Body Mass Index (BMI) is a simple screening measure — your weight divided by the square of your height — used by the NHS and the World Health Organization to flag whether someone's weight is likely to sit in a healthy range for their height. It isn't a diagnosis, and it has real limitations, but the chart below is the same one doctors and pharmacists actually work from.
The full BMI chart
These are the standard adult BMI categories, using the same 18.5 / 25 / 30 boundaries as the NHS and WHO:
| Category | BMI range |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25 – 29.9 |
| Obese — Class I | 30 – 34.9 |
| Obese — Class II | 35 – 39.9 |
| Obese — Class III | 40 and above |
This scale applies to adults — it isn't used for anyone under 18, whose healthy weight is instead judged against age- and sex-specific growth charts, since children's proportions change constantly as they grow.
How to calculate BMI by hand
The formula behind every BMI calculator, including Altto's:
Metric: BMI = weight (kg) ÷ height (m)²
Imperial: BMI = 703 × weight (lb) ÷ height (in)²Worked example, metric: someone who is 1.70m tall and weighs 70kg has a BMI of 70 ÷ (1.70 × 1.70) = 70 ÷ 2.89 = 24.2 — a healthy weight, just inside the 18.5–24.9 range.
Worked example, imperial: someone who is 5'8" (68 inches) and weighs 154lb has a BMI of 703 × 154 ÷ (68 × 68) = 108,262 ÷ 4,624 = 23.4 — also a healthy weight.
The 703 constant in the imperial formula exists purely to convert pounds and inches into the same kg/m² units the metric formula produces directly — the underlying calculation is identical either way.
Healthy weight range by height
Rather than a single BMI number, it's often more useful to know the actual weight range that counts as healthy for your height. This table shows the weight span corresponding to a BMI of 18.5–24.9 at common heights:
| Height | Healthy weight range |
|---|---|
| 150cm (4'11") | 41.6–56.0 kg (92–124 lb) |
| 155cm (5'1") | 44.4–59.8 kg (98–132 lb) |
| 160cm (5'3") | 47.4–63.7 kg (104–141 lb) |
| 165cm (5'5") | 50.4–67.8 kg (111–149 lb) |
| 170cm (5'7") | 53.5–72.0 kg (118–159 lb) |
| 175cm (5'9") | 56.7–76.3 kg (125–168 lb) |
| 180cm (5'11") | 59.9–80.7 kg (132–178 lb) |
| 185cm (6'1") | 63.3–85.2 kg (140–188 lb) |
| 190cm (6'3") | 66.8–89.9 kg (147–198 lb) |
| 195cm (6'5") | 70.3–94.7 kg (155–209 lb) |
| 200cm (6'7") | 74.0–99.6 kg (163–220 lb) |
Don't see your exact height? Altto's BMI Calculator works out the precise range for any height instantly, in metric or imperial.
Does the BMI chart mean the same thing for everyone?
Not quite, and it's worth knowing where the standard chart is less reliable:
- Muscle vs. fat: BMI can't distinguish between the two, so very muscular people (athletes, bodybuilders) are often classed as overweight or obese despite low body fat.
- Ethnic background: the standard 18.5–24.9 range was developed mostly from data on white European populations. NHS guidance notes that people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background can face increased health risks at a lower BMI, and some NHS services use adjusted thresholds for these groups — around 23 for overweight and 27.5 for obese, instead of 25 and 30.
- Age: older adults tend to carry more body fat for the same BMI as they lose muscle mass, which the standard chart doesn't account for.
- Pregnancy: BMI isn't used to assess weight during pregnancy, since healthy weight gain varies by starting weight and stage.
None of this makes the chart useless — it's still the same starting point the NHS and GPs use — but it's a screening tool, not a verdict, and a GP or pharmacist can put a result in proper context.
How to check yours
Altto's BMI Calculator takes your height and weight, in metric or imperial, and instantly shows your BMI, category and healthy weight range — entirely in your browser, with nothing ever uploaded.
- Choose metric (cm/kg) or imperial (ft, in/lb).
- Enter your height and weight.
- See your BMI, category and healthy weight range update instantly, checked against the same chart above.
Common questions
What BMI is considered a healthy weight?
Is the BMI chart the same for men and women?
How is BMI used by doctors?
What's the difference between BMI and healthy weight range?
Ready to check yours?
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