Body mass index compares your weight to your height, giving a single number that public-health bodies use to sort adults into weight categories. It is quick, free, and needs no equipment — but it measures size, not body composition, so read the result alongside the limitations below.
- What it measures
- Weight relative to height
- Formula
- BMI = weight (kg) ÷ height (m)²
- Applies to
- Adults aged 20 and over
- Healthy range
- 18.5 – 24.9
- Source
- CDC and WHO classification
Formula
WHO/CDC adult classificationBMI = weight (kg) ÷ height (m)²
- weight
- — Body weight in kilograms
- height
- — Height in metres (centimetres ÷ 100)
In pounds and inches the same calculation is BMI = 703 × weight (lb) ÷ height (in)². The 703 factor converts imperial units into the metric result — it is not a different formula.
BMI Calculator examples
| Input | Result | Category |
|---|---|---|
| 5'4" · 130 lb | 22.3 | Healthy weight |
| 5'4" · 160 lb | 27.5 | Overweight |
| 5'6" · 150 lb | 24.2 | Healthy weight |
| 5'8" · 170 lb | 25.8 | Overweight |
| 5'10" · 180 lb | 25.8 | Overweight |
| 5'10" · 220 lb | 31.6 | Obesity, class 1 |
| 6'0" · 200 lb | 27.1 | Overweight |
| 6'2" · 250 lb | 32.1 | Obesity, class 1 |
How do I calculate BMI?
Divide your weight in kilograms by the square of your height in metres. Someone 1.78 m tall weighing 81.6 kg has a BMI of 81.6 ÷ (1.78 × 1.78) = 25.8.
Working in pounds and inches, multiply by 703 instead: a person 70 inches tall weighing 180 lb has a BMI of 703 × 180 ÷ (70 × 70) = 25.8. Both routes give the same answer, because BMI is defined in metric and the 703 is purely a unit conversion.
What are the BMI ranges?
The CDC and WHO use the same adult cut-points. They apply to men and women alike — there is no separate BMI chart by sex.
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity, class 1 |
| 35.0 – 39.9 | Obesity, class 2 |
| 40.0 and above | Obesity, class 3 (severe) |
These thresholds were derived largely from populations of European ancestry. The WHO notes that people of South Asian, Chinese, and some other Asian backgrounds face raised cardiometabolic risk at lower BMIs, and several countries use a lower overweight cut-point of 23.0 for these groups.
What is a healthy BMI?
For adults aged 20 and over, 18.5 to 24.9 is classed as healthy weight. Within that band, mortality and cardiometabolic risk are broadly flat, so there is no advantage in aiming for the lower end.
Your result above converts this range back into pounds or kilograms for your specific height, which is usually more actionable than the BMI number itself.
What BMI does not tell you
BMI cannot distinguish muscle from fat, or tell where fat is stored. That produces predictable false readings:
- Muscular people — athletes, manual workers, regular lifters — often score in the overweight or obese range with low body fat.
- Older adults lose muscle and gain fat at a stable weight, so a normal BMI can hide a high body-fat percentage.
- Visceral fat around the organs drives more risk than fat on the hips and thighs, and BMI is blind to the difference.
- It is not valid during pregnancy, and children and teenagers need age-and-sex percentiles rather than fixed cut-points.
Waist circumference and waist-to-height ratio capture the fat distribution BMI misses, and body-fat percentage captures composition. Used together they give a far more complete picture than any one number.
BMI for children and teenagers
Under age 20, the same BMI number means different things at different ages, because body composition changes throughout growth. Paediatric BMI is therefore reported as a percentile against CDC growth charts for the child's exact age and sex.
| Percentile | Category |
|---|---|
| Below 5th | Underweight |
| 5th – 84th | Healthy weight |
| 85th – 94th | Overweight |
| 95th and above | Obesity |
What to do with your result
If your BMI sits outside the healthy range, the useful next step is context rather than alarm: check your waist circumference, and look at the trend rather than a single reading. A BMI that has climbed three points in two years matters more than one that has been stable for a decade.
A BMI of 30 or above, or 25 and above alongside a condition such as high blood pressure, prediabetes, or sleep apnoea, is the threshold at which clinical guidelines suggest discussing weight management with a clinician.
Frequently asked questions
Is BMI different for men and women?
No. The CDC and WHO use identical BMI cut-points for adult men and women. Women typically carry more body fat than men at the same BMI, so the same number can reflect different body composition — but the categories themselves do not change by sex.
What is a good BMI for my age?
For all adults aged 20 and over, 18.5 to 24.9 is the healthy range regardless of age. Some research suggests a slightly higher BMI may be protective after 65, but no official body publishes separate adult cut-points by age.
Why is my BMI high when I am not overweight?
BMI counts muscle and fat identically. If you carry significant muscle mass, your weight — and so your BMI — will be high while your body fat is low. A body-fat percentage or waist measurement gives a more accurate picture.
What is BMI Prime?
BMI Prime is your BMI divided by 25, the upper limit of the healthy range. A value of 1.00 sits exactly at that limit, 0.90 is 10% below it, and 1.20 is 20% above. It expresses how far you are from the threshold as a simple ratio.
How much should I weigh for my height?
Multiply your height in metres squared by 18.5 and by 24.9 to get the healthy weight range for your height. The calculator above does this and converts it into pounds or kilograms for you.
Can I use BMI while pregnant?
No. BMI is not valid during pregnancy because weight gain reflects the pregnancy itself. Clinicians use your pre-pregnancy BMI to set a recommended gain range instead, which is what the pregnancy weight gain calculator does.
Is BMI accurate?
It is accurate as a population screening tool and correlates reasonably with body fat across large groups. For an individual it can be substantially wrong in either direction, particularly for very muscular people, older adults, and some ethnic groups.
Sources
- About Adult BMI — Centers for Disease Control and Prevention, 2024
- Obesity and overweight — fact sheet — World Health Organization, 2024
- Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies — WHO Expert Consultation, The Lancet, 2004
- AMA adopts new policy clarifying role of BMI as a measure in medicine — American Medical Association, 2023
- Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults — National Heart, Lung, and Blood Institute, 1998