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BMI Calculator - adults, kids & youth

Accurate BMI calculator for adults, kids, and teens. Check your body mass index and see tailored ranges for every age.

Your body measurements

Enter your weight and height to receive a quick BMI estimate.

Your details
kg

Your current weight in kilograms.

cm

Your height in centimetres.

years

BMI guidance differs for adults and people under 20.

Used to reference BMI-for-age charts for children and teens.

Switch between the traditional squared-height formula and the modern "New BMI" exponent.

Results update instantly

Results

Example using default values

See how you compare to the recommended BMI range.

BMI (Traditional BMI)
23.5

Your body mass index rounded to one decimal place.

Healthy weight
Age: 30Sex: FemaleAdult BMI categories apply.

Weight span for this category: 56.7 - 76.6 kg

Your BMI is within the range that WHO considers healthy for most adults.

WHO adult thresholds: under 18.5 underweight · 18.5-24.9 healthy · 25.0-29.9 overweight · 30+ obesity.

Compare with New BMI (Trefethen)
23.1New BMI (Trefethen)

Shows the BMI value if you were to use the New BMI (Trefethen) formula instead.

Weight spans across WHO BMI classes

Converted to kilograms using your selected formula. Heavier or lighter ranges extend beyond the listed limits.

Severe thinnessBMI < 16.0
≤ 49.0 kg

Immediate medical attention is recommended to rule out illness or malnutrition.

Moderate thinnessBMI 16.0-16.9
49.0 - 52.1 kg

Monitor closely. Healthcare support can help explore nutrition or underlying conditions.

Mild thinnessBMI 17.0-18.4
52.1 - 56.7 kg

Slightly below the normal range. Review dietary intake and activity level.

Healthy rangeBMI 18.5-24.9
56.7 - 76.6 kg

WHO’s healthy interval for adults. Maintain balanced nutrition and activity.

This is the goal range for most adults.

Overweight (pre-obese)BMI 25.0-29.9
76.6 - 91.9 kg

Actionable zone for lifestyle adjustments to avoid long-term health risks.

Obesity class IBMI 30.0-34.9
91.9 - 107.2 kg

First obesity class. Medical guidance is recommended to plan sustainable changes.

Obesity class IIBMI 35.0-39.9
107.2 - 122.5 kg

Higher risk tier. Structured support and screening for co-morbidities are advised.

Obesity class IIIBMI ≥ 40.0
≥ 122.5 kg

Very high risk tier. Work with healthcare specialists for a tailored plan.

WHO adult BMI categories

WHO classification (via Wikipedia)
CategoryBMI (kg/m²)
Severe thinness< 16.0
Moderate thinness16.0-16.9
Mild thinness17.0-18.4
Normal range18.5-24.9
Overweight (pre-obese)25.0-29.9
Obesity class I30.0-34.9
Obesity class II35.0-39.9
Obesity class III≥ 40.0

Sources

Estimate, not a legal decision

Read methodology

Use the result as decision support and planning help. For high-stakes choices, confirm the details with the relevant authority, lender, employer, or adviser.

Analysis & guidance

  • Use BMI as guidance

    BMI does not factor in muscle mass or body composition—combine it with waist measurements for a fuller picture.

  • Set goals safely

    Work with healthcare professionals when planning to gain or lose weight.

  • Kids and teens use percentiles

    For under-20s, always interpret BMI using age- and sex-specific percentiles (P5-P85 is the healthy span).

  • Be consistent with method

    Traditional vs New BMI yields small differences. Track the same method over time for comparable values.

  • Small steps compound

    Incremental changes to activity, sleep, and eating habits often deliver the biggest long-run impact.

About this calculator

About BMI

Body mass index (BMI) is a simple indicator based on height and weight. It’s widely used as a quick screening tool for weight categories, but it does not measure body fat directly. In this article we explain what BMI is, how to interpret the result, and when you should be cautious.

What is BMI?

BMI is calculated as weight divided by height squared:

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

Because it normalizes weight by height, it makes it easier to compare people of different heights.

