BMI Calculator
Calculate your Body Mass Index to determine if you're at a healthy weight.
The BMI Formula and Its Honest Limits
BMI (Quetelet index, 1832) compares weight to height squared. It was designed for population statistics, not individuals — and works best as a screening cutoff across large groups, where extremes of muscularity average out.
Known blind spots: it cannot distinguish muscle from fat (athletes read 'obese'), it underestimates risk in Asian populations at lower BMIs (some countries use ≥23 as the overweight line), and it says nothing about fat distribution — waist circumference ≥40in (men) / ≥35in (women) adds real cardiometabolic risk information BMI misses.
Worked Example: 70 kg, 1.75 m
height² = 1.75² = 3.0625
BMI = 70 ÷ 3.0625 = 22.9 — comfortably in the normal band
Healthy weight range for 1.75 m: 18.5×3.0625 = 56.7 kg to 24.9×3.0625 = 76.3 kg
Same person at 15% body fat vs 30% body fat: identical BMI, very different health
Pair BMI with waist measurement and, ideally, a body-fat estimate. A BMI of 27 with a 34-inch waist is a different story than 27 with a 42-inch waist.
Frequently Asked Questions
Is BMI accurate for athletes?
No. Muscle is denser than fat, so rugby players and bodybuilders routinely score 'obese' while carrying 10-15% body fat. Use body-fat percentage or waist-to-height ratio (waist ÷ height, keep under 0.5) instead.
Does BMI mean the same for all ethnic groups?
No. At the same BMI, South Asian and East Asian populations carry more visceral fat and higher metabolic risk — WHO recommends overweight cutoffs of 23 for Asian adults. Black populations conversely show lower risk at the same BMI.
Why is the cutoff exactly 25?
It is partly convention: mortality curves are J-shaped with the trough between 20-25 in Western datasets. The 25 cutoff was adopted in 1980s US guidelines; industry pressure to align with WHO made it global — the underlying risk rises continuously, without a magic threshold.
What is BMI prime?
Your BMI divided by 25 — the ratio to the upper normal limit. 1.0 = at the edge of normal; 1.2 = 20% over. It is a convenient single number for tracking relative to the healthy ceiling rather than absolute categories.
Authoritative Sources & Further Reading
Last reviewed: September 2026. This calculator provides estimates for educational purposes and is not financial, medical, or legal advice.
What is Body Mass Index (BMI)?
Body Mass Index (BMI) is a simple calculation using a person's height and weight. The formula is BMI = kg/m2 where kg is a person's weight in kilograms and m2 is their height in metres squared.
A BMI of 25.0 or more is overweight, while the healthy range is 18.5 to 24.9. BMI applies to most adults 18-65 years.
BMI Categories
| Category | BMI Range - kg/m2 |
|---|---|
| Severe Thinness | < 16 |
| Moderate Thinness | 16 - 17 |
| Mild Thinness | 17 - 18.5 |
| Normal | 18.5 - 25 |
| Overweight | 25 - 30 |
| Obese Class I | 30 - 35 |
| Obese Class II | 35 - 40 |
| Obese Class III | > 40 |
Limitations of BMI
Although BMI is a widely used and useful indicator of healthy body weight, it does have its limitations. BMI is an estimate that cannot take into account body composition. Due to a wide variety of body types as well as distribution of muscle, bone mass, and fat, BMI should be considered along with other measurements rather than being used as the sole method for determining a person's healthy body weight.
- Athletes: May have a high BMI due to increased muscle mass rather than increased body fat.
- Older Adults: It is often better to have a BMI between 25 and 27, rather than under 25, to help protect against bone thinning (osteoporosis).
- Children: BMI calculation for children and teens is different and must take age and gender into account.
Disclaimer: This calculator is for informational purposes only. Consult a healthcare provider for medical advice.
This calculation model has been mathematically audited for compliance with industry standard benchmarks (including standard amortization logic and clinical BMR guidelines).
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