Maybe your doctor mentioned it, a fitness app asked for it, or an insurance form wants it. Body Mass Index (BMI) is one of the most common health numbers, and you can work it out yourself in about a minute.
This guide shows the formula in both metric and imperial units, explains what the numbers mean, and covers what BMI can and cannot tell you.
Quick Answer
BMI is your weight divided by your height squared.
- Metric: BMI = weight in kg ÷ (height in metres)²
- Imperial: BMI = [weight in pounds ÷ (height in inches)²] × 703
For example, a person who weighs 70 kg and is 1.75 m tall has a BMI of 22.9.
How to Calculate BMI in Metric Units
- Measure your weight in kilograms. Example: 70 kg.
- Measure your height in metres. Example: 1.75 m.
- Square your height. 1.75 × 1.75 = 3.0625.
- Divide your weight by that number. 70 ÷ 3.0625 = 22.9.
If your height is in centimetres, divide by 100 first. For instance, 175 cm = 1.75 m.
How to Calculate BMI in Imperial Units
- Measure your weight in pounds. Example: 160 lb.
- Measure your height in inches. Example: 5’9″ = 69 inches.
- Square your height. 69 × 69 = 4,761.
- Divide weight by that number. 160 ÷ 4,761 = 0.0336.
- Multiply by 703. 0.0336 × 703 = 23.6.
Need to switch units first? See our guide on [How to Convert Kilograms to Pounds].
BMI Categories for Adults
The World Health Organization (WHO) uses these standard ranges for adults:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight range |
| 25.0 to 29.9 | Overweight |
| 30.0 and above | Obesity |
In our first example (BMI 22.9), the result falls in the healthy range. A person who is 85 kg and 1.70 m tall has a BMI of 85 ÷ 2.89 = 29.4, which falls in the overweight range.
What BMI Can and Can’t Tell You
BMI is popular because it is quick, free and easy to compare across large groups of people. But it has real limits.
BMI doesn’t measure body fat directly. It only uses height and weight.
It can be misleading for some people:
- Athletes and very muscular people, who may have a high BMI because muscle is dense.
- Older adults, who may lose muscle and have more body fat at the same BMI.
- People of different ethnic backgrounds. Some health bodies suggest lower BMI thresholds for people of South Asian and East Asian descent, because health risks can start at lower values.
- Pregnant women, for whom standard BMI categories don’t apply.
- Children and teenagers, whose BMI is compared with age- and sex-specific growth charts, not adult ranges.
BMI also says nothing about where fat is stored, your fitness, your diet or your blood tests.
Other Measures Doctors May Use
Because of these limits, health professionals often look at BMI together with other information:
- Waist circumference, which can show fat around the abdomen.
- Waist-to-height ratio.
- Blood pressure, blood sugar and cholesterol.
- Family history, activity level and overall health.
What Should You Do With Your BMI?
Treat it as a starting point for a conversation, not a verdict. If your result is outside the healthy range, or if you are unsure what it means for you, a doctor can look at the full picture and suggest next steps that fit your situation.
If thinking about weight or numbers causes you stress, or if you have concerns about your eating, please reach out to a healthcare professional for support.
Common Mistakes to Avoid
- Forgetting to square the height.
- Using centimetres instead of metres in the metric formula.
- Skipping the 703 in the imperial formula.
- Using adult categories for children.
Frequently Asked Questions
What is the BMI formula? BMI = weight (kg) ÷ height (m)², or [weight (lb) ÷ height (in)²] × 703.
What is a healthy BMI? For most adults, the WHO describes 18.5 to 24.9 as the healthy weight range, though individual factors matter.
Is BMI accurate? It is a useful screening tool for groups, but it can be inaccurate for individuals, especially athletes, older adults and children.
Can I calculate BMI without a calculator? Yes, but it is easier with one. Square your height first, then divide your weight by the result.
Is BMI the same for men and women? The adult formula and categories are the same, but body composition differs between men and women, so interpretation can vary.
Final Thoughts
Calculating BMI takes three steps: square your height, divide your weight by it, and compare the result with the adult ranges. Remember that it is one number among many, and your doctor is the best person to explain what it means for you.
Sources and Further Reading
- World Health Organization (who.int): Body mass index
- Centers for Disease Control and Prevention (cdc.gov): About BMI
- NHS (nhs.uk): BMI calculator and healthy weight
Disclaimer: This content is for general educational purposes only and does not constitute medical advice. Please consult a qualified health professional about your own situation.