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BMI Calculator

Enter your height and weight in whichever units you know them in to calculate your body mass index (BMI), see which standard adult BMI category it falls in, see where the figure sits on the BMI scale, and see the body-weight interval corresponding to a BMI of 18.5 to 24.9 for your height.

Your measurements

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

Your BMI
22.9
BMI category
Healthy weight

A BMI of 22.9 is in the healthy-weight range for most adults.

A BMI of 22.9 on the adult BMI scale — Healthy weight.
Underweightunder 18.5
Healthy weight18.5–24.9
Overweight25.0–29.9
Obesity30.0 and above

For this height, a BMI of 18.5–24.9 corresponds to a body weight of 56.7 kg – 76.3 kg.

Screening estimate only. BMI is calculated from height and weight and does not measure body fat, muscle mass, fat distribution or overall health. It is not a diagnosis or a personal treatment target. BMI may be less useful during pregnancy, for people under 18, people aged 65 and over, and people with high muscle mass.

BMI reference bands may not apply equally across all ethnic backgrounds. For general information only, consider other measures such as waist-to-height ratio and advice from a qualified healthcare professional where appropriate.

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.

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

BMI = weight (kg) ÷ height (m)²
Weight at BMI 18.5 = 18.5 × height (m)²
Weight at BMI 24.9 = 24.9 × height (m)²
  • Weight is in kilograms.
  • Height is in metres, then squared.
  • Imperial input is converted to kilograms and metres before the division.
  • Unrounded values are used for category boundaries; only the displayed figures are rounded.

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.

Adult BMI reference bands and their screening terms
Adult BMI reference bandCommonly used screening term
Below 18.5Underweight
18.5–24.9Healthy weight
25.0–29.9Overweight
30.0 and aboveObesity

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:

lower = 18.5 × height²    upper = 24.9 × height²
  • At 1.75 m: 18.5 × 3.0625 = 56.7 kg and 24.9 × 3.0625 = 76.3 kg.
  • In imperial the calculator converts that interval to stone and pounds.

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 — itself a screening measure 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.

Frequently asked questions

What does my calculated BMI mean?

It is your weight in kilograms divided by the square of your height in metres, and it places you in one of the four standard adult BMI categories by its numeric value: Underweight below 18.5, Healthy weight 18.5–24.9, Overweight 25.0–29.9, and Obesity 30.0 and above. These are population screening terms for most adults, not a diagnosis or a statement about your individual health.

Is BMI a diagnosis of health?

No. BMI is a screening estimate calculated from height and weight alone. It does not measure body fat, muscle mass, fat distribution or overall health, and it is not a diagnosis or a personal treatment or weight target. The category names are widely used screening terms; a Healthy weight result does not confirm you are healthy, and an Overweight or Obesity result is not a diagnosis.

Does this calculator tell me my ideal or correct weight?

No. It shows the body-weight interval that corresponds to a BMI of 18.5 to 24.9 for your height, which is a reference interval rather than a personal target. It does not give diet, treatment or medication advice, and a weight inside or outside that interval is not a verdict on your health.

Why can BMI be less useful for muscular people, pregnancy, older adults or some ethnic backgrounds?

BMI uses only height and weight, so it cannot tell muscle from fat. Someone with high muscle mass can sit in a higher category while carrying little excess fat, and someone who has lost muscle can sit in a lower category while carrying more. During pregnancy, body weight rises for reasons the adult categories were never designed to describe. For adults aged 65 and over, the relationship between BMI and health differs from younger adults. And some health bodies apply lower thresholds for people of South Asian, Chinese and some other backgrounds, so the standard categories may not apply equally across all ethnic backgrounds. These are limitations of the measure, not diagnoses.

Is this calculator suitable for children or teenagers?

No. This calculator is designed for adults aged 18 and over. Children and teenagers are assessed using age- and sex-specific growth references rather than the fixed adult categories, because a healthy amount of body fat changes as a child grows. For concerns about a child or young person’s growth or weight, speak with a GP, school nurse or another qualified health professional.

How is the displayed body-weight interval calculated?

It is the BMI formula run backwards at each end of the 18.5–24.9 reference interval. The lower figure is 18.5 × height in metres squared and the upper figure is 24.9 × height in metres squared. For someone 1.75 m tall that is about 56.7 kg to 76.3 kg. Unrounded values are used internally and only the displayed figures are rounded.

Which category does a BMI of exactly 25 fall in?

Overweight. The calculator uses the unrounded BMI and 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. A value that displays as 25.0 after rounding but is actually 24.97 stays in the Healthy weight category.

Can I enter my height and weight in any units?

Yes. The height dropdown takes centimetres, metres, feet, inches, or feet and inches; the weight dropdown takes kilograms, pounds, or stones and pounds. Change a unit and the value you have already typed is converted, so the BMI, the category, the scale position and the body-weight interval are identical whichever units you use.