What is BMI?
BMI stands for Body Mass Index. It is a number calculated from your weight and height that gives you a rough idea of whether you are underweight, at a healthy weight, overweight, or obese.
Doctors and health organizations use BMI as a quick screening tool because it only requires two measurements that anyone can take at home. However, BMI is not a diagnosis - it is a starting point for conversation with your doctor.
How to Calculate BMI (The Formula)
The BMI formula is simple:
Step-by-step example: Let's calculate BMI for someone who weighs 75 kg and is 1.72 meters tall.
- First, square the height: 1.72 × 1.72 = 2.96
- Then divide weight by that number: 75 ÷ 2.96 = 25.3
This person has a BMI of 25.3, which falls into the "overweight" category (just barely over the 25.0 threshold).
Using pounds and inches? The formula changes slightly:
Example: 165 lbs at 5'9" (69 inches): (165 × 703) ÷ (69 × 69) = 115,995 ÷ 4,761 = 24.4
BMI Categories and What They Mean
| BMI | Category | What it means |
|---|---|---|
| Below 18.5 | Underweight | May indicate nutritional deficiency. Talk to a doctor about healthy weight gain. |
| 18.5 – 24.9 | Normal | Healthy range. Maintain through balanced diet and regular activity. |
| 25.0 – 29.9 | Overweight | Slightly elevated risk. Small lifestyle changes can make a big difference. |
| 30.0 – 34.9 | Obese (Class 1) | Higher risk of heart disease, diabetes. Consult a healthcare provider. |
| 35.0+ | Obese (Class 2/3) | Significantly elevated health risks. Medical guidance recommended. |
When BMI Gets It Wrong
BMI is a useful first check, but it has blind spots. Here are real situations where BMI can mislead you:
Athletes and gym-goers: A rugby player weighing 100 kg at 1.80m has a BMI of 30.9 (obese), but may have 12% body fat and excellent cardiovascular health. Muscle is denser than fat, so muscular people often show high BMI despite being fit.
Older adults: An 70-year-old woman with BMI of 23 might actually carry more body fat than a 30-year-old at the same BMI, because muscle mass naturally decreases with age.
Where you carry weight matters: Two people with BMI of 27 can have very different health risks. Fat around the belly (visceral fat) is more dangerous than fat on hips and thighs. That's why doctors often measure waist circumference too.
What to Do With Your BMI Result
If your BMI is under 18.5: You may not be getting enough calories or nutrients. This can weaken your immune system and bones. See a doctor to rule out underlying conditions, and consider working with a nutritionist.
If your BMI is 18.5-24.9: You are in the healthy range. Focus on maintaining this through regular physical activity (aim for 150 minutes of moderate exercise per week) and a balanced diet.
If your BMI is 25-29.9: You are slightly above the healthy range. The good news: losing just 5-10% of your body weight can significantly reduce health risks. For someone at 85 kg, that is only 4-8 kg.
If your BMI is 30+: Talk to your doctor. They can check for related conditions like high blood pressure or elevated blood sugar, and help create a safe weight management plan.
Frequently Asked Questions
What is BMI and why does it matter?
BMI (Body Mass Index) is a number calculated from your height and weight. It matters because studies show that people with very high or very low BMI tend to have more health problems on average. But BMI is just one piece of the puzzle - your doctor looks at many factors.
What is considered a healthy BMI?
For most adults, a BMI between 18.5 and 24.9 is considered healthy. But "healthy" looks different for everyone. A BMI of 26 with low body fat and good cardiovascular fitness may be healthier than a BMI of 22 with poor diet and no exercise.
My BMI says I am overweight but I exercise regularly. Is BMI wrong?
Possibly. If you lift weights or do strength training, you may have more muscle mass than average. Muscle weighs more than fat, so athletic people often have higher BMI despite being healthy. In this case, body fat percentage or waist-to-hip ratio are better measures.
How do I calculate my BMI manually?
Divide your weight in kilograms by your height in meters squared. Example: 70 kg ÷ (1.75m × 1.75m) = 70 ÷ 3.0625 = 22.9. For pounds/inches: multiply weight by 703, then divide by height in inches squared.
How much weight do I need to lose to get to a normal BMI?
Use this formula: Target weight = 24.9 × (your height in meters)². For example, at 1.70m tall: 24.9 × 2.89 = 72 kg. If you currently weigh 85 kg, you would need to lose about 13 kg to reach the top of the normal range.
Can children use this BMI calculator?
No. Children and teenagers need a different calculation that accounts for age and sex, because their bodies are still developing. Pediatricians use "BMI-for-age percentile" charts instead. This calculator is designed for adults aged 20 and older.
Is BMI the same for men and women?
The formula and categories are the same, but interpretation differs. Women naturally have higher body fat percentage than men at the same BMI. Some researchers suggest women can be healthy at slightly higher BMI, but the official categories remain the same for both sexes.