How to Calculate Your BMI Accurately (With Worked Examples)
Body Mass Index, or BMI, is probably the single most-quoted number in health screening. Doctors use it, insurance forms ask for it, fitness apps plot it on a chart. Yet almost nobody is taught how to calculate it properly — or, just as importantly, where the number breaks down.
This guide walks through the exact BMI formula, shows you a worked example in both metric and imperial units, lists the official World Health Organization (WHO) ranges, and then spends time on the thing most BMI explainers skip: why the number is a rough screening tool, not a diagnosis. By the end you should be able to calculate and interpret a BMI without overestimating what it tells you.
✨Key takeaways
- BMI = weight (kg) ÷ height (m)² — or (weight in lb ÷ height in in²) × 703 for imperial.
- WHO ranges: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, 30+ obese.
- BMI ignores body composition, so athletes and the very elderly are often misclassified.
- It is a population-level screen, not a medical diagnosis. Pair it with waist circumference and bloodwork.
The BMI formula in both unit systems
The metric formula is the cleanest: BMI = weight in kilograms divided by height in metres, squared. If you weigh 72 kg and stand 1.78 m tall, you divide 72 by (1.78 × 1.78) = 3.1684, giving a BMI of 22.7.
Imperial users multiply by 703 to keep the same numerical scale: BMI = (pounds ÷ inches²) × 703. A 160-lb person who is 5 feet 10 inches tall (70 inches) calculates 160 ÷ (70 × 70) × 703 = 22.96 — essentially the same number you get from the metric version after rounding.
Both formulas land on the same scale, which is why a BMI of 24 means "upper end of healthy" anywhere in the world, regardless of the units you started with.
A worked example, step by step
Say you are 68 kg and 1.70 m tall. Step one: square the height. 1.70 × 1.70 = 2.89. Step two: divide the weight by that number. 68 ÷ 2.89 ≈ 23.53. That puts you squarely in the healthy range.
Now repeat it in imperial. 68 kg is about 150 lb; 1.70 m is about 66.93 inches. 66.93 squared is 4479.6. 150 ÷ 4479.6 = 0.03349. Multiplied by 703, you get 23.54 — the same BMI to within rounding.
If you would rather skip the arithmetic, plug the same values into the BMI Calculator on this site and you will get an identical reading with one click.
The official BMI ranges
The WHO defines four adult ranges: under 18.5 is considered underweight, 18.5 up to (but not including) 25 is the healthy range, 25 up to 30 is overweight, and 30 or higher is classified as obesity (which itself is split into class I, II, and III at 35 and 40).
These cut-offs are based on long-running epidemiology showing increased risk of cardiovascular disease, type 2 diabetes, and all-cause mortality outside the 18.5–24.9 band. The effect sizes are real at the population level, which is why the scale stuck.
Worth noting: some health systems in East Asia use a lower overweight cut-off of 23, reflecting research that people of Asian heritage can develop metabolic disease at lower BMIs than European populations. If that applies to you, treat the standard ranges conservatively.
Where BMI gets it wrong
BMI only knows two numbers: your weight and your height. It has no concept of muscle, bone density, fat distribution, age, sex, or ethnicity. A 95 kg rugby player and a 95 kg sedentary office worker of the same height will get the same BMI, even though their metabolic profiles may be wildly different.
Specific situations where BMI misleads: muscular athletes (overestimates risk), the elderly with sarcopenia (underestimates risk), pregnant women (the formula does not apply), and growing children and teens (they use percentile charts, not adult BMI bands).
For a fuller picture, clinicians usually pair BMI with waist-to-height ratio, body fat percentage, and a basic blood panel. If your BMI falls outside the healthy range, that is a signal to dig deeper — not a verdict.
What to do once you know your BMI
A BMI in the healthy band is reassuring, but it is not a free pass to skip exercise, sleep, or nutrition basics. A BMI outside the healthy band is a prompt for three follow-ups: measure your waist (a circumference above 94 cm for men or 80 cm for women raises metabolic risk independently of BMI), talk to a GP about bloods, and look at daily calorie needs.
Our Calorie Calculator estimates your daily energy needs using the Mifflin-St Jeor equation, which is a good next step after a BMI check. Pair the two numbers with a realistic target and a trusted clinician.
Try the calculators referenced in this guide
Put the maths into practice — every calculator is free and runs entirely in your browser.
Frequently Asked Questions
Is BMI the same for men and women?
The formula and cut-offs are identical. However, at the same BMI, women tend to carry a higher proportion of body fat than men. This is why BMI is often paired with waist circumference or body fat percentage for a clearer picture.
Does BMI work for children?
No. Children and teenagers use age- and sex-specific BMI percentile charts, not the adult 18.5–24.9 band. A paediatrician will plot the value on a growth chart to interpret it correctly.
Can I lower my BMI quickly?
Sustained fat loss of around 0.5–1% of body weight per week is considered safe and realistic. Crash diets can lower weight quickly but often cost muscle mass, which does not improve the health outcomes BMI is supposed to track.
What is a good alternative to BMI?
Waist-to-height ratio is a strong and very simple alternative — keep your waist under half your height. DEXA scans, skinfold calipers, and bioelectrical impedance estimate body fat directly, though accuracy varies.
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