How to Calculate Your BMI and What the Number Really Means

Body Mass Index (BMI) is the most widely used screening number for weight. Doctors, insurers and fitness apps all use it because it takes seconds to calculate. It is also easy to misread. This guide shows how to calculate it in both kilograms and pounds, what each range means and when BMI can mislead you.

The BMI formula

Metric:

BMI = weight (kg) ÷ height (m)²

Imperial:

BMI = 703 × weight (lb) ÷ height (in)²

The 703 converts pounds and inches so both formulas give the same result.

Worked examples

Metric: 70 kg and 1.75 m tall.

• 1.75 × 1.75 = 3.0625
• 70 ÷ 3.0625 = 22.9

Imperial: 180 lb and 5 ft 10 in (70 inches) tall.

• 70 × 70 = 4,900
• 703 × 180 = 126,540
• 126,540 ÷ 4,900 = 25.8

BMI categories for adults (WHO)

BMI	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 ranges apply to adults aged 18 and over. For children and teenagers, BMI is interpreted with age- and sex-specific growth charts (percentiles), not the table above.

What BMI does not tell you

BMI only uses height and weight. It cannot tell muscle from fat or show where fat is stored, so:

• Muscular people (athletes, people who lift weights) can have a BMI in the "overweight" range while having low body fat.
• Older adults lose muscle with age, so a "normal" BMI can hide excess body fat.
• Fat distribution matters. Fat around the waist is linked to higher health risk than fat on the hips. A waist measurement or the waist-to-height ratio (keeping your waist below half your height) adds useful information.
• Different populations may have different risk at the same BMI. The WHO and several national health agencies use lower action points for people of Asian descent, for example.
• Pregnancy: BMI is not used to assess weight during pregnancy.

How to use your BMI result

Treat BMI as a starting point, not a diagnosis. If your result is outside the healthy range, it is a good reason to talk with a doctor, who can look at blood pressure, blood tests, waist size, family history and lifestyle together. Small, steady changes in sleep, activity and diet usually matter more than the exact number.

Healthy weight range for your height

You can reverse the formula to see the weight range that corresponds to a BMI of 18.5 to 24.9. For someone 1.75 m tall, that is about 56.7 kg to 76.3 kg (125 to 168 lb). Our BMI calculator shows your result on a color scale so you can see where you are within the ranges.

Frequently asked questions

What is a healthy BMI?

For adults, the WHO defines a BMI between 18.5 and 24.9 as the healthy weight range.

How do I calculate BMI in pounds?

Multiply your weight in pounds by 703 and divide by your height in inches squared. For 150 lb and 65 inches: 703 × 150 ÷ 4,225 ≈ 25.0.

Is BMI accurate for athletes?

Not always. Because BMI doesn't separate muscle from fat, muscular people can be classified as overweight even with low body fat.

Is BMI the same for men and women?

The formula and adult categories are the same for both sexes, although body fat at a given BMI tends to differ.

Informational content only. For legal, financial or health decisions, consult a qualified professional.