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Primax Tools

Adult BMI Calculator

Estimate adult body mass index in metric or imperial units, with clear context about what BMI can and cannot tell you.

Runs entirely in your browser. Nothing is uploaded.

Adult categories apply to people aged 20 and over.

Body mass index

—

  • Underweight < 18.5
  • Healthy weight 18.5–24.9
  • Overweight 25–29.9
  • Obesity ≥ 30

BMI is a screening estimate, not a diagnosis. It does not measure body fat directly or account for muscle mass, age, sex, ethnicity or pregnancy. Talk to a qualified healthcare professional about your health.

Step by step

How to use the BMI Calculator

  1. 1

    Choose metric (cm and kg) or imperial (feet, inches and pounds).

  2. 2

    Enter height and weight, and optionally your age.

  3. 3

    See your BMI, the adult category and where it falls on the scale.

  4. 4

    Read the healthy-weight range estimate for your height and the limitations below.

What it does well

Metric and imperial

Switch units at any time; values are converted so you do not lose your entries.

Visual range indicator

A scale marks the WHO adult categories with a marker at your result.

Healthy-weight estimate

Shows the weight range corresponding to a BMI of 18.5 to 24.9 at your height.

Responsible guidance

Adult categories are not shown for people under 20, and BMI is never presented as a diagnosis.

Who uses it

Adults tracking health
Get a quick screening number to discuss with a healthcare professional.
Fitness coaches
Provide context alongside better measures such as waist circumference.
Students
Learn how the BMI formula works in both unit systems.

How BMI is calculated

Body mass index is weight in kilograms divided by height in metres squared. In imperial units it is weight in pounds times 703, divided by height in inches squared. A person 175 cm tall weighing 70 kg has a BMI of 70 / 1.75 squared, which is about 22.9.

The World Health Organization classifies adult BMI as underweight below 18.5, healthy weight from 18.5 to 24.9, overweight from 25 to 29.9 and obesity at 30 or above. These thresholds were designed for population screening, not for judging an individual.

Important limitations

BMI does not distinguish muscle from fat, so athletes may score as overweight while being healthy. It does not show where fat is stored, and some ethnic groups face higher health risks at lower BMI values. It is not valid during pregnancy. For children and teenagers under 20, BMI must be interpreted with age- and sex-specific growth percentiles, so this calculator does not apply adult categories to them. Always speak to a qualified healthcare professional before making health decisions.

Frequently asked questions

Is BMI a diagnosis?

No. It is a screening estimate. Only a qualified professional can assess your health.

Can I use this for my child?

The number can be calculated, but adult categories do not apply under 20. Use a pediatric BMI-for-age chart with your doctor.

What is a healthy weight for my height?

The calculator shows the range that corresponds to an adult BMI of 18.5 to 24.9 at your height.

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