Body Mass Index (BMI) is one of the most widely used health screening tools in the world — used by doctors, gyms, insurance companies, and public health organisations in over 190 countries. It gives a quick estimate of whether your weight is in a healthy range for your height, and you can calculate it in under 10 seconds.
But BMI is also one of the most misunderstood numbers in healthcare. This guide explains exactly how to calculate it, what every category means, and — critically — when you should not rely on it alone.
The BMI Formula
BMI is calculated differently depending on whether you use metric or imperial measurements. Both formulas produce the same result.
Metric formula (weight in kilograms, height in metres):
BMI = weight (kg) ÷ height² (m²)
Imperial formula (weight in pounds, height in inches):
BMI = (weight (lbs) ÷ height² (in²)) × 703
Why 703? It's the conversion factor that makes the imperial formula produce the same BMI value as the metric formula. Without it, the numbers would be on a completely different scale.
Step-by-Step Calculation Examples
Metric Example: 70 kg, 1.75 m tall
- Square your height: 1.75 × 1.75 = 3.0625 m²
- Divide weight by height squared: 70 ÷ 3.0625 = 22.86
- Result: BMI of 22.86 → Normal weight ✅
Imperial Example: 154 lbs, 5 ft 9 in (69 inches) tall
- Convert height to inches: 5 × 12 + 9 = 69 inches
- Square your height: 69 × 69 = 4,761 in²
- Divide weight by height squared: 154 ÷ 4,761 = 0.03234
- Multiply by 703: 0.03234 × 703 = 22.74
- Result: BMI of 22.74 → Normal weight ✅
Quick Reference: BMI by Height and Weight (Metric)
| Height | Underweight (<18.5) | Normal (18.5–24.9) | Overweight (25–29.9) | Obese (30+) |
|---|---|---|---|---|
| 1.60 m | < 47.4 kg | 47.4 – 63.7 kg | 64 – 76.5 kg | > 76.8 kg |
| 1.65 m | < 50.3 kg | 50.3 – 67.7 kg | 68 – 81.4 kg | > 81.7 kg |
| 1.70 m | < 53.5 kg | 53.5 – 71.9 kg | 72.3 – 86.3 kg | > 86.7 kg |
| 1.75 m | < 56.7 kg | 56.7 – 76.3 kg | 76.6 – 91.6 kg | > 91.9 kg |
| 1.80 m | < 59.9 kg | 59.9 – 80.7 kg | 81 – 96.9 kg | > 97.2 kg |
| 1.85 m | < 63.3 kg | 63.3 – 85.2 kg | 85.6 – 102.3 kg | > 102.7 kg |
BMI Chart & Categories
The World Health Organization (WHO) defines six standard BMI categories for adults aged 18 and over:
| BMI Range | Category | Health Risk | What It Means |
|---|---|---|---|
| Below 18.5 | Underweight | Increased risk | May indicate malnutrition, eating disorders, or other health conditions |
| 18.5 – 24.9 | Normal weight | Low risk | Associated with the lowest risk of weight-related disease |
| 25.0 – 29.9 | Overweight | Moderate risk | Increased risk of heart disease, type 2 diabetes, and hypertension |
| 30.0 – 34.9 | Obese (Class I) | High risk | Significantly elevated risk of metabolic and cardiovascular conditions |
| 35.0 – 39.9 | Obese (Class II) | Very high risk | High risk; medical intervention often recommended |
| 40.0 and above | Obese (Class III) | Extremely high risk | Also called "severe" or "morbid" obesity; serious health implications |
Note: These categories apply to adults only. Children and teenagers use age- and sex-specific BMI percentile charts — a single number cutoff does not apply to them.
BMI for Children and Teenagers
For people under 18, BMI is calculated the same way but interpreted differently using BMI-for-age percentile charts.
| Percentile | Category |
|---|---|
| Below 5th | Underweight |
| 5th – 84th | Healthy weight |
| 85th – 94th | Overweight |
| 95th and above | Obese |
A child's BMI result is compared to other children of the same age and sex — not to a fixed number. This is because body fat naturally changes as children grow.
What Your BMI Score Means in Practice
BMI Below 18.5 — Underweight
Being underweight can be just as concerning as being overweight. It may be associated with:
- Nutritional deficiencies (iron, vitamin D, calcium)
- Weakened immune system
- Osteoporosis risk
- Fertility issues
- Muscle loss
If you are underweight, speak to a doctor or registered dietitian before making dietary changes.
