# BMI Calculator

> Body mass index from a height and a weight, in metric or imperial, with the band it falls in and the weight range that band covers at that height in both kilograms and pounds. Adults only: below the age of 20 the bands are withheld rather than guessed at, because BMI at that age is read against a growth reference and the adult categories give the wrong answer.

Use it: https://tessalor.com/en/health/bmi-calculator

This tool runs entirely in the browser. Nothing entered into it is uploaded.

## Formula

```
BMI = weight / height^2
    = kilograms / metres squared

Imperial input is converted first, and exactly:
  1 in = 2.54 cm            1 lb = 0.45359237 kg

The band is decided on the unrounded BMI:
  under 18.5            underweight
  18.5 to under 25      healthy weight
  25 to under 30        overweight
  30 and over           obesity

On the lowered action points the middle two
cut-offs move to 23 and 27.5.

A band's weight range is the same formula run
backwards:
  weight = BMI x metres squared

and each end is rounded inwards to a tenth -
the bottom up, the top down - so every weight
printed is inside the band it is listed under.
```

## Assumptions

- The band is decided on the unrounded BMI, not on the figure displayed. A BMI shown as 25.0 can still read as healthy weight, because the value before rounding was 24.97 and a cut-off is a number rather than a display.
- Below the age of 20 the adult bands are withheld. BMI at that age is read against a BMI-for-age reference, where the same figure means different things at 12 and at 17, so applying the adult bands to a child does not approximate the answer, it gives the wrong one.
- Each weight range is rounded inwards to a tenth, so every weight shown is genuinely inside its band. The consequence is a tenth of a kilogram between one band's top and the next band's bottom, and that gap is the boundary itself rather than a missing case.
- The kilogram and pound ranges are each rounded in their own unit rather than one being converted from the other, so the pound range is the tightest true pound range and is not always the exact conversion of the kilogram one.
- BMI carries no term for body composition, fat distribution or frame. Muscle is denser than the fat it replaces, so a trained athlete lands high on a body that is not overweight, and somebody who has lost muscle with age can land in the healthy band while carrying more fat than they used to.
- Pregnancy cannot be detected from a height and a weight, and the adult bands do not apply during it or while breastfeeding.
- The lowered cut-offs are points for public health action rather than a second classification. The 2004 WHO consultation kept the international bands unchanged and named 23.0 and 27.5 as triggers, leaving each country to decide where they apply.

## Inputs

| Name | Label | Type | Default | Range |
| --- | --- | --- | --- | --- |
| `height` | Height | number | 175 | 50 to 260 |
| `weight` | Weight | number | 70 | 2 to 500 |
| `age` | Age | integer | 30 | 2 to 120 |
| `thresholds` | Cut-off points | select | who |  |

## Outputs

- `bmi` — Body mass index (number), primary
- `classification` — Band (text)
- `weightRange` — Weight for that band at this height (text)
- `distance` — To reach that band (text)

## Questions

### Is my height and weight sent anywhere?

No. The arithmetic runs in this browser tab and there is no server to send it to. Nothing is stored either, beyond the unit preference the site remembers so you are not switching to imperial on every visit.

### Why is my BMI shown as 25.0 but called healthy weight?

Because the band is decided on the figure before it is rounded. A weight of 76.47 kg at 175 cm is a BMI of 24.9738, which displays as 25.0 to one decimal place and is below the cut-off. The alternative would be to classify the rounded figure, which quietly moves the real boundary to 24.95 — a number no health body has ever published.

### Why will it not give a band for a 17-year-old?

Because the adult bands are wrong at that age. A BMI of 21 is unremarkable at 30, high at 8 and low at 18, so BMI in childhood and adolescence is read as a percentile against a reference for the age and sex rather than against a fixed number. WHO publishes that reference for ages 5 to 19. The figure itself is still shown here, because it is the input to that reading.

### Does BMI work if I am muscular?

Not well. BMI is a ratio of total mass to height and has no way of telling muscle from fat, so a heavily trained body reads high on a composition that is not the one the band was named for. The same blindness runs the other way: muscle lost with age or illness can leave somebody inside the healthy band while carrying more fat than they did a decade earlier. It is a population screening measure that happens to be cheap on an individual, not a measurement of body fat.

### What are the lowered thresholds, and who are they for?

A 2004 WHO expert consultation found that Asian populations carry a higher proportion of body fat at the same BMI and face raised risk of type 2 diabetes and cardiovascular disease below a BMI of 25. It kept the international classification and named 23.0, 27.5, 32.5 and 37.5 as points for public health action, leaving each country to decide which apply. The option here applies the first two. It is a different kind of statement from a diagnosis, and the bands are labelled by risk to say so.

### Why does the pound range not convert exactly to the kilogram range?

Because each is rounded in its own unit. At 175 cm the healthy band tops out at 76.5 kg and at 168.7 lb; converting 76.5 kg gives 168.65 lb, and rounding that up to 168.7 would name a weight fractionally outside the band. Rounding each unit inwards on its own keeps both figures true, at the cost of the two not being exact conversions of one another.

### Should I aim for a particular number in the range?

The tool does not say, and neither does BMI. The healthy band at 165 cm is 50.4 kg to 68.0 kg — a spread of 17.6 kg, which is wide enough that any single target inside it is a preference rather than a finding. BMI was built to compare populations, and it tells you which band a body falls in, not what that body should weigh.

## Sources

- [Obesity and overweight](https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight) — World Health Organization, applies to 2026. Retrieved 2026-07-30.
- [Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies](https://pubmed.ncbi.nlm.nih.gov/14726171/) — WHO Expert Consultation, in The Lancet 363(9403):157-163, via PubMed, applies to 2004. Retrieved 2026-07-30.
- [Heart-Healthy Living: Aim for a Healthy Weight](https://www.nhlbi.nih.gov/health/heart-healthy-living/healthy-weight) — National Heart, Lung, and Blood Institute, National Institutes of Health, applies to the adult BMI categories at 18.5, 25 and 30. Retrieved 2026-07-30.
- [Growth reference data for 5-19 years — BMI-for-age](https://www.who.int/tools/growth-reference-data-for-5to19-years) — World Health Organization. Retrieved 2026-07-30.
- [NIST Guide to the SI (Special Publication 811), Appendix B: conversion factors](https://www.nist.gov/pml/special-publication-811) — National Institute of Standards and Technology. Retrieved 2026-07-30.

## Variants

- [What is a healthy weight for my height?](https://tessalor.com/en/health/bmi-calculator/healthy-weight-for-my-height)
- [Should I be using a lower BMI threshold?](https://tessalor.com/en/health/bmi-calculator/lower-bmi-thresholds)

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