What is BMI and How to Calculate It: A Complete Guide with Free Calculator
Learn what Body Mass Index (BMI) is, how to calculate it manually, what your BMI number means, and use our free online BMI calculator. Includes BMI charts, limitations, and health guidelines.
Body Mass Index (BMI) is the most widely used screening tool for determining whether a person's weight is appropriate for their height. Used by doctors, insurance companies, and health organizations worldwide, BMI provides a quick numerical indicator of potential weight-related health risks.
What is BMI?
BMI is a simple mathematical formula that divides your weight by the square of your height:
BMI = weight (kg) ÷ height² (m²)
For imperial units: BMI = (weight in lbs × 703) ÷ height² (inches²)
BMI Categories (WHO Classification)
| BMI Range | Category | Health Risk | |-----------|----------|-------------| | Below 18.5 | Underweight | Malnutrition, bone loss | | 18.5 - 24.9 | Normal weight | Low risk | | 25.0 - 29.9 | Overweight | Moderate risk | | 30.0 - 34.9 | Obesity Class I | High risk | | 35.0 - 39.9 | Obesity Class II | Very high risk | | 40.0+ | Obesity Class III | Extremely high risk |
How to Calculate BMI Manually
Metric System Example
Person: 75 kg, 1.75 m tall
- Square the height: 1.75 × 1.75 = 3.0625
- Divide weight by height squared: 75 ÷ 3.0625 = 24.5
- BMI = 24.5 (Normal weight)
Imperial System Example
Person: 165 lbs, 5'10" (70 inches) tall
- Square the height: 70 × 70 = 4,900
- Multiply weight by 703: 165 × 703 = 115,995
- Divide: 115,995 ÷ 4,900 = 23.7
- BMI = 23.7 (Normal weight)
Free BMI Calculator
Don't want to do the math? Use our free online BMI calculator:
Our calculator also shows:
- Your BMI category
- Healthy weight range for your height
- Calorie needs estimate
- Personalized health recommendations
BMI Chart
Quick Reference (Metric)
| Height | Underweight (<18.5) | Normal (18.5-24.9) | Overweight (25-29.9) | Obese (30+) | |--------|--------------------|--------------------|---------------------|-------------| | 150cm | <41.6kg | 41.6-56.0kg | 56.3-67.1kg | 67.5kg+ | | 160cm | <47.4kg | 47.4-63.7kg | 64.0-76.3kg | 76.8kg+ | | 170cm | <53.5kg | 53.5-71.9kg | 72.3-86.2kg | 86.7kg+ | | 175cm | <56.7kg | 56.7-76.2kg | 76.6-91.4kg | 91.9kg+ | | 180cm | <59.9kg | 59.9-80.6kg | 81.0-96.6kg | 97.2kg+ |
Limitations of BMI
What BMI Doesn't Account For
- Muscle mass — athletes may have high BMI but low body fat
- Bone density — heavier bones can skew results
- Age — same BMI can mean different things at different ages
- Gender — women naturally have more body fat than men at the same BMI
- Ethnicity — different ethnic groups have different risk thresholds
- Fat distribution — where you carry fat matters (belly vs. hips)
Better Alternatives for Some People
- Body fat percentage — more accurate for athletes
- Waist-to-hip ratio — better indicator of health risk
- Waist circumference — simple indicator of abdominal fat
- DEXA scan — gold standard for body composition
What to Do With Your BMI Number
Getting a number is easy; knowing what to do next is the useful part. Treat BMI as a first screen, then layer context:
- If you're in the normal range (18.5–24.9): BMI has done its job. Focus on habits — sleep, movement, diet quality — rather than chasing a specific number.
- If you're in the overweight range (25–29.9): Don't panic and crash-diet. Check your waist circumference too (a value over 94 cm for men / 80 cm for women signals elevated risk). Sustainable changes of 0.5–1 kg per week are both realistic and healthier than rapid loss.
- If you're in an obese range (30+): This is a signal to involve a professional rather than self-manage. A doctor can screen for related conditions (blood pressure, blood sugar, cholesterol) that matter far more than the BMI figure itself.
- If you're underweight (below 18.5): This deserves attention too — it's associated with nutrient deficiency and bone loss. Rule out an underlying cause with a professional.
Why Two People With the Same BMI Can Have Very Different Health
BMI treats a 90 kg rugby player and a 90 kg sedentary office worker of the same height identically — yet their health profiles are worlds apart. That's because BMI measures mass relative to height, not what that mass is made of. Muscle is dense and heavy; fat, especially visceral fat around the organs, is the real driver of metabolic risk.
This is why waist circumference and body-fat percentage often tell a more honest story. If your BMI says "overweight" but you strength-train regularly and your waist is well within range, the number is likely misreading muscle as risk. Conversely, a "normal" BMI with a large waistline — sometimes called "skinny fat" — can hide genuine risk. Use BMI to start the conversation, not to end it.
A Note on BMI Across Ethnic Groups
The standard WHO thresholds were derived largely from European populations. For people of South and East Asian descent, health risks tend to appear at lower BMI values, which is why several health authorities recommend a lower overweight threshold (around BMI 23) for these groups. If this applies to you, interpret the standard chart conservatively and prioritise waist measurement.
FAQ
Is BMI accurate for everyone?
BMI is a screening tool, not a diagnostic tool. It's accurate for most people but may be misleading for athletes, elderly individuals, and certain ethnic groups.
What's a healthy BMI for adults?
A BMI between 18.5 and 24.9 is considered healthy for most adults. However, optimal BMI can vary based on individual health factors.
Does BMI apply to children?
Children use BMI-for-age percentiles rather than fixed categories. A child's BMI is compared to other children of the same age and sex.
How often should I check my BMI?
Checking once a month is sufficient for most people. Daily weight fluctuations are normal and don't reflect meaningful changes in body composition.
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