Free BMI Calculator for Adults 18+

Use our free BMI calculator for adults to calculate your body mass index and find your general weight category.

Enter your height and weight to receive an instant BMI result. The calculator will show whether your result falls within the underweight, healthy weight, overweight or obesity range.

Your results may include:

• Body mass index

• General weight category

• Healthy weight range for your height

• Difference from the estimated healthy range

• Weight and height summary

• General health guidance

This calculator is designed for adults aged 18 and over. It is a simple weight assessment tool and should not be treated as a medical diagnosis.

Important: BMI is a screening measurement. It does not directly measure body fat, fitness, muscle mass or overall health.

Calculate Your Body Mass Index

Enter the following information:

Enter your height

Choose your preferred unit:

• Centimetres

• Metres

• Feet and inches

Enter your weight

Choose your preferred unit:

• Kilograms

• Pounds

• Stones and pounds

Select the calculation button to view your BMI and general weight classification.

What Is Body Mass Index?

Body mass index, commonly known as BMI, is a number calculated from an adult’s height and weight.

It is widely used as a simple health metric to place body weight into general categories. These categories can help identify whether a person may have a low or high weight for their height.

BMI is calculated using the following formula:

BMI = weight in kilograms ÷ height in metres²

For example, a person who weighs 70 kilograms and is 1.75 metres tall has a BMI of approximately 22.9.

BMI does not measure body fat directly. It is best understood as an initial health risk indicator rather than a complete assessment of an individual’s health.

How to Use the Adult BMI Calculator

The BMI calculator takes only a few seconds to use.

1. Enter Your Height

Enter your current height in centimetres, metres or feet and inches.

Measure your height without shoes and stand straight against a flat wall for a more accurate result.

2. Enter Your Weight

Enter your current weight in kilograms, pounds or stones and pounds.

For consistent weight tracking, use the same scale and weigh yourself under similar conditions.

3. Select Calculate

Select the calculation button to view your body mass index.

4. Review Your Weight Category

The calculator will compare your result with the standard adult BMI ranges.

5. Consider Other Health Measurements

Do not rely on BMI alone. Waist measurement, blood pressure, activity level, nutrition, body composition and medical history may provide additional health information.

Adult BMI Chart and Weight Classification

The following BMI chart shows the commonly used weight categories for adults:

The CDC describes BMI as a screening measure and recommends considering it together with other health factors.

A BMI category does not diagnose a disease. It indicates that a person may benefit from a more complete health assessment.

What Does an Underweight BMI Mean?

A BMI below 18.5 falls within the underweight category.

A low BMI may be related to:

• Naturally low body weight

• Low calorie intake

• Loss of appetite

• Digestive problems

• An overactive thyroid

• Long-term illness

• Unintentional weight loss

• An eating disorder

Some healthy adults naturally have a lower body weight. However, unexplained weight loss or difficulty maintaining weight should be discussed with a healthcare professional.

BMI alone cannot identify the reason for a low weight.

What Is a Healthy BMI Range?

For most adults, a BMI from 18.5 to 24.9 is generally described as the healthy-weight range.

Being within this range does not guarantee good health. A person can have a BMI within the healthy range while also having high blood pressure, poor nutrition, low fitness or excess abdominal fat.

Similarly, a person slightly outside the range may be physically active and have healthy clinical measurements.

Use your BMI as one part of a broader personal health assessment.

What Does an Overweight BMI Mean?

A BMI from 25.0 to 29.9 falls within the overweight category.

A result in this range means that body weight is higher than the standard healthy range for the person’s height.

BMI cannot show whether the additional weight comes from body fat, muscle, bone or another source. People with high muscle mass may receive an overweight result even when they have a relatively low body fat percentage.

Waist measurement and other health metrics may provide additional information.

What Does an Obesity BMI Mean?

A BMI of 30 or above falls within the obesity category. The World Health Organization defines overweight in adults as a BMI of at least 25 and obesity as a BMI of at least 30.

Obesity is divided into three general classes:

Class 1 Obesity

A BMI from 30.0 to 34.9.

Class 2 Obesity

A BMI from 35.0 to 39.9.

Class 3 Obesity

A BMI of 40.0 or higher.

A higher BMI can be associated with increased health risks, but individual risk cannot be determined from BMI alone. Blood pressure, cholesterol, blood sugar, waist size, lifestyle and family history may also need to be considered.

BMI and Healthy Weight Range

The calculator may estimate a healthy weight range using your height and a BMI from 18.5 to 24.9.

The lower end of the range is calculated using a BMI of 18.5, while the upper end is calculated using a BMI of 24.9.

This provides a range rather than one ideal weight because adults of the same height can have different healthy body weights.

