How to Calculate BMI
BMI is your weight in kilograms divided by your height in meters, squared.
> BMI = weight (kg) ÷ height (m)²
If you work in pounds and inches, multiply by 703 first:
> BMI = 703 × weight (lb) ÷ height (in)²
Worked examples
| Person | Weight | Height | Math | BMI |
|---|---|---|---|---|
| Woman | 62 kg | 165 cm | 62 ÷ 2.72 | 22.8 |
| Man | 80 kg | 175 cm | 80 ÷ 3.06 | 26.1 |
| Man | 95 kg | 180 cm | 95 ÷ 3.24 | 29.3 |
The idea goes back to the Belgian mathematician Adolphe Quetelet, who worked it out in the 1830s while studying population averages, which is why it was first called the Quetelet index. In 1972 the physiologist Ancel Keys gave it its modern name, body mass index, after finding that it tracked body fat in large groups better than other weight-for-height formulas.
BMI Chart for Adults
These are the standard categories from the World Health Organization (WHO) and the CDC. The CDC applies them to adults aged 20 and over, men and women alike.
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 to 34.9 | Obesity, class 1 |
| 35.0 to 39.9 | Obesity, class 2 |
| 40.0 and above | Obesity, class 3 |
A BMI in the healthy range does not guarantee good health, and a BMI above it does not guarantee poor health. It is a screening number that flags who might deserve a closer look.
Healthy Weight for Height
Flip the formula around and you get the weight range that sits between a BMI of 18.5 and 24.9 for your height.
| Height | Healthy weight (kg) | Healthy weight (lb) |
|---|---|---|
| 150 cm (4'11") | 41.6 to 56.0 kg | 91.8 to 123.5 lb |
| 160 cm (5'3") | 47.4 to 63.7 kg | 104.4 to 140.5 lb |
| 165 cm (5'5") | 50.4 to 67.8 kg | 111.0 to 149.5 lb |
| 170 cm (5'7") | 53.5 to 72.0 kg | 117.9 to 158.6 lb |
| 175 cm (5'9") | 56.7 to 76.3 kg | 124.9 to 168.1 lb |
| 180 cm (5'11") | 59.9 to 80.7 kg | 132.1 to 177.9 lb |
| 185 cm (6'1") | 63.3 to 85.2 kg | 139.6 to 187.9 lb |
The range is wide on purpose. People with a larger frame or more muscle usually sit toward the top, and people with a small frame toward the bottom. The calculator works out the exact range for your own height.
BMI Calculator for Women
The formula is the same for women and men, and so are the adult categories. What differs is what sits behind the number. At the same BMI, women typically carry roughly 10 percentage points more body fat than men, mostly because of hormones, fat storage linked to pregnancy and a lower average muscle mass.
BMI is also not a reliable guide during pregnancy. Weight gain in pregnancy follows separate ranges set by the Institute of Medicine (now the National Academy of Medicine), based on your BMI before pregnancy. Your doctor or midwife will track that for you.
After menopause, fat tends to shift toward the abdomen even when body weight stays stable, so a waist measurement becomes more informative than BMI alone.
BMI Calculator for Men
Men with the same BMI as a woman usually have more muscle and less body fat, but they store more of that fat around the abdomen, which is the pattern linked to higher heart and metabolic risk. A man at BMI 26 who lifts weights several times a week and has an 85 cm waist is in a different place from a man at BMI 26 with a 105 cm waist and no training.
The NIH flags a waist above 102 cm (40 inches) in men, and above 88 cm (35 inches) in women, as a sign of increased risk.
Is BMI Accurate? What It Gets Wrong
BMI measures weight relative to height. It does not measure fat, and that causes problems in a few groups.
- Muscular people and athletes. Muscle is denser than fat, so a lifter or rugby player can land in the overweight range with low body fat.
- Older adults. Muscle loss with age can hide excess fat, so a normal BMI can still hide too little muscle. Some studies also link a slightly higher BMI, around 25 to 27, with better outcomes in older adults, though the evidence is debated.
- Different ethnic groups. The same BMI reflects different body fat levels across populations, which is why some countries use adjusted cutoffs.
- Short and tall people. Critics point out that the formula can overstate fatness in tall people and understate it in short people.
- Children and teens. The adult table does not apply to them.
BMI is a reasonable first look at populations and a fair flag for individuals, but it works best alongside other measures.
BMI for Asians and Other Ethnic Groups
Studies in South Asian, East Asian and Southeast Asian populations show a higher risk of type 2 diabetes and heart disease at lower BMIs than in European populations. In response, a WHO expert consultation published in The Lancet in 2004 suggested extra public health action points, and many Asian countries use lower cutoffs.
| Asia-Pacific BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 22.9 | Healthy weight |
| 23.0 to 24.9 | Overweight, at risk |
| 25.0 to 29.9 | Obesity, class 1 |
| 30.0 and above | Obesity, class 2 |
India, Japan, China and Singapore each publish slightly different thresholds, so if you live in one of those countries, check your national guideline. Doctors may also use waist circumference as a second screen. The calculator has an Asia-Pacific switch that applies these cutoffs.
BMI for Children and Teens
For ages 2 to 19, BMI is read as a percentile compared with other children of the same age and sex, not as a fixed number. The CDC uses these bands.
| Percentile | Category |
|---|---|
| Below the 5th | Underweight |
| 5th to below the 85th | Healthy weight |
| 85th to below the 95th | Overweight |
| 95th and above | Obesity |
Children grow in spurts, so a pediatrician looks at the trend over time. Do not use the adult chart, or the adult category in this calculator, for a child.
