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BMI Calculator

Find your BMI from height and weight — with the WHO category, and an honest note on what BMI misses.

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Quick Answer

BMI Calculator divides your weight in kilograms by your height in metres squared — the Body Mass Index, or Quetelet index — and labels the result against the WHO adult categories: underweight, normal, overweight, or obese. Enter height in centimetres and weight in kilograms and the number appears instantly. BMI is a quick screen, not a diagnosis — it cannot tell muscle from fat — and nothing you enter is uploaded.

What the BMI Calculator Does

BMI is the most common quick check of whether your weight sits in a healthy range for your height. This tool computes it from two numbers and places you in one of the four standard WHO bands, with a colour cue for the healthy range.

It is deliberately simple, and that is both its strength and its limit. One ratio can flag whether weight might be worth a closer look, but it cannot see body composition — so the result is a starting point for a conversation, not a verdict.

How It Works

Your height in centimetres is converted to metres and squared; your weight in kilograms is divided by that figure. The result is your BMI, rounded to one decimal place.

The number is then compared against the WHO adult thresholds — 18.5, 25, and 30 — to assign a category, and the display colours the healthy range green so you can read it at a glance.

Methodology

  1. Step 1. Enter your height in centimetres and weight in kilograms.
  2. Step 2. Height is converted to metres (cm ÷ 100) and squared.
  3. Step 3. BMI = weight ÷ height², rounded to one decimal.
  4. Step 4. The value is matched to a WHO band: under 18.5, 18.5–24.9, 25–29.9, or 30 and over.
  5. Step 5. The BMI and its category display instantly, updating as you change a value.

BMI formula

BMI = weight (kg) ÷ height (m)²
Worked examples
70 kg, 1.70 m → 70 ÷ (1.70 × 1.70) = 70 ÷ 2.89 = 24.2
24.2 falls in 18.5–24.9 → Normal weight

Height is entered in centimetres and divided by 100 to get metres. The WHO adult bands: under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30 and over obese.

Assumptions

  • Uses the WHO adult cutoffs, which are the same for men and women aged roughly 20 and over.
  • Height is taken in centimetres and weight in kilograms (metric).
  • BMI is treated as a screening proxy for body fat, not a direct measurement of it.
  • The result is not adjusted for age, sex, ethnicity, muscle mass, or where fat is carried.

Technical Details

Formulaweight(kg) ÷ height(m)²
InputsHeight (cm), weight (kg)
OutputBMI to one decimal + WHO category
Categoriesunder 18.5, 18.5–24.9, 25–29.9, 30+
ScopeAdults (20+); not for children or pregnancy
ComputeLive in your browser — no upload

Standards & references

  • WHO BMI classification — the adult cutoffs used here: underweight below 18.5, normal 18.5–24.9, overweight 25–29.9, obese 30 and above.
  • Quetelet index — BMI's original name, defined in the 1830s as body mass divided by height squared.
  • Asian-population cutoffs — the WHO Western Pacific office proposes lower thresholds for Asian adults (overweight from 23, obesity from 25); this tool uses the standard global bands.

Accuracy & Limitations

BMI is a screen, not a diagnosis. It estimates whether weight is in a healthy range for height, but it cannot distinguish muscle from fat — a muscular athlete may read overweight, and someone with low muscle may read normal while carrying excess fat.

It ignores sex, age, ethnicity, frame size, and fat distribution, all of which affect health risk. A waist measurement or a body-fat method gives a fuller picture alongside BMI.

The categories are WHO adult thresholds. They are not appropriate for children and teenagers (who use age- and sex-specific percentiles), for pregnant people, or for the frail elderly. Some health bodies also use lower cutoffs for people of Asian descent.

The arithmetic is exact for your inputs; the result is rounded to one decimal, which is why a value can sit right on a boundary such as 25.0.

Real-World Use Cases

A quick health check

See roughly where your weight sits for your height before a check-up or a fitness plan.

Track a trend

Watch BMI move over time as a simple, consistent yardstick.

Set a target range

Find the weight range that puts you in the normal band for your height.

Context for a conversation

Bring a number to a discussion with a doctor or trainer, who can add what BMI misses.

When to use it — and when not to

Good for

  • A fast, standard weight-for-height screen
  • Tracking your own BMI over time
  • Finding your healthy-weight range
  • Adults of average build

Not the best choice for

  • Diagnosing health or body fat on its own
  • Athletes and very muscular builds
  • Children, teens, and pregnant people
  • Judging fat distribution or fitness

Pair BMI with a waist measurement or a body-fat estimate for a fuller picture; for children, use age- and sex-specific BMI-for-age percentiles.

Frequently Asked Questions

How is BMI calculated?
BMI is your weight in kilograms divided by the square of your height in metres. For example, 70 kg at 1.70 m is 70 ÷ 2.89 ≈ 24.2.
What is a healthy BMI?
The WHO normal range is 18.5 to 24.9. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is obese.
Is BMI accurate for athletes?
Not very. Muscle weighs more than fat, so a lean, muscular person can land in the overweight band despite low body fat. BMI cannot see body composition.
Does BMI work for children?
Not with these adult cutoffs. Children and teens are assessed with age- and sex-specific BMI-for-age percentiles, not the fixed adult thresholds.
Why does it ask for centimetres and kilograms?
BMI is defined in metric units — kilograms over metres squared. The tool takes height in centimetres and converts it to metres for you.
Should the cutoffs be different for Asian people?
Some health bodies think so: the WHO Western Pacific office suggests overweight from a BMI of 23 and obesity from 25 for Asian adults, because health risks can appear at lower BMIs. This tool uses the standard global bands.
Is BMI enough on its own?
No — treat it as a first screen. A waist measurement, body-fat percentage, blood pressure, and other markers give a much fuller view of health than BMI alone.
Does it account for muscle or fat distribution?
No. BMI is just weight and height; it cannot tell where weight comes from or where fat sits. Two people with the same BMI can have very different body compositions.
Is my height and weight uploaded?
No. The calculation happens entirely in your browser. Nothing you enter is sent to a server or stored.
What does the colour mean?
It is a quick visual cue: the normal range shows green, while underweight, overweight, and obese show amber or red. The colour just mirrors the category.

References

Built from the BMI formula and the WHO bands the tool actually uses — and upfront that BMI cannot tell muscle from fat.

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