Your most suitable weight may also depend on:

• Muscle mass

• Bone structure

• Age

• Sex

• Ethnicity

• Medical history

• Fitness level

• Body fat distribution

• Personal health goals

Do not treat either end of the range as a required target.

Is BMI a Body Fat Estimation Tool?

BMI is not a direct body fat estimation method.

It uses only height and weight and cannot distinguish between:

• Body fat

• Muscle

• Bone

• Body water

• Other body tissues

Two adults can have the same BMI while having very different body compositions.

More direct body fat estimation methods include skinfold measurements, bioelectrical impedance, air-displacement testing and medical imaging. Each method has its own limitations.

BMI for Athletes and Muscular Adults

BMI may be less informative for athletes, bodybuilders and adults with high muscle mass.

Muscle contributes to body weight, so a muscular person may have an overweight or obesity-range BMI without having excess body fat.

A fitness assessment for an athletic adult may also consider:

• Waist measurement

• Body fat percentage

• Lean body mass

• Strength

• Endurance

• Blood pressure

• Blood test results

• Exercise performance

BMI can still be recorded, but it should be interpreted in context.

BMI and Nutritional Status Evaluation

BMI can contribute to a general nutritional status evaluation, but it cannot show whether a person is eating a balanced diet.

A person may have a BMI in the healthy range while consuming too little:

• Protein

• Fibre

• Vitamins

• Minerals

• Healthy fats

• Fluids

A person with a higher BMI may also have a nutritious diet and good fitness habits.

A complete nutrition assessment may include food intake, symptoms, medical history, blood tests, weight changes and advice from a registered dietitian.

BMI as a Personal Health Tracker

You can use BMI as part of a personal health tracker when monitoring long-term changes in body weight.

However, small daily weight changes are normal and can be affected by:

• Food and drink

• Clothing

• Hydration

• Salt intake

• Digestion

• Exercise

• Time of day

Focus on longer-term patterns rather than one measurement.

BMI may be more useful when tracked together with waist size, activity, sleep, nutrition and other adult health metrics.

Who Should Not Rely on This BMI Calculator?

A standard adult BMI calculator may not be suitable as the main health assessment for:

• Pregnant women

• Adults with an eating disorder

• People with a condition that affects height

• People with limb loss

• Frail older adults

• Competitive athletes

• Bodybuilders

• People with significant fluid retention

• People with very high muscle mass

The NHS advises that its adult BMI tool should not be used during pregnancy, by people diagnosed with or concerned about an eating disorder, or by people with a condition affecting height.

When Should You Speak to a Healthcare Professional?

Consider seeking professional advice when:

• You are losing weight without trying

• Your weight changes rapidly

• You have difficulty eating

• You experience persistent tiredness

• Your BMI is very low or high

• You have diabetes or heart disease

• You are worried about an eating disorder

• You want to begin a major weight-loss programme

• You need a personalised nutrition or fitness plan

• Your result is causing distress or anxiety

A doctor or registered dietitian can interpret your weight together with your symptoms, medical history and other health measurements.

Frequently Asked Questions

What is BMI?

BMI is a measure of body weight relative to height. It is used to place adult weight into general screening categories.

How is adult BMI calculated?

Divide your weight in kilograms by your height in metres squared.

What is a healthy BMI for adults?

A BMI from 18.5 to 24.9 is generally classified as healthy weight for most adults.

Is BMI an accurate obesity measurement?

BMI is useful for population screening, but it does not directly measure body fat. Individual assessment may require additional measurements.

Does BMI consider age or sex?

The standard adult BMI formula uses only height and weight. Age and sex may still be relevant when interpreting health and body composition.

Can BMI calculate body fat percentage?

No. BMI and body fat percentage are different measurements.

Can I use this calculator during pregnancy?

No. Standard BMI categories are not designed to assess pregnancy-related weight changes.

Is this BMI calculator free?

Yes. This adult BMI calculator is free to use.

Can this calculator diagnose obesity?

It can display a BMI weight category, but it cannot provide a medical diagnosis.

How often should I calculate my BMI?

You can recalculate it when your weight changes meaningfully. There is usually no need to check it every day.

Calculate Your Adult BMI Now

Enter your height and weight to calculate your body mass index and general weight classification.

Your result may show:

• BMI score

• Weight category

• Estimated healthy weight range

• General health guidance

Use the result as a starting point for understanding your weight in relation to your height.

Calculate My BMI

Medical Disclaimer

This adult BMI calculator and all information on this page are provided for general educational purposes only.

The calculator does not provide medical advice and is not intended to diagnose, prevent or treat obesity, underweight, nutritional deficiencies or any other medical condition.

BMI is a screening measurement and does not directly measure body fat, muscle mass, fitness or overall health.

Always consult a qualified doctor, registered dietitian or another appropriate healthcare professional if you have concerns about your body weight, nutrition, symptoms or general health.