How to interpret BMI

Most BMI calculators classify results into broad categories (for adults): underweight, normal weight, overweight, and obesity. The exact cutoffs can vary slightly by guideline and context, but the purpose is the same: to provide a rough risk indicator.

Example: If you weigh 80 kg and are 1.80 m tall:

Example: BMI = 80 / (1.8^2) ≈ 24.7

Common limitations (important)

BMI is useful, but it has blind spots:

  • Muscle mass: People with high muscle mass can have a high BMI without excess body fat.
  • Body composition: BMI doesn’t tell you where fat is stored (waist circumference can matter).
  • Age and sex: Health risk patterns differ by age and sex.
  • Pregnancy and certain medical conditions: BMI may not be an appropriate measure.

How to use the BMI calculator

  1. Enter your height and weight.
  2. Review your BMI value and the category shown.
  3. If the result is borderline or unexpected, try re-checking your inputs and consider tracking trends over time.

Practical tips

  • Use BMI as a starting point, not a diagnosis.
  • Combine BMI with other measures like waist measurement, activity level, and how you feel day to day.
  • If you’re worried about your weight or health, talk to a healthcare professional.

How the calculation works

What is BMI?

Body Mass Index (BMI) is a measure used to assess whether a person's weight is within a healthy range relative to their height. BMI is calculated by dividing weight in kilograms by height in meters squared. A BMI between 18.5 and 24.9 is considered normal for adults. Our BMI calculator covers adults and kids alike, giving you a quick BMI value and the healthy weight range for your specific height.

Use BMI as guidance

BMI is a useful tool for getting an overall picture of healthy weight, but it has its limitations. BMI doesn't account for muscle mass, body composition or age. A person with high muscle mass may have a high BMI without being overweight. Therefore, it's important to combine BMI with other health indicators like waist measurement, fitness level and general wellbeing. For children and teens, always interpret BMI using age- and sex-specific percentiles.

Set healthy weight goals

If you're considering gaining or losing weight, our BMI calculator can help you set realistic and healthy goals. The calculator shows the recommended weight range for your height based on WHO guidelines, including tailored ranges for kids and teens. Remember that weight changes should be made gradually and preferably in consultation with healthcare professionals or a dietitian to ensure you do it in a healthy way.

BMI - frequently asked questions

How is BMI calculated?

BMI = weight (kg) / height (m)². Example: 72 kg and 1.75 m gives 72 / 1.75² ≈ 23.5. You can also pick the "New BMI" which uses an exponent of 2.5 - results tend to be slightly lower for taller heights and slightly higher for shorter heights.

What is a healthy BMI for adults?

WHO’s adult range is 18.5-24.9. 25.0-29.9 is overweight and 30.0+ is obesity. Adults 65+ may be advised to aim a bit higher in consultation with clinicians.

How do I interpret BMI for children and teens?

Use age- and sex-specific percentiles. Below 5th = underweight, 5th-85th = healthy, 85th-95th = overweight, ≥95th = obesity.

Is BMI fair for athletic or muscular people?

BMI does not separate muscle from fat. People with high muscle mass can score a high BMI without excess fat. Combine BMI with waist measures, fitness, and overall wellbeing.

Can I use BMI during pregnancy?

BMI is not appropriate for tracking weight during pregnancy. Follow guidance from your antenatal care team instead.

Traditional or New BMI - which should I use?

Both are fine as directional tools. Pick one and track it consistently over time for apples-to-apples comparisons. For kids/teens, percentiles drive interpretation regardless of method.

How to read this calculator

These results are meant as guidance. They are based on rules, assumptions, and simplified models that can differ from your exact real-world situation.

Estimate, not a legal decision

Use the result as decision support and planning help. For high-stakes choices, confirm the details with the relevant authority, lender, employer, or adviser.

Methodology

Each calculator uses defined inputs, assumptions, and logic. We explain the broader approach on the methodology page.

Read methodology

Sources and updates

Important calculators should be traceable back to official rules, public guidance, or other clearly stated references.

Read about sources

Sources

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