BMI Calculator

Last updated: April 2026

Unit system

Weight

Height

BMI Result

22.9

Normal weight

Details

Weight used70 kg

Height used1.75 m

Interpretation

Your BMI is within the normal weight range for adults.

BMI is a general screening tool and does not measure body fat directly.

Related calculators:

BMR Calculator
TDEE Calculator
Body Fat Percentage Calculator
Ideal Weight Calculator

What is BMI?

BMI stands for Body Mass Index. It is a simple height-to-weight screening measure used to group adults into broad weight categories.

How is BMI calculated?

BMI is calculated as weight in kilograms divided by height in metres squared: BMI = weight(kg) / height(m)^2.

What do BMI categories mean?

Underweight is below 18.5, normal weight is 18.5 to 24.9, overweight is 25.0 to 29.9, and obesity is 30.0 or above.

Limitations of BMI

BMI does not account for muscle mass, body composition, age, sex, or ethnicity. It is a general indicator, not a medical diagnosis.

About this calculator

The BMI Calculator estimates body mass index from height and weight. BMI is commonly used as a simple screening measure for adult weight categories, but it does not directly measure body fat, fitness, or health. It can be useful for a quick population-level indicator or as a starting point before looking at waist measurement, body composition, medical history, and professional health guidance.

BMI formula

BMI divides body weight by height squared. Metric inputs are the simplest version. Imperial inputs are converted before the BMI category is estimated.

BMI = weight(kg) / height(m)^2

How BMI is calculated

Enter height and weight.
The calculator converts units if needed.
Weight in kilograms is divided by height in metres squared.
The BMI value is matched to a broad category.
The result should be interpreted as a screening estimate, not a diagnosis.

Worked examples

Adult BMI example

Input: 70kg and 1.75m

Calculation: 70 / (1.75 x 1.75)

Result: BMI = 22.9

Higher BMI example

Input: 90kg and 1.75m

Calculation: 90 / 3.0625

Result: BMI = 29.4

Lower BMI example

Input: 55kg and 1.70m

Calculation: 55 / 2.89

Result: BMI = 19.0

What BMI measures

Body Mass Index compares weight with height to produce a simple screening number. It does not directly measure body fat, fitness, or health. Its value is that it is quick, repeatable, and easy to compare across large populations.

BMI is commonly used by health services and researchers as a broad weight-category tool, but it should be interpreted alongside age, sex, ethnicity, muscle mass, waist measurement, medical history, and clinical judgement.

BMI categories and interpretation

BMI categories are ranges, not diagnoses. A result in a higher category can indicate increased health risk for some people, while a result in a normal range does not automatically mean someone is healthy.

Underweight: A low BMI may suggest undernutrition or other health concerns, but it needs context and professional assessment.
Healthy weight range: A BMI in the commonly used healthy range is a screening result, not a complete measure of health.
Overweight or obesity ranges: Higher BMI ranges can be associated with higher health risk, but the result should not be used alone to diagnose a condition.

Adult BMI category	BMI range
Underweight	Below 18.5
Healthy weight	18.5 to 24.9
Overweight	25.0 to 29.9
Obesity class I	30.0 to 34.9
Obesity class II	35.0 to 39.9
Obesity class III	40.0 and above

When BMI is less reliable

BMI can be misleading for athletes, people with high muscle mass, older adults, pregnant people, children, and some ethnic groups. In these cases, waist-to-height ratio, body composition, clinical measurements, and professional advice may be more useful.

BMI calculator components

BMI uses only height and weight. This simplicity is why it is widely used, but it is also why the result needs careful interpretation. It cannot tell the difference between muscle, fat, bone, fluid, or changes caused by pregnancy or medical conditions.

Weight: Weight is the body mass entered into the formula. Hydration, clothing, recent meals, and normal daily fluctuation can change the number.
Height: Height is squared in the formula, so small height errors can noticeably change BMI. Use a measured height where possible.
BMI number: The BMI value is a ratio used for screening. It should not be read as body fat percentage.
BMI category: The category is a broad population-level label. It can help start a health conversation but cannot replace individual assessment.

BMI and health risk

BMI can be associated with health risk at population level, but risk is not determined by BMI alone. Waist measurement, blood pressure, blood glucose, cholesterol, smoking, family history, fitness, diet, sleep, and existing conditions all matter.

For some ethnic groups, health risk may increase at lower BMI values than the standard category labels suggest. This is one reason BMI should be treated as a starting point rather than a final answer.

Alternatives to BMI

Waist-to-height ratio, waist circumference, body fat estimates, progress photos, strength and fitness markers, and clinical blood markers can all add context. For children and teenagers, BMI is usually interpreted using age- and sex-specific percentiles rather than adult ranges.

BMI for children and teenagers

Adult BMI categories should not be applied directly to children and teenagers. Because children are still growing, BMI is usually compared with age- and sex-specific centiles. A child’s BMI centile shows how their BMI compares with children of the same age and sex.

Parents should avoid using an adult BMI result to label a child’s health. Growth pattern, puberty stage, height velocity, family history, and clinical context all matter.

Child BMI interpretation	Typical centile range
Underweight	Below the 2nd or 5th centile, depending on the reference used
Healthy weight	Broad middle centile range
Overweight	High centile range
Very overweight	Highest centile range

BMI Prime and Ponderal Index

Some BMI tools also show BMI Prime and Ponderal Index. BMI Prime compares a person’s BMI with the upper end of the usual healthy adult range. A BMI Prime of 1 means BMI is exactly 25. A value below 1 is below that upper limit; a value above 1 is above it.

Ponderal Index is similar to BMI but uses height cubed rather than height squared. It can sometimes behave better for very short or very tall people, but it is still only a body-size index and not a direct health diagnosis.

Measure	Formula	What it adds
BMI	weight(kg) / height(m)^2	Common adult screening measure
BMI Prime	BMI / 25	Shows BMI relative to upper healthy-range limit
Ponderal Index	weight(kg) / height(m)^3	Alternative size index for height extremes

Common mistakes and edge cases

BMI can overstate risk for very muscular people.
BMI can understate risk where abdominal fat is high.
Adult BMI categories are not the same as child BMI percentiles.
Pregnancy changes weight and makes standard BMI interpretation less useful.
Ethnicity and age can affect risk at the same BMI.

Limitations

This calculator provides an estimate only and is not a medical diagnosis.

BMI does not measure body fat directly.
BMI does not account for muscle mass, pregnancy, age, ethnicity, or body composition.
Speak to a qualified healthcare professional for health concerns.

Frequently asked questions

What is a healthy BMI?

For many adults, a BMI from 18.5 to 24.9 is commonly described as the healthy range, but individual risk varies.

Is BMI accurate for athletes?

It can be misleading for athletes because muscle increases weight without necessarily increasing health risk.

Can BMI diagnose obesity?

No. BMI is a screening measure. Diagnosis and health assessment require wider clinical context.

Is BMI different for children?

Children are usually assessed using age- and sex-specific percentiles rather than adult BMI categories.

Should I use waist measurement too?

Waist measurement can add useful context, especially for abdominal fat risk.

Related calculators

BMR Calculator
TDEE Calculator
Body Fat Percentage Calculator
Healthy Weight Calculator

What does this mean?

This calculator is designed to help you understand the likely number before you make a decision or start an application.

Your result should be checked against official UK guidance, especially if your circumstances include dependants, exemptions, prior leave, or a complex immigration history.

Treat the figure as a planning tool rather than legal advice. Where the answer affects an application deadline or major payment, speak to an authorised adviser.