BMI — Body Mass Index — is one of those numbers people check once and then either feel relieved or vaguely anxious about for weeks. I wanted to write this guide because the number itself is easy to calculate, but what it means is more nuanced than most resources let on. So let's go through the formula, the chart, and the parts that doctors don't always explain.
BMI is weight divided by the square of height. That's it. The formula comes in two flavours depending on which unit system you use:
Metric (kg and cm): BMI = weight(kg) ÷ height(m)²
Imperial (lb and inches): BMI = (weight(lb) × 703) ÷ height(in)²
The 703 factor converts pounds and inches into the same ratio as the metric version. Let me walk through an example for both:
Metric example: Someone who weighs 72 kg and is 1.75 m tall. 1.75² = 3.0625. 72 ÷ 3.0625 = 23.5 — well within healthy range.
Imperial example: Someone who weighs 158 lbs and is 5'9" (69 inches). 158 × 703 = 111,074. 69² = 4,761. 111,074 ÷ 4,761 = 23.3 — same person, same result.
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese (Class I) |
| 35.0 – 39.9 | Obese (Class II) |
| 40.0 and above | Obese (Class III) |
These cut-offs come from the World Health Organization and are the most widely used reference globally. The BMI calculator on this site uses these standard WHO thresholds (25 = overweight, 30 = obese). Some countries and research institutions use slightly different thresholds — for example, Asian populations tend to carry higher body fat at the same BMI, so the WHO suggests lower cut-off points (23 for overweight, 27.5 for obese) for many Asian countries. These Asian-specific thresholds are different from the global standard above and are not currently applied in the calculator.
BMI works reasonably well as a quick screen for large populations. If you're a healthcare system trying to flag likely health risks across millions of people, BMI is cheap, non-invasive, and correlates well with adverse outcomes at the extremes. That's why it stuck around.
But it was never designed to assess individuals, and it shows at the edges:
BMI is a useful starting point but not the whole picture. Here are measurements that give more context:
None of this means BMI is useless — it's still worth knowing, especially if you're far outside the healthy range. But treat it as one data point, not a verdict.
If your BMI is in the healthy range and you feel well, there's not much to do — keep up whatever you're doing. If you're above 25, the question worth asking is whether weight reduction would genuinely improve your health markers (blood pressure, blood glucose, cholesterol) or whether you're active and metabolically healthy despite the number. That's a conversation worth having with a GP, not just a calculator.
If you're underweight, that's often overlooked in health conversations. A BMI below 18.5 is associated with bone density loss, immune suppression, and hormonal disruption — and deserves the same attention as a high BMI.
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