Body mass index (BMI) is your weight in kilograms divided by the square of your height in meters. A person who weighs 70 kg and is 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 22.9, which is in the adult normal-weight range of 18.5 to 24.9. In US units, multiply your weight in pounds by 703 and divide by your height in inches squared: 160 lb at 5 ft 9 in gives a BMI of 23.6.
Our free BMI calculator takes metric or imperial measurements and shows your BMI and its category next to the standard adult ranges. This guide covers the formula, what the numbers mean, the healthy weight range for your height and what BMI cannot tell you.
How to calculate BMI
Metric: BMI = weight (kg) ÷ height (m)²
Imperial: BMI = 703 × weight (lb) ÷ height (in)²The 703 converts pounds and inches into the metric result, so both formulas give the same BMI for the same person.
Worked example in metric units
- Convert height to meters: 175 cm ÷ 100 = 1.75 m.
- Square it: 1.75 × 1.75 = 3.0625.
- Divide the weight by that number: 70 ÷ 3.0625 = 22.86.
- Round to one decimal place: BMI 22.9, normal weight.
Worked example in pounds and inches
- Convert height to inches: 5 ft 9 in is 5 × 12 + 9 = 69 inches.
- Square it: 69 × 69 = 4,761.
- Multiply the weight by 703: 160 × 703 = 112,480.
- Divide: 112,480 ÷ 4,761 = 23.6. BMI 23.6, normal weight.
BMI categories for adults
The calculator uses the World Health Organization's adult categories:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal weight |
| 25 to 29.9 | Overweight |
| 30 or more | Obese |
Clinicians often split the obese range further: 30 to 34.9 is class 1, 35 to 39.9 is class 2 and 40 or more is class 3. The calculator reports all of these as Obese.
Healthy weight range for your height
To find the normal-weight range for a height, multiply 18.5 and 24.9 by the height in meters squared. At 1.75 m that is 18.5 × 3.0625 = 56.7 kg up to 24.9 × 3.0625 = 76.3 kg.
| Height | Normal weight (kg) | Normal weight (lb) |
|---|---|---|
| 160 cm (5 ft 3 in) | 47.4 – 63.7 | 104 – 141 |
| 165 cm (5 ft 5 in) | 50.4 – 67.8 | 111 – 149 |
| 170 cm (5 ft 7 in) | 53.5 – 72.0 | 118 – 159 |
| 175 cm (5 ft 9 in) | 56.7 – 76.3 | 125 – 168 |
| 180 cm (5 ft 11 in) | 59.9 – 80.7 | 132 – 178 |
| 185 cm (6 ft 1 in) | 63.3 – 85.2 | 140 – 188 |
| 190 cm (6 ft 3 in) | 66.8 – 89.9 | 147 – 198 |
What BMI does not tell you
BMI is a quick screening number, not a diagnosis. It uses only height and weight, so it misses several things that matter for health:
- Muscle versus fat. Muscle is dense, so a muscular athlete can have a BMI over 25 with little body fat. An older adult who has lost muscle can have a normal BMI and still carry excess fat.
- Where fat sits. Fat around the waist is linked to higher health risk than fat on the hips and thighs, and BMI cannot see the difference. A waist measurement adds that information.
- Ethnicity. Health risks rise at lower BMIs in some groups, and some health bodies use lower cut-offs for people of South Asian, Chinese and other Asian backgrounds.
- Children and teens. From ages 2 to 19, BMI is compared with growth charts for age and sex as a percentile. The adult categories above do not apply.
- Pregnancy. Weight gain during pregnancy makes BMI unsuitable as a measure.
For a fuller picture, pair BMI with a body fat estimate. The body fat calculator uses the U.S. Navy tape-measure method, based on your neck and waist (plus hips for women), so it reflects how your weight is made up. If your BMI falls outside the normal range, or you have concerns about your weight, talk to a doctor.
How to use the BMI calculator
- Open the BMI calculator.
- Choose Metric or Imperial units.
- Enter your Weight in kilograms or pounds.
- Enter your Height in centimeters, or in total inches for imperial. There is no separate feet field, so convert first: 5 ft 9 in is 69 inches.
- Read your Body mass index, rounded to one decimal place, and its Category. The BMI categories table sits beside the result for comparison.
Tips for an accurate reading
- Weigh yourself consistently. Body weight can shift by a kilogram or more during the day. Weigh in the morning, after using the bathroom and before eating, in light clothing.
- Measure height properly. Stand barefoot with your back against a wall and mark the top of your head with a flat object. Guessing can move BMI by half a point or more.
- Convert units carefully. If your scale shows stones or your height is in feet and inches, use the weight converter and length converter before entering the numbers.
- Watch the trend. One BMI reading says little on its own. A steady change over months is more meaningful than a single number.
Frequently asked questions
How do I calculate my BMI?
Divide your weight in kilograms by your height in meters squared. At 70 kg and 1.75 m, BMI is 70 ÷ 3.0625 = 22.9. A BMI calculator does the math for you in metric or imperial units.
What is a healthy BMI for adults?
A BMI from 18.5 to 24.9 is classed as normal weight for adults. Below 18.5 is underweight, 25 to 29.9 is overweight and 30 or more is obese, according to the World Health Organization.
How do I calculate BMI in pounds and inches?
Multiply your weight in pounds by 703, then divide by your height in inches squared. For 160 lb at 69 inches: 160 × 703 ÷ 4,761 = 23.6.
Is BMI accurate for muscular people?
Not very. BMI cannot tell muscle from fat, so athletes and people who lift weights can be labeled overweight while having low body fat. A body fat estimate or a waist measurement gives better context.
Can children use the BMI calculator?
The categories in the calculator are for adults. For children and teens, BMI is plotted on age- and sex-specific growth charts as a percentile, so ask a pediatrician to interpret it.
What is the difference between BMI and body fat percentage?
BMI compares weight with height and says nothing about what the weight is made of. Body fat percentage estimates how much of your weight is fat, which is why the two can disagree for very muscular or very inactive people.