BMI by Hand: The Formula, Worked Examples and Adult Categories
By Akmal Ali · Published · Updated · 4 min read
Divide your weight in kilograms by your height in metres squared: 85 kg at 1.80 m is 85 ÷ 3.24 = 26.2. In pounds and inches, multiply the weight by 703 and divide by the height in inches squared. For adults, 18.5 to 24.9 is the healthy range, but BMI is a screening measure, not a diagnosis.
Body mass index (BMI) compares your weight with your height. It’s quick to calculate by hand, and health services use it to screen adults for weight categories that may affect health. What it can’t do is tell muscle from fat, which is why the number is a starting point and not a verdict.
The BMI formula
BMI = weight (kg) ÷ height (m)²
“Height squared” means height multiplied by itself. Use metres, not centimetres: 175 cm is 1.75 m. If you measure in pounds and inches, the formula needs a conversion factor:
BMI = 703 × weight (lb) ÷ height (in)²
The 703 comes from converting units: one pound is 0.45359237 kg and one inch is 0.0254 m, and 0.45359237 ÷ 0.0254² is 703.07. Rounding it to 703 changes the result by less than 0.01.
In kilograms and metres
- Convert your height to metres: divide centimetres by 100.
- Square it: multiply the height by itself.
- Divide your weight in kilograms by that number.
- Round to one decimal place.
Example: 70 kg and 175 cm. The height is 1.75 m, and 1.75 × 1.75 = 3.0625. Then 70 ÷ 3.0625 = 22.86, so the BMI is 22.9.
In pounds and inches
- Convert your height to inches: feet × 12, plus the remaining inches.
- Square it.
- Multiply your weight in pounds by 703, then divide by the squared height.
- Round to one decimal place.
Example: 154 lb and 5 ft 9 in. The height is 5 × 12 + 9 = 69 inches, and 69 × 69 = 4761. Then 703 × 154 = 108,262, and 108,262 ÷ 4761 = 22.74, so the BMI is 22.7.
More worked examples
| Weight | Height | Calculation | BMI |
|---|---|---|---|
| 52 kg | 170 cm | 52 ÷ 1.70² = 52 ÷ 2.89 | 18.0 |
| 60 kg | 165 cm | 60 ÷ 1.65² = 60 ÷ 2.7225 | 22.0 |
| 85 kg | 180 cm | 85 ÷ 1.80² = 85 ÷ 3.24 | 26.2 |
| 95 kg | 172 cm | 95 ÷ 1.72² = 95 ÷ 2.9584 | 32.1 |
| 180 lb | 5 ft 10 in | 703 × 180 ÷ 70² = 126,540 ÷ 4900 | 25.8 |
BMI Calculator
Enter your weight and height in kg and cm, or lb, ft and in. You get your BMI, its adult category and the healthy-weight range for your height.
What are the BMI categories for adults?
The World Health Organization, the CDC and the NHS use the same cut-offs for adults:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight (WHO: normal weight) |
| 25 to 29.9 | Overweight |
| 30 to 34.9 | Obesity class 1 |
| 35 to 39.9 | Obesity class 2 |
| 40 or above | Obesity class 3 |
In the examples above, 18.0 is underweight, 22.0 and 22.9 are in the healthy range, 25.8 and 26.2 are overweight, and 32.1 is obesity class 1. Tables like this are usually read with the BMI rounded to one decimal, so a value of 24.96 becomes 25.0; a result that close to a boundary is borderline either way.
The NHS notes that people from South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds have a higher risk of some health problems at a lower BMI. For them, it uses 23 for overweight and 27.5 for obesity.
What weight gives a particular BMI?
You can rearrange the formula to find the weight that gives a particular BMI at your height:
weight (kg) = BMI × height (m)²
At 1.75 m, a BMI of 18.5 is 18.5 × 3.0625 = 56.7 kg and a BMI of 24.9 is 24.9 × 3.0625 = 76.3 kg. So the healthy-weight range at that height is roughly 56.7 to 76.3 kg. The BMI calculator shows this range for your own height.
What can’t BMI tell you?
BMI is a screening measure, not a diagnosis. It was designed to describe groups of people, and it has clear limits for individuals:
- It doesn’t measure body fat. Muscle is denser than fat, so muscular people can have a high BMI without excess fat.
- It ignores where fat is stored. Fat around the waist carries more health risk than fat elsewhere, which is why health services also use waist measurements.
- It’s for adults. For children and teenagers, BMI is compared with growth charts for their age and sex. Use a child BMI calculator from a health service instead.
- It doesn’t apply during pregnancy.
A BMI outside the healthy range is a reason to look more closely, not a verdict on your health. If your result worries you, or you’re planning to change your weight, talk to a doctor, nurse or other qualified health professional, who can take your whole health into account.
Frequently Asked Questions
How accurate is a BMI worked out at home?
As accurate as your measurements. At 70 kg, a height of 1.77 m instead of 1.75 m changes the BMI from 22.9 to 22.3, so measure your height without shoes and weigh yourself at the same time of day.
Is BMI the same for men and women?
For adults, the formula and the categories are the same for men and women. For children and teenagers, BMI is compared with growth charts for their age and sex.
What should I do if my BMI is outside the healthy range?
Treat it as a reason to find out more, not as a diagnosis. A doctor, nurse or other health professional can look at your overall health and explain what the number means for you.