BMI Calculator Widget

Add a BMI calculator to your website. Visitors enter height and weight in metric or imperial units and see their BMI, the WHO adult category and the weight range that corresponds to a BMI of 18.5 to 24.9 for their height.

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<iframe src="https://a2z.tools/embed/w/bmi-calculator" title="BMI Calculator by A2Z Tools" width="100%" height="520" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>

A plain iframe. Works everywhere, including site builders that strip scripts. Adjust height if your content needs more room.

Works with

How it works

The widget uses the same BMI engine as the A2Z BMI Calculator: weight in kilograms divided by the square of height in metres, the definition WHO gives. Imperial heights are entered as feet plus inches and converted with the exact international definitions (1 in = 2.54 cm, 1 lb = 0.45359237 kg), which is why a 150 lb, 5 ft 5 in adult gets 24.96 - the same figure CDC's worked example reaches with its 703 factor. The WHO category is decided on the unrounded BMI, so 24.96 stays in the healthy band even though it displays as 25.0, and no 'above the range' line is shown for it. Obesity is split into WHO classes I (30-34.9), II (35-39.9) and III (40+), and a coloured scale from 15 to 40 marks the result with its label in text, so the category never depends on colour alone. The healthy range is the BMI formula solved for weight at 18.5 and 24.9 for the entered height; when the weight is outside it and the category is not healthy, the widget states the gap in kg or lb.

Calculation method

  • BMI = weight (kg) / height (m)^2, shown to 1 decimal place; the category uses the unrounded value
  • Imperial: height (in) = feet x 12 + inches; cm = in x 2.54; kg = lb x 0.45359237 (exact definitions)
  • WHO adult categories: below 18.5 underweight; 18.5-24.9 normal (shown as healthy weight); 25.0-29.9 pre-obesity (overweight); 30.0-34.9 obesity class I; 35.0-39.9 class II; 40 and above class III
  • Healthy range for a height h (m) = 18.5 x h^2 to 24.9 x h^2 kg, shown to 1 decimal place in kg or lb
  • Gap = entered weight minus the nearer end of that range, shown only when the category is not healthy

Worked examples

Metric, healthy weight

Inputs: 175 cm, 70 kg

Result: BMI 22.9, WHO adult category Healthy weight; healthy range for 175 cm is 56.7 to 76.3 kg

70 / 1.75^2 = 22.86. The range is 18.5 x 3.0625 = 56.66 kg to 24.9 x 3.0625 = 76.26 kg.

Imperial, just under the overweight cut-off

Inputs: 5 ft 5 in, 150 lb (imperial)

Result: BMI 25.0 displayed, category Healthy weight; healthy range 111.2 to 149.6 lb

65 in = 1.651 m and 150 lb = 68.04 kg give 24.96, below 25, so the category stays healthy and no gap line appears even though 150 lb is 0.4 lb above the 24.9 bound.

Metric, obesity class I

Inputs: 175 cm, 100 kg

Result: BMI 32.7, Obesity class I; above the healthy range by 23.7 kg

100 / 3.0625 = 32.65, inside the WHO 30.0-34.9 band; 100 - 76.26 = 23.7 kg.

An estimate for general information only - not medical advice. Talk to a doctor or registered dietitian about your health.

Limitations

  • Adults only: children and teenagers need age-and-sex BMI percentiles (for example the WHO or CDC growth charts), which the widget does not provide.
  • BMI cannot distinguish muscle from fat or show where fat is stored, so muscular athletes may read as overweight and older adults with low muscle may read as healthy.
  • Not valid during pregnancy, when weight gain is expected and assessed against pre-pregnancy BMI instead.
  • Uses the global WHO cut-offs only; it does not apply the lower Asian action points or any national variant.

Where publishers use it

  • A gym, personal trainer or nutrition coach site where prospects check a starting point before booking
  • Health and wellness articles about weight, where readers can try the numbers themselves
  • Clinic or pharmacy websites that want a simple, clearly caveated screening number
  • School and college biology or PE lessons on how BMI is calculated and where it falls short

Questions

Why do some countries use lower BMI cut-offs?

A 2004 WHO expert consultation found that many Asian populations carry higher health risk at lower BMI, and suggested public-health action points such as 23 and 27.5. Some national guidelines, including in India and Singapore, use these lower thresholds.

Which BMI categories does the widget use?

The World Health Organization adult cut-offs: below 18.5 underweight, 18.5 to 24.9 normal weight, 25 to 29.9 pre-obesity (overweight) and 30 or more obesity, split into classes I, II and III at 35 and 40.

Does BMI work for children or athletes?

Not well. Children and teenagers are assessed with age-and-sex BMI percentiles rather than adult cut-offs, and muscular people can have a high BMI without excess fat. The widget states both limits under the result.

Why does 150 lb at 5 ft 5 in show 25.0 but still say healthy weight?

The exact BMI is 24.96, which is below the WHO overweight cut-off of 25. The display rounds to one decimal place, but the category is decided on the unrounded number - the same example CDC uses to explain the imperial formula.

Can I show imperial units by default?

Yes. Add units=imperial to the embed URL, or pick Imperial in the builder. Visitors can still switch between metric and imperial.

Why do some health services use different cut-offs?

A 2004 WHO expert consultation found that many Asian populations carry higher diabetes and heart-disease risk at a BMI below 25 and proposed extra action points at 23, 27.5, 32.5 and 37.5. National guidelines that adopt them classify the same BMI differently from this widget.

Sources

  1. A healthy lifestyle - WHO recommendations (BMI classification for adults) - WHO Regional Office for Europe . BMI table: underweight, normal, pre-obesity, obesity classes I-III
  2. Obesity and overweight - fact sheet - World Health Organization . Definition BMI = kg/m2; overweight >= 25, obesity >= 30 for adults
  3. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies - WHO Expert Consultation, The Lancet 363:157-163 (2004) . Why some national guidelines use lower cut-offs
  4. Adult BMI categories - US Centers for Disease Control and Prevention . Imperial worked example: 150 lb at 5 ft 5 in = 24.96
  5. NIST Guide to the SI, Appendix B.8 - conversion factors - National Institute of Standards and Technology (SP 811) . Inch = 0.0254 m and pound = 0.45359237 kg exactly

Cite or recommend this tool

If you reference this tool in an article, course or documentation, these formats are ready to copy. They are optional - nothing is added to your site unless you paste it.

A2Z Tools BMI Calculator
https://a2z.tools/bmi-calculator

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