How BMI is calculated
Body mass index is weight in kilograms divided by the square of height in metres (WHO, CDC). With height in centimetres it is weight ÷ height ÷ height × 10,000; in US units, weight in pounds ÷ height in inches squared × 703. The NHS gives a worked example: 70 kg at 1.70 m is 70 ÷ (1.70 × 1.70) = 70 ÷ 2.89 = 24.22, shown to one decimal place as 24.2.
The category follows the figure shown. A BMI that works out at 24.96 is displayed as 25.0, so it counts as overweight — some calculators label it “healthy” because they compare the unrounded number. This matches the CDC’s own example: at 5 ft 9 in, 125 to 168 pounds is a healthy weight, and 169 pounds is not.
Adult BMI ranges — the same for every age and sex
A “BMI calculator for women”, “for men” or “by age” gives the same answer for every adult. The WHO states that “BMI values are age-independent and the same for both sexes”, and the CDC uses its categories for adults 20 and older “regardless of age, sex, or race”. Age and sex only change the result for children and teens (below).
| BMI (adults) | WHO / CDC category |
|---|---|
| Under 18.5 | Underweight (WHO: severe thinness under 16, moderate 16–16.99, mild 17–18.49) |
| 18.5 to 24.9 | Healthy weight (WHO: normal range) |
| 25 to 29.9 | Overweight (WHO: pre-obese) |
| 30 to 34.9 | Obesity class 1 |
| 35 to 39.9 | Obesity class 2 |
| 40 or more | Obesity class 3 (the CDC: severe obesity) |
The NHS uses BMI with adults aged 18 and over; the CDC from 20. NICE advises reading BMI “with caution in people aged 65 and over”, allowing for “the possible protective effect of having a slightly higher BMI when older”.
Lower thresholds for some ethnic groups
A WHO expert consultation (The Lancet, 2004) found that the BMI at which risk rises “varies from 22 kg/m2 to 25 kg/m2 in different Asian populations”, kept the international cut-offs and added “public health action points” at 23, 27.5, 32.5 and 37.5. The UK’s NICE guideline (NG246, updated January 2026) applies them: people with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African–Caribbean background “are prone to central adiposity and their cardiometabolic risk occurs at lower BMI”, so overweight is BMI 23 to 27.4 and obesity 27.5 or above, with obesity classes 2 and 3 lowered by 2.5. Choose those thresholds in the calculator.
A frequent misreading: 23 is where overweight begins, not the top of the healthy range. With these thresholds the healthy range is 18.5 to 22.9.
Children and teens: BMI-for-age percentiles
For ages 2 to 19 a BMI only means something compared with children of the same age and sex. The Child & teen tab places the BMI on the CDC growth charts, using the CDC’s own LMS data; above the 95th percentile it switches to the CDC’s 2022 extended method, because the original charts “do not extend beyond the 97th percentile”. The CDC categories:
| BMI-for-age (CDC, ages 2–19) | Category |
|---|---|
| Below the 5th percentile | Underweight |
| 5th to below the 85th | Healthy weight |
| 85th to below the 95th | Overweight |
| 95th or above | Obesity |
| 120% of the 95th percentile or more, or a BMI of 35 or more | Severe obesity |
Other systems draw the lines elsewhere. In the UK, NICE uses the UK-WHO charts with overweight from the 91st centile, clinical obesity from the 98th and severe obesity from the 99.6th — which is why an NHS letter and a US calculator can describe the same child differently. The WHO’s references for ages 5 to 19 call overweight more than 1 standard deviation above the median (“equivalent to BMI 25 kg/m2 at 19 years”) and obesity more than 2. This calculator deliberately gives no weight target for a child: they are still growing, and any change should be planned with a doctor or nurse.
What weight should I be? The healthy range for your height
Turning the formula round gives the weights for a BMI of 18.5 to 24.9 at a height — the range the calculator shows for yours, with how far you are from it. If you are above it, the NHS points out that “just losing a small amount of weight (5 to 10% of your body weight) can improve your health”, and suggests aiming for 0.5 to 1 kg (1 to 2 lb) a week.