BMI vs Body Fat Percentage vs Waist Measurements
If BMI is your starting point, these measures fill in the gaps.
| Measure | What it tells you | How to check |
|---|---|---|
| BMI | Weight relative to height | Scale and tape |
| Waist circumference | Abdominal fat, linked to heart and metabolic risk | Tape at navel level, after a normal exhale |
| Waist-to-height ratio | Central fat relative to height | Waist ÷ height. Below 0.5 is a common target |
| Body fat percentage | Fat versus lean mass | DEXA scan, calipers or a smart scale (less precise) |
The waist-to-height ratio is gaining attention because it is simple and works across ages and ethnic groups. NICE, the UK's health guidance body, includes it alongside BMI in its updated obesity guideline. A rough rule is to keep your waist below half your height.
For body fat, the American Council on Exercise (ACE) lists these ranges.
| Category | Women | Men |
|---|---|---|
| Essential fat | 10 to 13% | 2 to 5% |
| Athletes | 14 to 20% | 6 to 13% |
| Fitness | 21 to 24% | 14 to 17% |
| Average | 25 to 31% | 18 to 24% |
| Obesity | 32% and above | 25% and above |
What Your BMI Result Means
- Below 18.5: you may be underweight, which can go with low energy, weaker bones and a weaker immune system. Possible causes include not eating enough, a fast metabolism or a medical condition. If you have lost weight without trying, see a doctor.
- 18.5 to 24.9: your weight is in the range linked to the lowest average health risk. Keep an eye on waist size and fitness as well.
- 25 to 29.9: risk goes up gradually. If you are muscular, this may be fine. If your waist is above the risk line, losing 5 to 10 percent of your weight can lower blood pressure, blood sugar and cholesterol.
- 30 and above: the risk of type 2 diabetes, heart disease, sleep apnea and some cancers rises. A doctor can check blood pressure, blood sugar and blood fats, which say more than BMI does.
How to Move Toward a Healthy BMI
Small, steady changes beat crash plans.
- Set a realistic target. Aim for 5 to 10 percent of your body weight first, which is already enough to improve many health markers.
- Find your calorie number. Work out your maintenance calories with our macro calculator, then adjust by 15 to 20 percent for fat loss or 5 to 10 percent for weight gain.
- Keep protein high. About 1.6 to 2.2 g per kg protects muscle during weight loss and helps build it during weight gain.
- Lift weights twice a week and walk daily. The CDC baseline for adults is at least 150 minutes of moderate activity a week plus two days of muscle-strengthening.
- Sleep seven to nine hours. Short sleep raises appetite hormones.
- Recheck every four to six weeks, not every morning.
To gain weight in a healthy way, eat a modest surplus, favor calorie-dense whole foods such as oats, nuts, olive oil and dairy, and train with weights so the gain is mostly muscle.
Related Calculators
BMI is one number in a larger picture. Your basal metabolic rate (BMR) shows what you burn at rest, and your total daily energy expenditure (TDEE) shows what you burn in a normal day. Macro targets turn those numbers into grams of protein, carbs and fat. Used together, they give you a plan that BMI alone cannot.
- Macro calculator for calories and macros for any goal.
- Weight loss macro calculator to set a safe deficit.
- Muscle gain macro calculator to plan a lean bulk.
- Macro calculator for women for cycle, age and life stage.
Where These Numbers Come From
- WHO classification of BMI for adults, and the CDC adult BMI categories and child BMI-for-age percentile bands.
- The 2004 WHO expert consultation on appropriate BMI for Asian populations, published in The Lancet.
- NIH (NHLBI) clinical guidelines on waist circumference, and the NICE guideline on waist-to-height ratio.
- American Council on Exercise body fat categories, and the Deurenberg equation behind the calculator's rough body fat estimate.
- Institute of Medicine (National Academy of Medicine) pregnancy weight gain guidance, and the Mifflin-St Jeor equation for resting energy.
Results are estimates, not medical advice. Please read our disclaimer.
FAQs
What is a good BMI?
For most adults, 18.5 to 24.9 is the healthy range. In Asian populations, 18.5 to 22.9 is often used.
What is the BMI formula?
Weight in kilograms divided by height in meters squared. In pounds and inches, multiply by 703 first.
Is BMI the same for men and women?
The formula and the adult categories are the same. Women usually carry more body fat at the same BMI, so the number means slightly different things.
Is BMI accurate for athletes?
Not always. Muscle weighs more than fat, so athletes can score as overweight with low body fat. Use waist size or body fat percentage as well.
What is a healthy BMI for my age?
The adult categories apply from age 20. Some research suggests a BMI slightly above 25 may be fine for older adults, but talk to your doctor about your own case.
What BMI is considered obese?
A BMI of 30 or above falls in the obesity range under WHO and CDC categories. In Asia-Pacific guidelines, the cutoff is 25.
What is the ideal weight for my height?
Use the range that matches a BMI of 18.5 to 24.9 for your height, as in the table above. Where you sit within it depends on frame size and muscle.
Can I use a BMI calculator for my child?
No. Children need BMI-for-age percentiles, which your pediatrician or the CDC growth charts provide.
Is waist-to-height ratio better than BMI?
It does a better job of showing abdominal fat, which carries most of the risk. Using both gives the clearest picture.
How often should I check my BMI?
Every month or two is plenty. Weight swings daily with water and food, so short-term changes do not mean much.
Final Words
BMI is a starting point, not a verdict. Use it to see roughly where you stand, then look at your waist, your strength and your energy for the rest of the story. If your number lands outside the healthy range, or you are worried about your weight, talk to a doctor, who can order the tests that BMI cannot replace.