How to Use the BMI Calculator

Choose either metric or imperial measurements, then enter your height and weight. The BMI calculator automatically divides your weight by your height squared using the appropriate BMI formula.

The calculator displays:

• Your calculated BMI;
• Your adult BMI category;
• Your position on a visual BMI scale;
• A healthy-weight range based on your height;
• Your BMI Prime value; and
• An indication of whether your current weight is inside, below or above the standard healthy BMI range.

What Is BMI?

BMI stands for Body Mass Index. It is a simple calculation that compares a person’s body weight with their height.

BMI is commonly used as a screening measurement for adults because it provides a quick way to classify weight relative to height.

BMI does not directly measure body fat percentage, muscle mass or body composition.

BMI Formula Using Kilograms and Metres

When using metric units, BMI is calculated as:

For example, a person who weighs 70 kg and is 1.75 metres tall has:

BMI Formula Using Pounds and Inches

For imperial measurements:

The calculator automatically performs the required conversion when Imperial is selected.

BMI Calculation Example

What Is a Healthy BMI?

For adults, a BMI between approximately 18.5 and 24.9 is commonly classified as the healthy-weight range.

Adult BMI categories are generally:

• Below 18.5 — Underweight;
• 18.5 to 24.9 — Healthy weight;
• 25.0 to 29.9 — Overweight;
• 30.0 to 34.9 — Obesity Class I;
• 35.0 to 39.9 — Obesity Class II; and
• 40 or higher — Obesity Class III.

How Is Healthy Weight Calculated From Height?

The calculator reverses the BMI formula using the lower and upper values of the healthy adult BMI range.

For someone who is 1.75 metres tall:

Lower estimate ≈ 56.7 kg

Upper estimate ≈ 76.3 kg

This is a BMI-based range only and is not a personalised medical target.

What Is BMI Prime?

BMI Prime is a ratio comparing a person’s BMI with the upper boundary of the standard healthy BMI range.

For example:

• BMI Prime below 1.0 means BMI is below 25;
• BMI Prime of 1.0 corresponds to BMI 25; and
• BMI Prime above 1.0 means BMI is above 25.

Does BMI Measure Body Fat?

No. BMI is based only on height and weight.

It does not directly measure:

• Body fat percentage;
• Muscle mass;
• Bone density;
• Fat distribution; or
• Waist circumference.

Two people with the same BMI can have different body compositions.

Can Athletes Have a High BMI?

Yes. A person with substantial muscle mass may have a relatively high BMI even if their body-fat level is not high.

Because muscle contributes to body weight, BMI can sometimes classify highly muscular individuals above the healthy-weight range even when BMI alone does not accurately describe their body composition.

BMI vs Body Fat Percentage

BMI and body-fat percentage measure different things.

BMI

BMI uses only height and weight.

Body fat percentage

Body-fat percentage estimates the proportion of total body weight made up of fat tissue.

For a more body-composition-focused estimate, you can use the Body Fat Calculator.

BMI vs Ideal Weight

BMI categories provide broad ranges rather than a single ideal body weight.

A person’s suitable weight can depend on factors beyond height, including:

• Body composition;
• Age;
• Muscle mass;
• Bone structure;
• Overall health; and
• Individual circumstances.

Use the Ideal Weight Calculator for additional mathematical weight estimates.

Does BMI Change With Age?

The standard adult BMI calculation itself does not include age.

BMI interpretation for children and teenagers is different from adult BMI because growth and development need to be considered. Paediatric BMI is generally interpreted using age- and sex-specific growth references.

Does BMI Differ for Men and Women?

The standard adult BMI formula is the same for men and women.

However, BMI does not account for differences in body composition, muscle mass or fat distribution between individuals.

Is BMI Accurate?

BMI can be useful as a general population-level and screening measure, but it has limitations when applied to an individual.

It does not distinguish between:

• Fat and muscle;
• Different body frames;
• Different fat distribution patterns; or
• Other health indicators.

BMI should therefore be considered alongside other relevant health information rather than used as a standalone diagnosis.

BMI and Waist Circumference

Waist circumference can provide additional information about abdominal fat distribution that BMI does not capture.

A healthcare professional may consider BMI together with waist measurement, medical history, blood pressure, blood tests and other health indicators.

Can BMI Be Used During Pregnancy?

Standard BMI based on current pregnancy weight is generally not an appropriate way to interpret weight changes during pregnancy because pregnancy itself changes body weight substantially.

For pregnancy-related planning, use the Pregnancy Weight Gain Calculator where appropriate.

Can BMI Be Used for Older Adults?

BMI can still be calculated for older adults, but changes in muscle mass, bone density and body composition can affect how useful BMI is as an individual health measure.

Clinical interpretation may therefore require additional information.

Why Can BMI Be Misleading?

BMI can be misleading when weight is strongly influenced by factors other than body fat.

Examples can include:

• High muscle mass;
• Fluid changes;
• Pregnancy;
• Changes in muscle mass with ageing; and
• Differences in body composition between individuals.

How to Interpret Your BMI Result

Treat BMI as a screening number rather than a diagnosis.

A result outside the standard healthy range does not by itself determine whether a person is healthy or unhealthy.

If you are using BMI to make decisions about weight management, nutrition, exercise or health, individual medical and lifestyle factors should also be considered.

BMI Calculator Assumptions

This calculator:

• Uses the standard BMI height-and-weight formula;
• Uses kilograms and metres internally;
• Supports centimetres and kilograms for metric input;
• Supports feet, inches and pounds for imperial input;
• Uses 18.5 to 24.9 as the displayed standard adult healthy BMI range;
• Calculates a mathematical healthy-weight range from height;
• Does not calculate body-fat percentage;
• Does not adjust BMI for muscle mass or body frame;
• Is intended for adult BMI screening;
• Does not provide a medical diagnosis; and
• Does not replace individual healthcare assessment.

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