BMI 18.5–24.9 — Normal Weight
This range is associated with the lowest statistical risk of weight-related health conditions. However, "normal BMI" does not automatically mean you are healthy — body composition, fitness level, diet, sleep, and stress all matter enormously.
BMI 25.0–29.9 — Overweight
Research links this range with a moderately increased risk of:
- Type 2 diabetes
- High blood pressure
- High cholesterol
- Sleep apnoea
- Joint problems
Even modest weight loss of 5–10% of body weight has been shown to meaningfully reduce these risks.
BMI 30 and Above — Obese
Obesity is associated with significantly elevated risks of serious conditions including cardiovascular disease, stroke, certain cancers, and type 2 diabetes. Medical guidance is strongly recommended.
Limitations of BMI
BMI is a useful population-level screening tool, but it has significant limitations when applied to individuals:
| Limitation | Why It Matters |
|---|---|
| Doesn't measure body fat | A muscular athlete can have a BMI of 28 (overweight) with very low body fat |
| Ignores fat distribution | Abdominal (visceral) fat is far more dangerous than fat stored elsewhere — BMI can't distinguish between them |
| Age differences | Older adults naturally carry more body fat at the same BMI as younger adults |
| Sex differences | Women typically have more body fat than men at the same BMI |
| Ethnicity | Asian populations face higher health risks at lower BMI values; WHO recommends lower cutoffs (23 for overweight, 27.5 for obese) |
| Tall and short people | BMI tends to overestimate fatness in tall people and underestimate it in shorter people |
| Pregnancy | BMI is not a valid health measure during pregnancy |
Important: BMI is a screening tool, not a diagnosis. A high or low BMI is a signal to investigate further — not a verdict on your health.
Better Metrics to Use Alongside BMI
For a more complete picture of your health, combine BMI with these additional measurements:
1. Waist Circumference
Measures abdominal fat directly — a stronger predictor of cardiovascular risk than BMI alone.
| Risk Level | Women | Men |
|---|---|---|
| Low risk | < 80 cm (31.5 in) | < 94 cm (37 in) |
| Increased risk | 80–88 cm (31.5–34.6 in) | 94–102 cm (37–40 in) |
| High risk | > 88 cm (34.6 in) | > 102 cm (40 in) |
2. Waist-to-Height Ratio (WHtR)
A simple rule: your waist should be less than half your height.
- WHtR below 0.5 = lower risk
- WHtR above 0.5 = increased cardiometabolic risk
Example: Height 175 cm → waist should be under 87.5 cm.
3. Body Fat Percentage
Measured by DEXA scan, bioelectrical impedance, or skinfold calipers.
| Category | Women | Men |
|---|---|---|
| Essential fat | 10–13% | 2–5% |
| Athletic | 14–20% | 6–13% |
| Fitness | 21–24% | 14–17% |
| Acceptable | 25–31% | 18–24% |
| Obese | 32%+ | 25%+ |
4. Resting Heart Rate
A resting heart rate of 60–100 bpm is normal; well-trained athletes often sit at 40–60 bpm. Consistently elevated resting heart rate is a cardiovascular risk indicator independent of BMI.
Frequently Asked Questions
Q: Is a BMI of 25 considered overweight? Yes — a BMI of 25.0 or above is classified as overweight by the WHO. However, context matters: muscle mass, age, sex, and ethnicity all affect what this number means for an individual.
Q: What is a healthy BMI for women? The same WHO range applies: 18.5–24.9. However, women naturally carry more body fat than men at the same BMI, so some clinicians use slightly adjusted interpretations.
Q: Can you have a normal BMI but still be unhealthy? Yes. This is called "normal weight obesity" or being "skinny fat" — normal BMI with excess body fat and low muscle mass. It carries similar metabolic risks to overweight BMI.
Q: How often should I check my BMI? For most adults, once every 6–12 months is sufficient unless you are actively managing your weight, in which case monthly tracking alongside other metrics is more informative.
Q: Does BMI differ by age for adults? The WHO uses the same cutoffs for all adults 18+, but some research suggests slightly higher BMI values (up to 27) may be acceptable for adults over 65 without increased health risk.
Quick Reference Summary
| Task | Formula |
|---|---|
| Metric BMI | weight (kg) ÷ height² (m²) |
| Imperial BMI | (weight (lbs) ÷ height² (in²)) × 703 |
| Normal range | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese | 30.0 and above |
| Waist-to-height rule | Waist < 50% of height |
Use our free BMI Calculator to get your result instantly in both metric and imperial — no manual calculation needed.
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