How do you calculate BMI?
Divide your weight by your height squared. Body mass index compares weight with height. It was designed to sort large groups of adults into broad weight bands, and it is still the quickest screening measure used worldwide.
BMI (kg/m²) = weight (kg) ÷ height (m)²
The calculator also runs the formula backwards to show the healthy weight for this height. It multiplies your height in metres squared by 18.5 and by 24.9, the lower and upper limits of the WHO healthy range.
Healthy weight range (kg) = 18.5 × height² to 24.9 × height²
How to use the BMI calculator
- Enter your Weight in kilograms. Weighing yourself in light clothing, ideally at the same time of day, gives a more consistent figure.
- Enter your Height in centimetres, measured without shoes.
- Read your BMI, your WHO category and the Healthy weight for this height.
- Check the working underneath to see each step: height in metres squared, then the division.
Your BMI is shown to one decimal place, which is all the precision it really has. The category follows the number you see: a BMI of 24.96 shows as 25 and counts as Overweight.
Worked examples
Example 1 – healthy weight. 70 kg and 175 cm. Height squared: (175 ÷ 100)² = 3.0625 m². BMI: 70 ÷ 3.0625 = 22.9 kg/m², in the Healthy weight category. Healthy weight range for 175 cm: 18.5 × 3.0625 to 24.9 × 3.0625 = 56.7–76.3 kg.
Example 2 – overweight range. 95 kg and 180 cm. Height squared: 1.8² = 3.24 m². BMI: 95 ÷ 3.24 = 29.3 kg/m², in the Overweight category. Healthy weight range for 180 cm: 59.9–80.7 kg.
Example 3 – underweight range. 50 kg and 170 cm. Height squared: 1.7² = 2.89 m². BMI: 50 ÷ 2.89 = 17.3 kg/m², in the Underweight category. Healthy weight range for 170 cm: 53.5–72 kg.
WHO BMI categories for adults
| BMI (kg/m²) | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
These thresholds apply to adults aged 18 and over, men and women alike. Some health services use lower action points for people of South Asian, Chinese and other Asian backgrounds, because health risks start to rise at a lower BMI in these groups.
What is a normal BMI?
For adults, a normal BMI is 18.5 to 24.9 kg/m². The WHO calls this range normal weight; the calculator labels it Healthy weight, as the NHS does. The same range applies at every adult age and to both sexes.
A normal BMI tells you that your weight is in proportion to your height. It does not say how much of that weight is fat or muscle, or where the fat sits.
What is a healthy weight for my height?
A healthy weight is any weight that puts your BMI between 18.5 and 24.9. The table shows that range for common adult heights, as the calculator gives it under Healthy weight for this height. Feet, inches and pounds are converted from the metric values.
| Height | In feet and inches | Healthy weight (kg) | Healthy weight (lb) |
|---|---|---|---|
| 150 cm | 4 ft 11 in | 41.6–56 kg | 92–124 lb |
| 155 cm | 5 ft 1 in | 44.4–59.8 kg | 98–132 lb |
| 160 cm | 5 ft 3 in | 47.4–63.7 kg | 104–141 lb |
| 165 cm | 5 ft 5 in | 50.4–67.8 kg | 111–149 lb |
| 170 cm | 5 ft 7 in | 53.5–72 kg | 118–159 lb |
| 175 cm | 5 ft 9 in | 56.7–76.3 kg | 125–168 lb |
| 180 cm | 5 ft 11 in | 59.9–80.7 kg | 132–178 lb |
| 185 cm | 6 ft 1 in | 63.3–85.2 kg | 140–188 lb |
| 190 cm | 6 ft 3 in | 66.8–89.9 kg | 147–198 lb |
| 195 cm | 6 ft 5 in | 70.3–94.7 kg | 155–209 lb |
For example, at 190 cm (about 6 ft 3 in) the healthy range is 66.8 to 89.9 kg, or about 147 to 198 lb. The limits are the same for men and women and at every adult age, though some health services use lower action points for people of Asian backgrounds, as explained above. Drug doses sometimes use a different measure, ideal body weight, which the ideal body weight calculator works out with the Devine formula.
Is BMI different for men and women?
No. The formula and the adult categories are the same for both sexes. What differs is body composition. At the same BMI, women on average carry more body fat and men more muscle. Men also tend to store more fat around the waist, which carries more health risk than fat on the hips.
