BMI explained: how it's calculated, what the ranges mean
Work it out with your own numbersBMI calculator: BMI, healthy weight range, waist-to-height ratio and lower cut-offs for some backgrounds.Open the calculator →Body mass index (BMI) is your weight in kilograms divided by your height in metres squared. For most adults, a BMI from 18.5 to 24.9 is the healthy range, 25 to 29.9 is overweight and 30 or more is obesity. Someone 175 cm tall weighing 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9, in the healthy range, and the healthy weight for that height is about 57 to 76 kg.
This guide covers the formula in metric and imperial units, what the ranges mean, the lower cut-offs used for some family backgrounds, your healthy weight range, waist-to-height ratio, and where BMI can mislead. To skip the maths, use the BMI calculator with your own height and weight, in any mix of units.
How is BMI calculated?
BMI compares your weight with your height. It is a quick screening measure used by doctors and health services, not a diagnosis.
BMI = weight in kg ÷ (height in m × height in m)
BMI = 703 × weight in lb ÷ (height in inches × height in inches)
healthy weight range = 18.5 × height in m² to 24.9 × height in m²
BMI Prime = BMI ÷ 25
The 703 is just a unit conversion, so both lines give the same answer, give or take a little rounding. Our calculator turns pounds and inches into kilograms and metres first and then uses the metric formula, so you can mix units, for example feet and inches with kilograms.
Two extra figures sometimes appear next to BMI. BMI Prime is your BMI divided by 25, the top of the standard healthy range, so anything above 1.0 is over that range. The Ponderal Index divides weight by height cubed instead of squared (kg ÷ m³), which stays steadier for very tall or very short people; for adults it is usually about 11 to 15.
What do the BMI ranges mean?
The standard adult ranges come from the World Health Organization. The WHO also splits underweight into three grades of thinness and obesity into three classes.
| Range | Standard (WHO) | Lower cut-offs (NICE) |
|---|---|---|
| Underweight | below 18.5 | below 18.5 |
| Healthy weight | 18.5 – 24.9 | 18.5 – 22.9 |
| Overweight | 25 – 29.9 | 23 – 27.4 |
| Obesity class 1 | 30 – 34.9 | 27.5 – 32.4 |
| Obesity class 2 | 35 – 39.9 | 32.5 – 37.4 |
| Obesity class 3 | 40 and above | 37.5 and above |
Underweight is graded as mild thinness (17 to 18.4), moderate thinness (16 to 16.9) and severe thinness (below 16). These ranges are for adults aged 18 and over.
One small detail: the range is judged on your BMI rounded to one decimal place, the figure you actually see. A BMI of 24.96 shows as 25.0, so it counts as overweight, not healthy.
Why are the cut-offs lower for some backgrounds?
For people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, health problems such as type 2 diabetes start at a lower BMI. So the UK's NICE guideline NG246 (January 2025), also used by the NHS, sets lower cut-offs: overweight from 23 and obesity from 27.5. The underweight line stays at 18.5.
In practice this mostly changes the top of your healthy range. At 175 cm, the top drops from 76.3 kg (BMI 24.9) to 70.1 kg (BMI 22.9). In the BMI calculator, choose your family background under Advanced to switch to these ranges.
What is a healthy weight for my height?
Turn the formula round: multiply your height in metres squared by 18.5 for the bottom of the range and by 24.9 for the top (or 22.9 with the lower cut-offs).
