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BMI Calculator

Calculate your Body Mass Index using height and weight to assess whether you are underweight, healthy, overweight or obese using standard BMI classifications.

Last reviewed 15 August 2026Reviewed after a methodology change

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Calculator inputs

When to use this calculator

  • Before setting a new health goal or checking whether a plan is realistic.
  • When you want a quick baseline before discussing the result with a professional.
  • When you are starting a fitness or diet programme and want a starting measurement.
  • When you want to recheck the numbers after several weeks of change.

A realistic planning example

Use these sample inputs as a quick scenario test, then change one variable at a time to compare outcomes.

A realistic planning example
InputValue
Weight (kg)80 kg
Height (cm)178 cm
Age35
Waist Measurement84 cm

After entering these figures, review bmi, healthy weight range and waist-to-height together rather than in isolation. Then rerun the tool with one input adjusted.

How to read your results

BMI

Treat BMI as a broad screening marker rather than a clinical diagnosis, and read it alongside the other health indicators the tool produces.

Healthy Weight Range

A planning range based on standard BMI thresholds, not a prescriptive personal target.

Waist-to-Height

Waist measurement adds context to a headline BMI result and can indicate metabolic risk even when overall weight appears normal.

Method & assumptionsAuthoritative sources

Body Mass Index (BMI) is calculated by dividing your weight in kilograms by the square of your height in metres (kg/m²). It was developed in the 19th century as a statistical tool for comparing body size across populations, and it remains one of the most widely used screening metrics in public health. However, BMI was never designed as a diagnostic tool for individuals. It takes no account of age, sex, muscle mass, bone density, or fat distribution — all factors that influence health risk. Use your BMI result as a starting point for a broader conversation about health, not as a definitive verdict. A GP or registered dietitian can provide context based on your full medical history.

Common mistakes

  • !Using an assumption that is not supported by a current local quote, bill, statement or official source.
  • !Treating a generic estimate as a lender, provider, payroll or tax authority decision.
  • !Mixing monthly and annual inputs without converting them consistently.
  • !Testing only one scenario instead of checking how a cautious assumption changes the result.

What to do next

  • Run a second scenario with a cautious assumption so you can see the downside clearly.
  • Compare the result with the related calculators below before making a decision.
  • Check current local rules, eligibility and provider terms before applying or committing money.
  • Keep a record of the assumptions so you can update the estimate when a quote, bill or pay figure changes.

Keep exploring

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Frequently asked

The WHO and WHO classify BMI as: underweight below 18.5, healthy 18.5-24.9, overweight 25-29.9, obese class 1 30-34.9, obese class 2 35-39.9, and obese class 3 (severely obese) 40+. Lower thresholds apply for people of South Asian, Chinese, Middle Eastern, Black African and African-Caribbean descent (overweight from 23, obese from 27.5) because cardiometabolic risk rises at lower BMI.

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