So the same BMI can mean slightly different things for a man and a woman. That is one reason to read it together with a waist measurement. For children and teenagers, BMI is read on separate growth charts for boys and girls.
Limits of BMI
- Children and teenagers. BMI changes naturally as young people grow, so it is read on growth charts as a percentile for age and sex. The adult categories do not apply under 18.
- Pregnancy. Weight gain in pregnancy is normal, and BMI categories do not apply.
- Muscle versus fat. BMI cannot tell them apart. Athletes and people who do heavy physical work can score high with little fat. Older adults who have lost muscle can score normal with more fat than is ideal.
- Where fat is stored. Fat around the waist carries more health risk than fat on the hips and thighs, and BMI does not see the difference. A waist measurement, or waist-to-height ratio, adds useful information.
- Ethnicity and body build. The same BMI can mean different levels of risk in different populations.
BMI is a screening tool, not a diagnosis. A health professional who knows your history can tell you what your number means for you.
Related calculators
- The calorie calculator estimates how many calories a day keep your weight stable, from the same weight and height plus age, sex and activity.
- The ideal body weight calculator gives a dosing estimate used in hospitals. It is not a target weight, and it differs from the healthy weight range shown here.
Common misconceptions
- “A normal BMI means I’m healthy.” It is one piece of the puzzle. Blood pressure, blood sugar, fitness, sleep and smoking matter just as much.
- “The healthy range is my target weight.” The range is wide on purpose. It simply shows which weights fall between BMI 18.5 and 24.9 for your height. Any personal goal is best agreed with a doctor or dietitian.
- “BMI is calculated differently for women.” The formula and the cut-offs are the same. Only the interpretation alongside other measures may differ.
- “A small change in BMI is a big deal.” A difference of a few tenths is within normal day-to-day weight variation from meals, fluids and clothing.
Frequently asked questions
How do you calculate BMI?
Divide your weight in kilograms by your height in metres squared. At 70 kg and 1.75 m, that is 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 kg/m², which falls in the healthy weight range. The calculator shows each step, so you can follow the sum and check it yourself.
What is a healthy BMI?
For adults, the World Health Organization uses 18.5 to 24.9 as the healthy range. Below 18.5 is classed as underweight, 25 to 29.9 as overweight and 30 or more as obesity. These are cut-offs for whole populations, not a verdict on any one person’s health.
How is the healthy weight range worked out?
By running the BMI formula backwards. The calculator multiplies your height in metres squared by 18.5 and by 24.9, the limits of the WHO healthy range. At 1.70 m the range is 53.5 to 72 kg. It is deliberately wide, and it is not a precise number to aim for.
What is a healthy BMI for my age?
For adults the same categories apply at every age, and the formula does not change with age or sex. Some doctors accept slightly higher values in older people, who lose muscle and height over time. Children and teenagers are read on growth charts by age and sex, so this calculator is not for them.
Is there a separate BMI calculator for men?
No. Adult BMI uses one formula and one set of WHO categories for men and women, so this calculator works for both. What differs is body composition: at the same BMI, men tend to carry more muscle and less fat than women. A waist measurement adds what BMI cannot see.
Can I enter pounds, feet and inches?
Not directly: the calculator works in kilograms and centimetres. Convert first: 1 lb = 0.4536 kg and 1 inch = 2.54 cm. For example, 154 lb is 69.9 kg, and 5 ft 9 in (69 inches) is 175.3 cm. Keep one decimal place when you enter the converted values.
Is BMI accurate for athletes and muscular people?
Not very. BMI only knows your total weight, so it cannot tell muscle from fat. A rugby player or bodybuilder can have a BMI in the overweight range with little body fat. A waist measurement or a body composition test gives a fuller picture.
Can I use BMI during pregnancy?
No. Weight gain in pregnancy is expected, and the BMI categories do not apply. Midwives and doctors use your BMI from before pregnancy, usually recorded at the first antenatal appointment, to plan care. If you are pregnant, our due date calculator gives your weeks and trimester instead.
What does BMI stand for?
BMI stands for body mass index. In Italian and some other languages it is called IMC, from indice di massa corporea. Both names describe the same number, expressed in kg/m². It is also known as the Quetelet index, after the 19th-century statistician who first described it.
My BMI is outside the healthy range. What should I do?
It is best seen as a prompt for a conversation, not a diagnosis. Your GP, a practice nurse or a dietitian can look at your BMI alongside your waist, blood pressure, blood tests, lifestyle and family history. They can then help you decide whether anything needs to change.