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Formula
BMI = weight (kg) ÷ height (m)² = 703 × weight (lb) ÷ height (in)²
- BMI
- body mass index, in kg/m²
- weight
- body weight in kilograms (or pounds in the US version)
- height
- standing height in meters (or inches in the US version)
What BMI measures
Body mass index (BMI) compares your weight with your height. It is weight in kilograms divided by the square of height in meters, so the unit is kg/m². Because it needs only two measurements, doctors and public-health agencies use it as a quick screen for weight categories that may be linked to health problems.
BMI does not measure body fat directly. The CDC describes it as one possible health indicator, to be read alongside other information such as waist size, blood pressure, cholesterol, activity and family history.
How to use the BMI calculator
- Enter your weight and pick the unit: pounds, kilograms or stones.
- Enter your height. In feet, put the feet in the height box and any remaining inches in Extra inches (5 ft 7 in is 5 and 7). You can also enter height in inches, centimeters or meters.
- Optionally, enter your age. Under 20, the calculator still shows the BMI but tells you that adult categories do not apply.
The result shows your BMI to one decimal place, your CDC category, the weight range that would put you between a BMI of 18.5 and 24.9 at your height, and a table of every category converted to weights at your height.
CDC adult BMI categories
These categories apply to adults aged 20 and older, regardless of sex:
| Category | BMI |
|---|---|
| Underweight | below 18.5 |
| Healthy weight | 18.5 to under 25 |
| Overweight | 25 to under 30 |
| Obesity, class 1 | 30 to under 35 |
| Obesity, class 2 | 35 to under 40 |
| Obesity, class 3 (severe) | 40 or more |
The World Health Organization uses the same cut-offs of 25 for overweight and 30 for obesity in adults.
How to calculate BMI in pounds and kilograms
Take someone who is 5 ft 4 in tall and weighs 140 lb.
Metric method. Convert first: 140 lb × 0.45359237 = 63.50 kg, and 64 in × 2.54 = 162.56 cm, or 1.6256 m. Square the height: 1.6256² = 2.6426. Divide: 63.50 ÷ 2.6426 = 24.0.
US formula. BMI = 703 × weight (lb) ÷ height (in)². Here, 703 × 140 ÷ 64² = 98,420 ÷ 4,096 = 24.0.
Both give a BMI of 24.0, in the healthy weight range. The factor 703 is a rounded conversion constant, so the US formula can differ from the exact metric result in the second decimal place. The calculator uses exact conversions.
Healthy weight range by height
Weights were calculated for BMIs of 18.5 and 24.9 (the healthy range) and 30 (the start of obesity), rounded to 0.1 lb.
| Height | Healthy weight range | Obesity starts at |
|---|---|---|
| 4 ft 10 in | 88.5 – 119.1 lb | 143.5 lb |
| 5 ft 0 in | 94.7 – 127.5 lb | 153.6 lb |
| 5 ft 2 in | 101.1 – 136.1 lb | 164.0 lb |
| 5 ft 4 in | 107.8 – 145.1 lb | 174.8 lb |
| 5 ft 6 in | 114.6 – 154.3 lb | 185.9 lb |
| 5 ft 8 in | 121.7 – 163.8 lb | 197.3 lb |
| 5 ft 10 in | 128.9 – 173.5 lb | 209.1 lb |
| 6 ft 0 in | 136.4 – 183.6 lb | 221.2 lb |
| 6 ft 2 in | 144.1 – 193.9 lb | 233.7 lb |
| 6 ft 4 in | 152.0 – 204.6 lb | 246.5 lb |
The range spans about 33 lb at 5 ft 0 in and about 53 lb at 6 ft 4 in, because the healthy band scales with the square of height.
BMI for children and teens
For ages 2 through 19, BMI is interpreted with sex-specific BMI-for-age growth charts, because body fat changes as children grow and differs between boys and girls. The result is a percentile, not one of the adult categories. A BMI of 22 could be healthy for one 15-year-old and high for a 10-year-old.
This BMI calculator does not compute those percentiles. Use the CDC Child and Teen BMI Calculator, or ask a pediatrician, who can also track the trend over time.
Limits of BMI
BMI is a useful population screen, but it has known blind spots for individuals:
- Muscle. BMI cannot tell muscle from fat. Muscular athletes can fall in the overweight or obesity range with little body fat.
- Fat location. Fat around the waist carries more health risk than fat elsewhere, and BMI cannot see where fat is. Waist circumference adds this information.
- Age. Older adults often have more body fat than younger adults at the same BMI, as muscle declines.
- Body build and background. The same BMI can reflect different body fat levels across sexes and ethnic groups.
- Pregnancy. Weight gain in pregnancy is assessed differently, so adult categories do not apply.
For a direct estimate of fatness, the body fat calculator uses tape measurements, and the ideal weight calculator compares several published target-weight formulas.
What to do with your result
A single BMI is a starting point for a conversation, not a verdict, which is why the BMI calculator adds a note whenever your result is outside the healthy range. If you are outside the healthy range, a doctor can weigh it against your other risk factors and suggest whether any change is worthwhile. If you plan to change your weight, the calorie calculator estimates daily calories for a chosen weekly rate, and the BMR calculator shows how many calories your body uses at rest.
