BMI Calculator: Body Mass Index and WHO Categories

Enter your weight and height to get your BMI and its WHO category, from underweight to overweight and obesity.

How to calculate and read your BMI with WHO cut-offs

Body mass index is weight in kilograms divided by height in metres squared. For adults, the WHO defines underweight below 18.5, overweight from 25 and obesity from 30; between 18.5 and 25 the result is in the normal range.

Formula

BMI = weight (kg) / height (m)². The tool rounds the result to one decimal and places it on a gauge running from 10 to 45.

Example

For 70 kg and 1.75 m: 1.75² = 3.0625; 70 / 3.0625 = 22.857, shown as 22.9, which is in the normal range. At 95 kg and 1.80 m the BMI is 29.3 (overweight); at 55 kg and 1.70 m it is 19.0.

Cut-offs used

  • below 18.5: underweight;
  • 18.5 to under 25: normal;
  • 25 to under 30: overweight;
  • 30 and above: obesity (the tool does not split classes I, II and III, which start at 30, 35 and 40).

Assumptions and limits

BMI is a population screening measure. It ignores muscle mass and fat distribution: a muscular athlete can exceed 25 without excess fat, and a large waist does not show in the number. Adult cut-offs do not apply to children and teenagers, who are assessed with age and sex charts, nor to pregnancy. The sliders cover 100-250 cm and 30-200 kg. To estimate body fat itself, a tape-measure method such as the US Navy formula is better suited. The result is an estimate and does not replace a doctor's opinion.

Sources and dates: WHO adult definitions of overweight (BMI ≥ 25) and obesity (BMI ≥ 30), fact sheet updated on 7 October 2026 (WHO, Obesity and overweight); underweight threshold of 18.5 in the Global Health Observatory indicators (WHO, Global Health Observatory, body mass index) (checked on 14/10/2026)

Frequently Asked Questions

Is BMI reliable for athletes and bodybuilders?

No, BMI does not distinguish between fat mass and muscle mass. A muscular athlete may be classified as 'overweight' by BMI despite having optimal body composition.

What is the BMI formula?

BMI = Weight (in kg) divided by the square of Height (in meters). Example: 70 kg / (1.75 m × 1.75 m) = 22.86, rounded to 22.9.

What is a normal BMI for an adult?

According to the WHO, a BMI from 18.5 to under 25 is in the normal range. Below 18.5 is underweight, 25 and above is overweight and 30 and above is obesity. The same cut-offs apply to adult men and women.

Can I use this BMI for a child?

No: the formula is the same, but for children and teenagers the result is read on BMI-for-age charts by sex. The adult cut-offs of 18.5, 25 and 30 only apply to adults.

Is this tool really free?

Yes, all tools on Kristools are 100% free and accessible without registration.

Is my data protected?

Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.