How to Calculate BMI by Hand: Metric and Imperial Formulas With Examples
October 21, 2026
Body mass index (BMI) is a simple ratio of weight to height squared. It was developed in the 19th century by the Belgian statistician Adolphe Quetelet and is now used worldwide for screening whether weight is in a healthy range for height. The calculation is easy enough to do on any calculator, in either metric or imperial units, but the unit conversion in the imperial formula catches people out.
The metric formula is BMI = weight in kilograms ÷ (height in meters)². The imperial formula is BMI = 703 × weight in pounds ÷ (height in inches)². The 703 converts pounds per square inch into the metric kilogram-per-square-meter scale.
Skip the math and get your answer instantly:
Open the BMI Calculator →Metric Example
A person who weighs 80 kg and is 1.80 m tall: height squared is 1.80 × 1.80 = 3.24. BMI = 80 ÷ 3.24 = 24.7. If the height is given in centimeters, divide by 100 first: 180 cm becomes 1.80 m. The most common metric error is to use height in centimeters directly, which gives a number that is 10,000 times too small.
Another example: 65 kg and 1.65 m. Height squared = 2.7225, BMI = 65 ÷ 2.7225 = 23.9.
Imperial Example
A person who weighs 180 pounds and is 5 feet 10 inches (70 inches) tall: height squared = 4,900. BMI = 703 × 180 ÷ 4,900 = 126,540 ÷ 4,900 = 25.8. Converting to metric gives the same answer: 180 lb is 81.65 kg, 70 inches is 1.778 m, and 81.65 ÷ 3.161 = 25.8.
Remember to convert feet to inches first: feet × 12 + inches.
The Standard Categories
The World Health Organization classifies adult BMI as follows. These apply to adults and not to children, who use age- and sex-specific percentiles, and some guidance uses lower cutoffs for people of Asian descent.
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal range |
| 25.0 to 29.9 | Overweight |
| 30.0 to 34.9 | Obesity class I |
| 35.0 to 39.9 | Obesity class II |
| 40 and above | Obesity class III |
What BMI Can and Cannot Tell You
BMI is a quick screening tool and correlates with health risk at the population level. It does not distinguish muscle from fat, does not show where fat is stored, and ignores age, sex, and ethnicity. A muscular athlete may have a BMI in the overweight range with low body fat, while an older person with low muscle mass can have a normal BMI yet high body fat.
Use it together with waist circumference, blood pressure, blood sugar, and cholesterol. A clinician considers these in context rather than relying on a single number.
Working Backward
You can find the weight range for a normal BMI at your height: weight = BMI × height². For 1.75 m: 18.5 × 3.0625 = 56.7 kg and 24.9 × 3.0625 = 76.3 kg. In pounds, this is about 125 to 168 lb. That is a more useful way to see the healthy range than a single BMI number.
Frequently Asked Questions
What is the BMI formula?
Weight in kilograms divided by height in meters squared, or 703 × pounds ÷ inches squared.
Why is there a 703 in the imperial formula?
It converts the units from pounds and inches to kilograms and meters.
What BMI is considered normal?
18.5 to 24.9 for adults, per WHO.
Is BMI accurate for athletes?
Not always, since muscle adds weight. Body fat measurements are more informative for them.
How do I find the weight for a BMI of 22?
Multiply 22 by your height in meters squared.