BMI Explained: How to Calculate It and What Counts as Healthy
What's a healthy BMI, and how do you work yours out? This plain guide explains the BMI formula, the weight categories, why thresholds differ by region, and BMI's limits — with a free BMI calculator.
BMI (Body Mass Index) is the quickest way to gauge whether your weight sits in a generally healthy range — but it's widely misunderstood, both how to read it and what its limits are. Here's the plain version, with a calculator to do the maths for you.
How to calculate BMI
The formula uses only height and weight:
BMI = weight (kg) ÷ height (m)²
Example: 65 kg, 170 cm → 65 ÷ (1.7 × 1.7) ≈ 22.5.
Don't want to do the maths? Use our BMI calculator — enter height and weight to get your BMI and category instantly, plus the healthy weight range for your height.
The weight categories
| BMI | Category (WHO) |
|---|---|
| Under 18.5 | Underweight |
| 18.5 – 24.9 | Normal |
| 25 – 29.9 | Overweight |
| 30 and above | Obese |
Regional differences: some health authorities use stricter cut-offs for Asian populations — for example, overweight at 24 and obese at 27 — because risks like diabetes and heart disease can appear at lower BMIs. Our calculator uses these stricter Asian thresholds. Check which standard applies to you.
BMI's limits — read this
BMI is a screening tool, not a diagnosis. It uses only height and weight, so it can't tell muscle from fat or where fat sits. It doesn't suit:
- Pregnant women
- Under-18s (different growth charts apply)
- Athletes / very muscular people (muscle pushes BMI up without excess fat)
For these, BMI alone is misleading — pair it with body-fat percentage, waist measurement and professional assessment.
A healthier way to use BMI
- Treat it as a starting point. In range is reassuring; out of range is a prompt to look closer, not a verdict.
- Look at more than weight. Body-fat percentage and waist-to-height ratio say more about health than BMI alone.
- Track trends, not one number. How your numbers move over time matters more than a single reading.
Health note: this article is general information, not medical advice. For health concerns, consult a qualified professional.
The bottom line
BMI = weight ÷ height² is a fast way to see if your weight sits in a generally healthy range — normal is about 18.5–24.9 (some regions use stricter 24/27 cut-offs for Asian populations). But it's a screening tool with real limits: it can't tell muscle from fat and doesn't suit athletes, pregnant women or under-18s. Use the BMI calculator for your number, then look at the bigger picture for the full story.
FAQ
How is BMI calculated?
BMI = weight (kg) ÷ height (m) squared. For example, 65 kg and 170 cm: 65 ÷ (1.7 × 1.7) ≈ 22.5. It's a quick screening number based only on height and weight, used to gauge whether your weight sits in a generally healthy range. A calculator does it instantly if you'd rather not do the maths.
What BMI is considered healthy?
By the common WHO scale, 18.5–24.9 is normal, 25–29.9 overweight and 30+ obese, with under 18.5 underweight. Some regions use stricter cut-offs for Asian populations (for example, overweight at 24 and obese at 27), because health risks can appear at lower BMIs. Our calculator uses these stricter Asian thresholds; check which standard applies to you.
Is BMI accurate for everyone?
BMI is a fast screening tool, not a full health assessment. It doesn't suit pregnant women, under-18s, athletes or very muscular people, because muscle weighs more than fat and can push BMI up without excess fat. Use BMI as a rough indicator alongside body-fat percentage, waist measurement and professional advice.
My BMI is in range but I still feel out of shape — why?
Because BMI only uses height and weight; it can't tell muscle from fat or where fat sits. Two people with the same BMI can have very different body compositions. If your BMI looks fine but you're concerned, look at body-fat percentage and waist-to-height ratio, and consider a professional assessment — BMI is a starting point, not the whole picture.