BMI Calculator: How to Calculate and Understand Your Body Mass Index
Kordu Team · 2026-03-31
Key Takeaways
- BMI is a screening tool, not a diagnosis. It estimates body fat from height and weight but cannot tell you if you are healthy.
- A BMI between 18.5 and 24.9 is generally considered healthy for most adults.
- BMI does not distinguish muscle from fat -- athletes and older adults often get misleading results.
- If your BMI is outside the healthy range, talk to your GP before making major changes.
Calculate Your BMI
Enter your height and weight below to get your BMI instantly.
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Your BMI
22.9
Normal
Enter your weight and height to calculate your BMI.
BMI is a general indicator and does not account for muscle mass, bone density, age, or sex. Consult a healthcare professional for personalised advice.
What BMI Actually Tells You
BMI divides your weight (in kg) by your height (in metres) squared. A 70 kg person at 1.75 m tall gets a BMI of 22.9 — “healthy weight.”
That is genuinely all it is: a ratio. It was designed in the 1830s as a population-level statistics tool, not a personal health assessment. It stuck around because it is fast, free, and requires no equipment. But it has real blind spots, which we will get to.
The formula:
- Metric: BMI = weight (kg) / height (m) squared
- Imperial: BMI = (weight (lbs) x 703) / height (inches) squared
BMI Categories
The WHO defines these ranges for adults over 20:
| Category | BMI Range | Health Risk |
|---|---|---|
| Underweight | Below 18.5 | Nutritional deficiencies, weakened immunity |
| Healthy weight | 18.5 -- 24.9 | Lowest risk of weight-related issues |
| Overweight | 25.0 -- 29.9 | Moderately increased risk of heart disease, type 2 diabetes |
| Obese (Class I) | 30.0 -- 34.9 | High risk of cardiovascular disease, joint problems |
| Obese (Class II) | 35.0 -- 39.9 | Very high risk of serious complications |
| Obese (Class III) | 40.0 and above | Extremely high risk; often qualifies for surgical intervention |
These thresholds are not universal. Research in The Lancet shows South Asian, Chinese, and Japanese populations face elevated metabolic risk at lower BMIs. The WHO suggests using 23 (not 25) as the “overweight” threshold for these groups.
Children and Teenagers
BMI works differently for under-20s. Instead of fixed thresholds, paediatric BMI is expressed as a percentile relative to age and sex. 85th percentile = overweight. 95th = obese. The adult categories above do not apply.
Where BMI Falls Short
BMI is useful for a quick sanity check, but it has well-documented problems. Knowing them helps you interpret your result honestly.
It cannot tell muscle from fat. A rugby player with significant lean mass may register as “obese” while carrying minimal body fat. Someone with a “healthy” BMI but low muscle mass may carry dangerous amounts of visceral fat. Same number, completely different health profiles.
It ignores fat distribution. Abdominal fat around your organs is far more dangerous than fat on your hips and thighs. Two people with identical BMIs can have very different cardiovascular risk depending on waist circumference.
It shifts with age. Older adults lose muscle and gain fat even at a stable weight. A BMI of 24 at age 70 represents a different body composition than the same BMI at 30.
BMI is a screening tool, not a diagnosis
A BMI number alone cannot tell you whether you are healthy. It does not measure body fat directly, account for muscle mass, or consider blood pressure, cholesterol, or blood sugar. Treat it as one data point, not the final word.
Better Measures to Use Alongside BMI
- Waist-to-hip ratio: A WHR above 0.85 (women) or 0.90 (men) indicates higher cardiovascular risk.
- Waist circumference: The NHS flags increased risk at 94 cm+ for men and 80 cm+ for women.
- Body fat percentage: Measured via DEXA scan, bioelectrical impedance, or skinfold callipers. More accurate but requires equipment.
- Blood markers: Fasting glucose, HbA1c, lipid panel, and blood pressure give direct metabolic insight regardless of weight.
When to See a Doctor
BMI below 18.5: If you are underweight with fatigue, hair loss, frequent illness, or irregular periods, book an appointment. Unintentional weight loss always warrants investigation.
BMI 25—29.9: Common and not necessarily dangerous — especially if your waist circumference and blood markers are normal. Worth discussing if you also have high blood pressure, elevated cholesterol, or family history of diabetes.
BMI 30+: The statistical association with chronic disease is strong enough that a GP review makes sense even if you feel fine. Early intervention beats waiting for symptoms.
Any sudden change: A significant BMI shift in either direction over a short period without obvious explanation is itself clinically relevant.
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The Bottom Line
BMI tells you where your weight sits relative to your height. That is it. Use it alongside waist measurements, blood work, and how you actually feel. If your result concerns you, a conversation with your doctor will always be worth more than a number on a screen.