BMI Calculator

How to use this calculator

1. Enter your height, and pick its unit — centimetres, metres, feet, inches, or feet and inches.
2. Enter your weight, and pick its unit — kilograms, pounds, or stones and pounds. Choosing stones and pounds shows a second box for the pounds.
3. Your BMI updates as you type, along with its standard BMI category, the marker on the BMI scale and the body-weight interval for BMI 18.5–24.9 at your height.
4. Use Reset to return to the starting values.

Changing a unit converts the figure you have already entered rather than clearing it, so you can switch freely. Nothing is submitted anywhere and your measurements are not saved.

This calculator is designed for adults aged 18 and over. Children and teenagers are assessed using age- and sex-specific growth references instead.

What do I need to enter?

• Height — in centimetres, metres, feet, inches, or feet and inches. Accepted from 50 cm to 275 cm, whichever unit you use.
• Weight — in kilograms, pounds, or stones and pounds. Accepted from 2 kg to 500 kg, whichever unit you use.

What do the results show?

• Your BMI — the main figure: weight in kilograms divided by height in metres squared.
• BMI category — Underweight, Healthy weight, Overweight or Obesity, using the standard adult thresholds.
• BMI scale — a marker showing where your BMI sits across the bands.
• Weight range for BMI 18.5–24.9 — the body weights that would put someone of your height in the healthy-weight band.

How the BMI calculation works

1. Your height and weight are converted to metric units — metres and kilograms.
2. BMI is your weight in kilograms divided by your height in metres squared.
3. The unrounded BMI is placed in one of the four standard adult categories — Underweight, Healthy weight, Overweight or Obesity — by its numeric value, using the threshold values 18.5, 25.0 and 30.0.
4. The BMI formula is run backwards at BMI 18.5 and BMI 24.9 to give the body-weight interval that corresponds to the healthy-weight BMI range for your height.

BMI is a population screening estimate for most adults, with known limitations. It is calculated from height and weight only, so it does not measure body composition — body fat, muscle mass or fat distribution — and it is not a measure of overall health. Treat the category as a starting point, not a verdict.

The BMI formula

Example: 70 kg and 175 cm. Height in metres is 1.75, so BMI = 70 ÷ (1.75 × 1.75) = 22.86, which displays as 22.9 and is in the Healthy weight category. For that height, a BMI of 18.5–24.9 corresponds to a body weight of about 56.7 kg to 76.3 kg.

Adult BMI reference bands

The calculator compares your calculated BMI with four adult reference bands. In population screening, these bands are commonly described using the terms in the second column. These are widely used screening terms, not diagnoses, and the calculator does not assess overall health.

Classification uses the unrounded BMI with a strict less-than test at each boundary, so a BMI of 18.5 is Healthy weight, 25.0 is Overweight, and 30.0 is Obesity.

Body-weight interval for BMI 18.5–24.9

For a given height, this is the span of body weights that corresponds to a BMI between 18.5 and 24.9 — the standard adult BMI reference interval used by the NHS and WHO. Multiply each of those BMI figures by your height in metres squared:

This interval is a BMI-based reference, not a personal medical target. Muscle mass, fat distribution, age, ethnicity and other individual factors are not reflected in BMI, so a weight inside or outside the interval is not a statement about your health. The BMI-based weight range calculator shows this interval from your height alone.

Limitations of BMI

• It does not measure body fat and cannot tell muscle from fat.
• It can place very muscular people in a higher category, and people who have lost muscle in a lower one, than their body fat would suggest.
• It is not adjusted for pregnancy, and it treats fat stored around the middle the same as fat elsewhere.
• For people under 18 and people aged 65 and over, the adult categories are a poorer guide.
• The categories may not apply equally across all ethnic backgrounds — some health bodies use a lower threshold for people of South Asian, Chinese and some other backgrounds.

Treat BMI as one screening figure among several. For general information only, consider other measures such as waist-to-height ratio or a direct estimate from the Body Fat Percentage Calculator — these are themselves screening measures rather than a diagnostic test — alongside advice from a qualified healthcare professional where appropriate.

Adults versus children and teenagers

This calculator and its categories are for adults aged 18 and over. For anyone under 18, BMI is interpreted against age- and sex-specific centile charts, because the amount of body fat that is healthy changes as a child grows. A child on the 90th BMI centile, not a fixed number, is the kind of result a clinician works from. For concerns about a child or young person’s growth or weight, speak with a GP, school nurse or another qualified health professional.

Sources

The formula and category names follow the NHS adult BMI calculator guidance and the WHO obesity and overweight fact sheet, which defines BMI as weight in kilograms divided by height in metres squared, with overweight at a BMI of 25 or more and obesity at 30 or more.

Frequently asked questions