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BMI Calculator

BMI Calculator helps you estimate bmi and healthy weight range for Ireland using Weight (kg), Height (cm), and Age. Use it to compare scenarios before making a final decision.

Last reviewed: 14 June 2025Source: WHO — BMI classification
BMI Calculator · IEHealth & Lifestyle

Rates & sources

Body Mass Index uses weight (kg) ÷ height (m)². WHO categories apply globally.

Underweight< 18.5
Healthy18.5 – 24.9
Overweight25.0 – 29.9
Obese≥ 30.0

Source: WHO — BMI classification — figures refreshed at the start of each tax year.

When to use this calculator

  • Before setting a new health goal or checking whether a plan is realistic.
  • When you want to compare different assumptions without tracking them manually.
  • When you need a quick baseline before discussing the result with a professional.

A realistic Ireland planning example

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

Weight (kg)

80 kg

Height (cm)

178 cm

Age

35

Waist Measurement

84 cm

After entering these figures, compare bmi, healthy weight range and waist-to-height before deciding which scenario looks strongest.

How to read your results

BMI

Treat this as a screening marker rather than a diagnosis, and read it alongside the other health indicators.

Healthy Weight Range

This is a planning range, not a personal target in isolation, so use it with context and common sense.

Waist-to-Height

This adds context to the headline result and can help you spot whether body composition changes the picture.

Method & assumptionsAuthoritative sources

Body Mass Index (BMI) is calculated by dividing your weight in kilograms by the square of your height in metres. The result places you within a standard WHO classification: Underweight (below 18.5), Healthy Weight (18.5-24.9), Overweight (25-29.9), or Obese (30 and above).

BMI is a widely used screening tool because it requires only two measurements and correlates broadly with body fat percentage across large populations. However, it does not distinguish between fat and muscle, so very muscular individuals may show a high BMI without excess body fat.

Use this calculator as a general guide only. A GP or healthcare professional can provide a more detailed assessment of your weight and overall health using additional measures such as waist circumference and blood pressure.

Common mistakes

  • !Using optimistic assumptions without testing a more cautious scenario as well.
  • !Comparing outputs from different tools without checking that the inputs match.
  • !Treating the result as a final quote instead of a planning estimate.

What to do next

  • Try at least one more scenario so you can compare a realistic range instead of a single estimate.
  • Use the related calculators below to cross-check the decision from another angle.
  • Open one of the linked guides if you need more context before you act on the result.

Frequently asked

The NHS 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.

Use arrow keys to navigate items, Enter or Space to expand/collapse.

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