Healthy weight range by height
| Height | About | Healthy range (BMI 18.5–24.9) | In pounds | Top with lower cut-offs (BMI 22.9) |
|---|---|---|---|---|
| 150 cm | 4 ft 11 in | 41.6 – 56.0 kg | 92 – 124 lb | 51.5 kg |
| 155 cm | 5 ft 1 in | 44.4 – 59.8 kg | 98 – 132 lb | 55.0 kg |
| 160 cm | 5 ft 3 in | 47.4 – 63.7 kg | 104 – 141 lb | 58.6 kg |
| 165 cm | 5 ft 5 in | 50.4 – 67.8 kg | 111 – 149 lb | 62.3 kg |
| 170 cm | 5 ft 7 in | 53.5 – 72.0 kg | 118 – 159 lb | 66.2 kg |
| 175 cm | 5 ft 9 in | 56.7 – 76.3 kg | 125 – 168 lb | 70.1 kg |
| 180 cm | 5 ft 11 in | 59.9 – 80.7 kg | 132 – 178 lb | 74.2 kg |
| 185 cm | 6 ft 1 in | 63.3 – 85.2 kg | 140 – 188 lb | 78.4 kg |
| 190 cm | 6 ft 3 in | 66.8 – 89.9 kg | 147 – 198 lb | 82.7 kg |
The range is wide on purpose. Classic "ideal weight" formulas such as Devine or Robinson give one figure for your height and sex, but they were written mostly to work out medicine doses. The healthy BMI range is what most health services use today. The ideal weight calculator shows both side by side, with frame size.
Worked examples
Example 1: 175 cm and 70 kg
- Height squared: 1.75 × 1.75 = 3.0625
- BMI: 70 ÷ 3.0625 = 22.9, in the healthy range
- Healthy range: 18.5 × 3.0625 = 56.7 kg up to 24.9 × 3.0625 = 76.3 kg
- BMI Prime: 22.86 ÷ 25 = 0.91, under 1.0
- Ponderal Index: 70 ÷ (1.75 × 1.75 × 1.75) = 70 ÷ 5.36 = 13.1
With the lower cut-offs this person is still in the healthy range, because 22.9 is just under 23. But the top of their healthy weight drops to 22.9 × 3.0625 = 70.1 kg.
Example 2: 5 ft 10 in and 190 lb
- Height in inches: 5 × 12 + 10 = 70 in, and 70 × 70 = 4,900
- BMI: 703 × 190 = 133,570, and 133,570 ÷ 4,900 = 27.3, in the overweight range
- Healthy range for 70 in: 18.5 × 4,900 ÷ 703 = 129 lb up to 24.9 × 4,900 ÷ 703 = 173.6 lb, about 174 lb
- Distance from the top of the range: 190 − 173.6 = about 16.4 lb
With the lower cut-offs, 27.3 is still overweight (just under 27.5), and the top of the healthy range becomes 22.9 × 4,900 ÷ 703 = about 160 lb.
Example 3: 165 cm, 66 kg, lower cut-offs and an 80 cm waist
- Height squared: 1.65 × 1.65 = 2.7225
- BMI: 66 ÷ 2.7225 = 24.2
- Standard ranges: healthy (under 25). Lower cut-offs: overweight (23 to 27.4)
- Top of the healthy range with lower cut-offs: 22.9 × 2.7225 = 62.3 kg, about 3.7 kg below 66 kg
- Waist-to-height ratio: 80 ÷ 165 = 0.48, in NICE's healthy band. Half the height is 165 ÷ 2 = 82.5 cm, so the waist is 2.5 cm under the line
This shows why the background setting matters, and why a waist measurement is a useful second check: the BMI is just over the lower line, but the waist is within the healthy band.
What is waist-to-height ratio, and why use it with BMI?
Fat around the middle is linked to a higher risk of problems such as type 2 diabetes and heart disease, and BMI cannot see where you carry weight. Waist-to-height ratio can. Divide your waist by your height, both in the same unit.
waist-to-height ratio = waist ÷ height (both in cm, or both in inches)
| Waist ÷ height | What it suggests (NICE, adults) |
|---|---|
| 0.4 – 0.49 | Healthy |
| 0.5 – 0.59 | Increased health risk |
| 0.6 and above | High health risk |
The simple rule is to keep your waist to less than half your height. The same line is used for men and women and for people of all ethnic backgrounds. NICE suggests it alongside BMI for adults with a BMI under 35, including adults with high muscle mass. Below 0.4 is not a NICE category; if you are unsure what it means for you, talk to a doctor.
Measure halfway between your lowest rib and the top of your hip bone, standing up, after a normal breath out, with the tape level and snug, not tight. The waist-to-height ratio calculator shows the largest waist for your height and adds waist-to-hip ratio.
Where does BMI mislead?
- Muscle. BMI does not separate muscle from fat, so very muscular people can show as overweight while carrying little fat. A waist-to-height ratio or a body fat estimate gives a fuller picture.