| Height | Healthy weight (BMI 18.5–24.9) | With NICE’s lower thresholds (up to 22.9) |
|---|---|---|
| 150 cm | 41.6 – 56.0 kg | up to 51.5 kg |
| 160 cm | 47.4 – 63.7 kg | up to 58.6 kg |
| 165 cm | 50.4 – 67.8 kg | up to 62.3 kg |
| 170 cm | 53.5 – 72.0 kg | up to 66.2 kg |
| 175 cm | 56.7 – 76.3 kg | up to 70.1 kg |
| 180 cm | 59.9 – 80.7 kg | up to 74.2 kg |
| 190 cm | 66.8 – 89.9 kg | up to 82.7 kg |
| Height | Healthy weight (whole pounds) |
|---|---|
| 5 ft 0 in | 95 – 127 lb |
| 5 ft 4 in | 108 – 145 lb |
| 5 ft 6 in | 115 – 154 lb |
| 5 ft 8 in | 122 – 164 lb |
| 5 ft 9 in | 125 – 168 lb |
| 5 ft 10 in | 129 – 173 lb |
| 6 ft 0 in | 137 – 183 lb |
| 6 ft 2 in | 144 – 194 lb |
Waist-to-height ratio: what BMI cannot see
BMI “does not indicate where in the body that people carry fat” (CDC). Waist-to-height ratio does. NICE bands it as 0.4 to 0.49 healthy, 0.5 to 0.59 increased and 0.6 or more high central adiposity, for people with a BMI under 35 “of both sexes and all ethnicities, including adults with high muscle mass” — so the simple rule is to keep your waist to less than half your height. Measure midway between the bottom of the ribs and the top of the hips (“just above the belly button”), breathing out naturally. NICE’s example: a 96.5 cm waist and 170 cm height give 0.57. In England in 2024, 70% of adults had a ratio above 0.5 (Health Survey for England).
The US Army made the same switch: since 7 July 2026 waist-to-height ratio is its “sole body composition assessment”, replacing the height and weight tables and the tape test. A ratio “of less than, but not equal to 0.55” passes, measured at the navel in inches.
What BMI cannot tell you
- It is not a measure of body fat. The CDC: BMI “does not distinguish between fat, muscle, and bone mass”, though it is “moderately to strongly associated” with measures of body fat. NICE asks for caution “in adults with high muscle mass”.
- The American Medical Association (2023) recommends using it “in conjunction with other valid measures of risk”, noting that it “loses predictability when applied on the individual level”.
- A 2025 Lancet Diabetes & Endocrinology commission on clinical obesity recommends BMI “only as a surrogate measure of health risk at a population level … or for screening purposes, rather than as an individual measure of health”, with excess fat confirmed by another measure such as waist-to-height ratio — except at a BMI over 40.
The calculator therefore shows BMI as a screening figure. A result outside the healthy range is a reason to talk to a doctor or nurse, not a diagnosis.
BMI and pregnancy
The NHS calculator is not for use in pregnancy, and the adult categories do not describe a pregnant body. What matters is the BMI before pregnancy, which sets the recommended weight gain (US Institute of Medicine, 2009, for one baby):
| Pre-pregnancy BMI | Total gain |
|---|---|
| Under 18.5 | 12.5–18 kg (28–40 lb) |
| 18.5–24.9 | 11.5–16 kg (25–35 lb) |
| 25.0–29.9 | 7–11.5 kg (15–25 lb) |
| 30.0 or more | 5–9 kg (11–20 lb) |
Where BMI came from
The Belgian statistician Adolphe Quetelet observed in the 1830s that adult weight rises roughly with the square of height, and the ratio was long called the Quetelet index. Ancel Keys and colleagues named it the “body mass index” in 1972, after comparing weight indices in 7,424 men in 12 cohorts across five countries.
Alternatives exist. The Oxford mathematician Nick Trefethen proposed a “new BMI” of 1.3 × weight ÷ height^2.5, which raises height to the power 2.5 instead of 2 — but he notes the formula is “not based on epidemiological studies”. The calculator shows it for comparison only.
BMI in numbers
- Worldwide in 2024, 2.6 billion adults (45%) were overweight, 940 million of them living with obesity; 1 in 7 people of all ages lived with obesity, and adult obesity has more than doubled since 1990 (WHO).
- In the United States, 40.3% of adults had obesity in August 2021–August 2023, and 9.4% severe obesity (CDC); among children and teens it was 19.7% in 2017–March 2020.
- In England in 2024, 30% of adults were living with obesity and 66% were overweight or living with obesity; the average adult BMI rose from 25.8 in 1993 to 27.8 (Health Survey for England).
Calculating BMI by hand, in Excel or in code
- On a calculator: divide your weight in kilograms by your height in metres, then divide by the height again. In pounds and inches: weight × 703 ÷ height ÷ height.
- Excel or Google Sheets, height in cm in A2 and weight in kg in B2:
=B2/(A2/100)^2 - Python:
bmi = weight_kg / (height_cm / 100) ** 2; JavaScript:const bmi = kg / (cm / 100) ** 2; - The usual bugs: centimetres used as metres (a BMI 10,000 times too small), the height not squared, and a category taken from the unrounded value so that 24.95 matches no range or the wrong one.