce long-term results. If your BMI is below 18.5, focus on nutrient-dense foods, strength training and consulting a professional to rule out any underlying cause.

Frequently asked questions

Can BMI be misleading for children? Yes — standard adult ranges should not be applied to children and teenagers; healthcare providers use BMI-for-age percentiles instead. Does age change your BMI? For adults the same thresholds apply, but body composition shifts with age, so BMI becomes less representative in older adults. Is a high BMI always unhealthy? Not necessarily, but it is correlated with increased risk and should be discussed with a doctor alongside other checks such as waist measurement and blood tests.

BMI and sex, age and ethnicity

The standard BMI thresholds were developed using data from large adult populations and work reasonably well as averages. However, research has shown that body composition differs between men and women — women naturally carry a higher percentage of body fat — and that healthy ranges can vary between ethnic groups. Many Asian health organisations use lower thresholds for overweight (23 and above) and obesity (27 and above) because people of Asian descent tend to accumulate abdominal fat at lower BMIs, which carries higher metabolic risk. Rather than treating a single cut-off as a universal truth, it is wise to interpret your BMI alongside your waist circumference, activity level and family history.

How to measure your inputs correctly

The accuracy of your BMI depends entirely on the accuracy of your inputs. Weigh yourself at the same time of day, ideally in the morning before breakfast, using a properly calibrated scale. Stand barefoot for your height measurement and read it to the nearest centimetre. Measuring on a hard surface with your heels together and looking straight ahead gives the most reliable figure. Small errors in height matter more than most people realise, because height is squared in the formula: being a few centimetres out changes your BMI by a noticeable amount.

A worked example

Consider two people with different builds. A woman who is 1.62 metres tall and weighs 52 kilograms has a BMI of 52 ÷ (1.62 × 1.62), which is approximately 19.8 — comfortably inside the normal range. A man who is 1.80 metres tall and weighs 95 kilograms has a BMI of 95 ÷ (1.80 × 1.80), which is approximately 29.3 — inside the overweight range and close to obesity. The same tool lets you experiment with your own numbers and see how much a change of even a couple of kilograms moves the result and the category.

When should you recheck your BMI?

Weight changes slowly, so there is little point checking every day. A useful rhythm is to track your BMI once a month at the same point in your routine. If you are actively trying to gain or lose weight, monthly readings show a clear trend without the daily noise of water weight fluctuations. If your BMI crosses into a new category, that is a sensible prompt to review your diet, sleep and exercise habits or to speak with a professional.

This calculator is a screening and educational tool, not a diagnostic instrument, and it cannot measure body fat percentage, fitness level or overall health. Consult a qualified healthcare professional before making significant diet or exercise changes.