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⚖️ BMI Calculator

By ToolNimba Health Team · Reviewed by ToolNimba Editorial Review, health content · Updated 2026-06-19

BMI is a screening tool, not a diagnosis, and it does not measure body fat directly. It is not suitable for assessing children, pregnant people or elite athletes the same way. Use it as a starting point and speak to a doctor before making decisions about your health.

Your BMI
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Category
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Body Mass Index (BMI) estimates whether your weight is in a healthy range for your height. Enter your height and weight below, in metric or imperial, to get your BMI, the category it falls in, and the healthy-weight range for your height. BMI is a quick first screen, not a diagnosis, so read the notes below to understand what your number does and does not tell you.

What is the BMI Calculator?

BMI is a single number calculated from your weight and height: weight in kilograms divided by height in metres squared. It was developed in the 1830s by Belgian statistician Adolphe Quetelet to describe populations, not individuals, which is the root of both its usefulness and its limits. Because it is cheap, fast and needs no equipment, health bodies like the WHO and CDC use it as a population-level screening tool.

What BMI cannot do is tell muscle from fat, or show where fat sits on your body. A muscular athlete and a sedentary person can share a BMI of 27 with very different health profiles. Fat around the abdomen carries more risk than fat on the hips, and BMI is blind to that, which is why a waist measurement is often used alongside it. BMI is also read differently for children and teens (against age and sex percentiles), and some health bodies use lower thresholds for people of South Asian descent, who face higher risk at a given BMI.

When to use it

  • Checking where you fall before a routine check-up or an insurance form.
  • Tracking the direction of change over months as you adjust diet or activity.
  • Setting a rough healthy-weight target range for your height.

How to use the BMI Calculator

  1. Choose metric (cm/kg) or imperial (ft-in/lb) units.
  2. Enter your height and your current weight.
  3. Read your BMI value, your category, and the healthy weight range for your height.

Formula & method

Metric: BMI = weight(kg) ÷ height(m)2.   Imperial: BMI = 703 × weight(lb) ÷ height(in)2.

Worked examples

You weigh 70 kg and are 1.75 m tall.

  1. height² = 1.75 × 1.75 = 3.0625
  2. BMI = 70 ÷ 3.0625 = 22.9

Result: BMI 22.9, Healthy weight

You weigh 154 lb and are 5 ft 7 in (67 in).

  1. BMI = 703 × 154 ÷ (67 × 67)
  2. = 108,262 ÷ 4,489 = 24.1

Result: BMI 24.1, Healthy weight

BMI categories for adults (WHO)

BMI rangeCategory
Below 18.5Underweight
18.5 – 24.9Healthy weight
25.0 – 29.9Overweight
30.0 – 34.9Obese (Class I)
35.0 – 39.9Obese (Class II)
40.0 and aboveObese (Class III)

Common mistakes to avoid

  • Treating BMI as a body-fat percentage. It estimates a category from height and weight only; it never measures fat directly.
  • Using adult thresholds for children. Kids and teens are assessed on age- and sex-specific percentiles, not the 18.5–24.9 adult band.
  • Mixing units. Entering height in cm but weight in pounds (or vice versa) throws the result off by a wide margin.

Glossary

BMI
Weight (kg) divided by height (m) squared, a screening estimate of weight category.
Obesity class
Sub-bands (I, II, III) within the obese range used clinically to gauge risk.
Waist-to-height ratio
A complementary measure that captures where fat is carried, which BMI ignores.

Frequently asked questions

What is a healthy BMI range?

For most adults, a BMI of 18.5 to 24.9 is considered the healthy range. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is in the obese range.

What BMI is considered obese?

A BMI of 30 or above is classified as obese. It is split into Class I (30–34.9), Class II (35–39.9) and Class III (40+), which doctors use to gauge health risk.

Is BMI different for men and women?

The BMI formula and adult category thresholds are the same for men and women. However, at the same BMI women tend to have more body fat than men, so BMI is interpreted alongside other measures rather than in isolation.

Why is BMI inaccurate for athletes?

Muscle is denser than fat, so very muscular people can have a high BMI while carrying little fat. For them BMI overestimates health risk, which is why body-fat or waist measurements are used instead.

How do I calculate BMI by hand?

In metric, divide your weight in kilograms by your height in metres squared. In imperial, multiply your weight in pounds by 703 and divide by your height in inches squared.

Does this calculator store my data?

No. The calculation happens entirely in your browser and nothing is sent anywhere or saved.

Sources