- Older adults. For older adults, a BMI a little above 25 is not always a concern; a doctor will look at the whole picture.
- Children and teenagers. For ages 2 to 17, BMI is compared with growth charts for exact age and sex, so the adult ranges do not apply. Our calculator still shows the number if you enter an age under 18, but not an adult range; the NHS and CDC child and teen BMI calculators do the growth-chart check.
- Pregnancy. BMI is not meant for pregnant women, whose weight changes for other reasons.
- Family background. The standard ranges can understate risk for South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds, which is why the lower cut-offs exist.
Common mistakes
- Forgetting to square the height. 70 kg ÷ 1.75 m is 40, not a BMI. You must divide by 1.75 × 1.75.
- Using centimetres instead of metres. 175 cm is 1.75 m. Squaring 175 gives a BMI 10,000 times too small.
- Leaving out the 703. In pounds and inches, weight ÷ inches² alone gives a tiny number. Multiply by 703.
- Using adult ranges for a child. Under 18, use growth charts instead.
- Treating BMI as a diagnosis. It is a screening measure. Muscle, age, background and where you carry fat all matter.
- Picking one "perfect" weight. At 175 cm the healthy range spans almost 20 kg (56.7 to 76.3 kg).
What to do with your result
- Check your range with the right cut-offs. If your family background is one of those listed above, use the lower cut-offs.
- Add your waist. A waist-to-height ratio under 0.5 is a useful second check, especially if you are muscular.
- Look at the distance, not just the label. The calculator shows how far you are from the healthy range for your height. Even a small loss of 5% of body weight can help health if you are above the range.
- Talk to a doctor about what your BMI means for you, and see one if you are losing weight without trying. This guide is general information, not medical advice.
Quick summary
- BMI = kg ÷ m², or 703 × lb ÷ in².
- Adults: under 18.5 underweight, 18.5 to 24.9 healthy, 25 to 29.9 overweight, 30+ obesity (WHO).
- Lower cut-offs of 23 and 27.5 for South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds (NICE NG246).
- Healthy weight = 18.5 to 24.9 × height in metres squared.
- Keep your waist to less than half your height.
- Not for children, pregnancy or as a diagnosis; muscle can push BMI up.
Work out your own figures with the BMI calculator, the waist-to-height ratio calculator or the ideal weight calculator. For the full NICE waist advice, see NICE guideline NG246.
Common questions
How is BMI calculated?
Divide your weight in kilograms by your height in metres squared. In pounds and inches it is 703 × weight in pounds ÷ (height in inches × height in inches). Someone 1.75 m tall weighing 70 kg has a BMI of 70 ÷ 3.0625 = 22.9.
What is a healthy BMI for adults?
For most adults, 18.5 to 24.9. Below 18.5 is underweight, 25 to 29.9 is overweight and 30 or more is obesity. For South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean backgrounds, NICE (UK) uses lower cut-offs of 23 and 27.5, so the healthy range is 18.5 to 22.9.
What is the healthy weight for my height?
Multiply your height in metres squared by 18.5 for the bottom of the range and by 24.9 for the top. At 175 cm that is 56.7 kg to 76.3 kg. With the lower cut-offs, the top is 22.9 × your height squared, or 70.1 kg at 175 cm.
Is BMI accurate for muscular people?
Not always. BMI does not separate muscle from fat, so very muscular people can show as overweight while carrying little fat. Adding a waist-to-height ratio or a body fat estimate gives a fuller picture.
Can I use adult BMI ranges for children?
No. For children and teenagers aged 2 to 17, BMI is compared with growth charts for their exact age and sex, so the adult ranges do not apply. Use the NHS or CDC child and teen BMI calculators for that check.
What is a good waist-to-height ratio?
Divide your waist by your height, both in the same unit. NICE puts 0.4 to 0.49 in the healthy band, 0.5 to 0.59 as increased health risk and 0.6 or more as high health risk. The simple rule is to keep your waist to less than half your height.
This guide explains the rules in plain English for planning. It is not legal, financial or flight-operations advice; check your contract, the official source linked above, your employer or your operator.