How to Calculate BMI

6 min read · Updated September 26, 2026 · by the SolveCalcPro editorial team

Body mass index, or BMI, is a single number that compares your weight with your height. It is quick to calculate, which is why doctors, health agencies and researchers use it as a first screening step. This guide shows the formula in metric and imperial units, walks through examples and explains how to read the result sensibly — including what BMI can’t tell you.

The BMI formula

The metric formula is BMI = weight in kilograms ÷ (height in metres)². If you use pounds and inches, multiply by 703: BMI = 703 × weight in pounds ÷ (height in inches)². The 703 simply converts the units so both versions give the same answer.

The squared height is the part that surprises people. Dividing by height squared rather than height means that two people of different heights can be compared fairly: a taller person is expected to weigh more, and the weight needed grows roughly with the square of height.

Worked example in metric units

Take someone who weighs 70 kg and is 175 cm tall.

Convert height to metres: 175 cm = 1.75 m.
Square it: 1.75 × 1.75 = 3.0625.
Divide weight by that: 70 ÷ 3.0625 = 22.9.

That falls in the healthy weight band. For this height the healthy range works out to 56.66 kg – 76.26 kg.

Worked example in pounds, feet and inches

Now someone who weighs 154 lb and is 5 ft 9 in tall.

Convert height to inches: 5 × 12 + 9 = 69 in.
Square it: 69 × 69 = 4,761.
Multiply weight by 703: 154 × 703 = 108,262.
Divide: 108,262 ÷ 4,761 = 22.7.

The metric and imperial routes agree because both describe the same person.

Reading the result: WHO adult categories

BMI	Category
Below 18.5	Underweight
18.5 – 24.9	Healthy weight
25.0 – 29.9	Overweight
30.0 and above	Obesity (classes 1–3 at 30, 35 and 40)

These cut-offs come from the World Health Organization and are used for adults of both sexes. They are population-level screening bands, not individual verdicts.

What BMI cannot tell you

Muscle vs fat: a muscular athlete can have a high BMI with low body fat.
Fat distribution: fat carried around the waist carries more health risk than the same weight carried elsewhere, and BMI cannot see it.
Age and ancestry: older adults, children and some ethnic groups need different interpretation; children use age-and-sex growth charts.

Pair BMI with other measures such as waist circumference, and talk to a clinician about anything that concerns you. This guide is educational, not medical advice.

Practice questions

Test yourself, then tap to check. Answers are calculated by the tools linked below.

A person weighs 82 kg and is 178 cm tall. What is their BMI and category?
Show answer
BMI: 25.9 · Category (adult WHO ranges): Overweight
A 28-year-old woman, 62 kg, 168 cm, lightly active, wants to maintain her weight. What are her BMR and TDEE?
Show answer
Daily calorie target: 1,882 kcal · Maintenance calories (TDEE): 1,882 kcal
Convert 12 stone to kilograms.
Show answer
12 st in kg: 76.20351816

Frequently asked questions

What is the BMI formula?

Weight in kilograms divided by height in metres squared. In pounds and inches, multiply weight by 703 and divide by height in inches squared.

What is a good BMI?

For most adults, 18.5 to 24.9 is considered the healthy range by the WHO and the NHS.

Is BMI the same for men and women?

The adult categories are the same for both sexes, though body composition differs between them, so BMI is only a rough guide.

Can I calculate BMI without a calculator?

Yes, using the steps above: square your height, then divide weight by it. A calculator just does the arithmetic instantly and shows your healthy weight range.