Common mistakes
- Typing 5.7 for 5 ft 7 in. 5.7 ft is 5 ft 8.4 in. Enter 5 in the height box and 7 in Extra inches instead.
- Mixing units. Pounds with centimeters works only if each unit selector matches what you typed.
- Using adult categories for a teenager. Under 20, read the BMI as a percentile.
- Treating 25.0 as a diagnosis. The cut-offs are screening thresholds, not a line between healthy and unhealthy.
Worked examples
US units: 5 ft 10 in and 185 lb
An adult man, 5 ft 10 in tall, weighing 185 lb.
Inputs: Weight = 185; Weight unit = Pounds; Height = 5; Height unit = Feet; Extra inches = 10 in; Age (optional) = 45 years
185 lb = 83.91 kg; 5 ft 10 in = 1.778 m BMI = 83.91 ÷ 1.778² = 83.91 ÷ 3.1613 = 26.5
Answer: BMI 26.5 (overweight) for 185 lb at 5 ft 10 in. Estimate, not medical advice.
Metric units: 168 cm and 58 kg
An adult, 168 cm tall, weighing 58 kg.
Inputs: Weight = 58; Weight unit = Kilograms; Height = 168; Height unit = Centimeters; Age (optional) = 29 years
168 cm = 1.68 m BMI = 58 ÷ 1.68² = 58 ÷ 2.8224 = 20.5
Answer: BMI 20.5 (healthy weight) for 58 kg at 168 cm. Estimate, not medical advice.
A 15-year-old (adult categories do not apply)
A 15-year-old, 5 ft 5 in and 130 lb. The BMI is shown, but the category must come from a BMI-for-age percentile chart.
Inputs: Weight = 130; Weight unit = Pounds; Height = 5; Height unit = Feet; Extra inches = 5 in; Age (optional) = 15 years
130 lb = 58.97 kg; 5 ft 5 in = 1.651 m BMI = 58.97 ÷ 1.651² = 58.97 ÷ 2.7258 = 21.6
Answer: BMI 21.6 for 130 lb at 5 ft 5 in. Under 20, BMI must be read as a BMI-for-age percentile, not with adult categories. Estimate, not medical advice.
Assumptions and limitations
- Results are an estimate, not medical advice. BMI is a screening measure and cannot diagnose body fatness or health.
- Categories are the CDC adult categories for ages 20 and older (underweight below 18.5, healthy weight 18.5 to under 25, overweight 25 to under 30, obesity 30 or more, split into classes 1, 2 and 3 at 35 and 40).
- The BMI is rounded to one decimal place before it is categorized, so the category always matches the number shown.
- Under age 20, the adult categories do not apply. The calculator shows the BMI but does not compute the BMI-for-age percentile that children and teens need.
- The calculation uses exact unit conversions (1 lb = 0.45359237 kg, 1 in = 2.54 cm). The healthy weight range is the weight at a BMI of 18.5 to 24.9 for your height.
- Weight must be between 20 and 400 kg (44 to 880 lb) and height between 100 and 250 cm. BMI is not valid during pregnancy.
Frequently asked questions
How do you calculate BMI in pounds and inches?
Multiply your weight in pounds by 703, then divide by your height in inches squared. For 165 lb at 5 ft 9 in (69 in): 703 × 165 ÷ 69² = 115,995 ÷ 4,761 = 24.4.
How do you calculate BMI with cm and kg?
Convert height to meters, square it and divide your weight in kilograms by the result. For 72 kg and 180 cm: 1.8² = 3.24, and 72 ÷ 3.24 = 22.2.
Is BMI calculated differently for women and men?
No. Adults 20 and older use the same formula and the same CDC categories whatever their sex. Women tend to have more body fat than men at the same BMI, which is one of the measure’s known limits.
Does the BMI calculator take age into account?
For adults, no: the categories are the same at 25 or 75. Age matters for children and teens aged 2 to 19, whose BMI is judged as a percentile for their age and sex. This calculator flags ages under 20 but does not compute that percentile.
How do I find a BMI percentile for my child?
Use the CDC Child and Teen BMI Calculator or ask your child’s doctor. It compares the BMI with growth charts for the same age and sex; a BMI at or above the 95th percentile is classed as obesity in children.
Why does BMI say I’m overweight when I’m muscular?
BMI only uses height and weight, so it cannot tell muscle from fat. Muscular people can have a BMI of 25 or more with little body fat. A waist measurement or a body fat estimate gives a better picture.
Can I use BMI during pregnancy?
No. Pregnancy weight gain is expected and is tracked against guidance based on your BMI before pregnancy. Ask your prenatal care provider how much gain is right for you.
Sources
- Adult BMI Categories — Centers for Disease Control and Prevention (CDC). Checked October 10, 2026.
- About Body Mass Index (BMI) — Centers for Disease Control and Prevention (CDC). Checked October 10, 2026.
- BMI Frequently Asked Questions — Centers for Disease Control and Prevention (CDC). Checked October 10, 2026.
- Child and Teen BMI Calculator — Centers for Disease Control and Prevention (CDC). Checked October 10, 2026.
- Obesity and overweight (fact sheet) — World Health Organization. Checked October 10, 2026.
- Calculate Your BMI — National Heart, Lung, and Blood Institute (NIH). Checked October 10, 2026.