How BMI Is Calculated, What the Categories Mean, and When to Ignore It

Body Mass Index has been around since the 1830s, when a Belgian statistician named Adolphe Quetelet was looking for a simple way to describe the “average man”. It was never designed to diagnose anyone. Nearly two centuries later it is still the first number a doctor, insurer or fitness app reaches for, mostly because it needs only a height and a weight. Knowing how it is calculated, what the categories mean and where it breaks down lets you use it for what it is good at and ignore it where it isn’t.

The formula

Metric: BMI = weight (kg) ÷ height (m)²

Imperial: BMI = 703 × weight (lb) ÷ height (in)²

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Example: 70 kg and 1.75 m tall → 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9.

The same person in imperial units is 154 lb and 69 in: 703 × 154 ÷ (69 × 69) = 108,262 ÷ 4,761 = 22.7. The small difference is rounding in the unit conversion. The BMI Calculator accepts either system and shows the category alongside the number.

The standard categories (adults)

BMI	WHO category
Below 18.5	Underweight
18.5 – 24.9	Healthy weight
25.0 – 29.9	Overweight
30.0 – 34.9	Obesity class I
35.0 – 39.9	Obesity class II
40 and above	Obesity class III

These cut-offs come from population studies linking BMI ranges to health outcomes. They are averages across very large groups, which is precisely why they can be wrong for an individual.

Why the cut-offs differ by population

The WHO thresholds were derived largely from European data. Research in the 2000s found that people of South Asian, Chinese and Japanese ancestry tend to carry more body fat, and higher metabolic risk, at a given BMI. As a result, many health bodies use lower thresholds for these groups: overweight from 23 and obesity from 27.5 (the UK’s NICE guidance and India’s national guidelines, for example). If you are in one of these populations, a BMI of 24 sits in the “take notice” range rather than the middle of “healthy”.

Where BMI gets it wrong

• Muscle. BMI cannot tell fat from muscle. A rugby forward at 1.85 m and 100 kg has a BMI of 29.2 and is “overweight” on paper. Anyone who trains seriously with weights should treat BMI as close to meaningless for themselves.
• Height extremes. The formula divides by height squared, but body mass scales closer to height cubed. It slightly overstates BMI for tall people and understates it for short people.
• Age. Older adults lose muscle and gain fat at the same weight; a “healthy” BMI at 70 can hide poor body composition. Some research suggests the lowest-risk range shifts upward with age.
• Fat distribution. Fat around the organs (visceral fat) carries far more risk than fat under the skin, and BMI cannot see the difference. Two people with identical BMI can have very different waistlines and very different risk.
• Children and teenagers. Adult categories do not apply. Paediatric BMI is read from age- and sex-specific percentile charts, because healthy BMI changes throughout growth.
• Pregnancy. BMI is not used during pregnancy beyond the initial booking measurement.

Better numbers to pair it with

None of these require a lab:

• Waist circumference. Measured at the navel, relaxed, at the end of a normal breath. Increased risk above roughly 94 cm (37 in) for men and 80 cm (31.5 in) for women, with lower thresholds again for South and East Asian populations.
• Waist-to-height ratio. Waist ÷ height, aiming for under 0.5. Simple, and it tracks visceral fat far better than BMI. Keep your waist under half your height is a rule most clinicians would endorse.
• Trend, not snapshot. A BMI that has moved from 23 to 27 over three years tells you more than either number alone.

If you are trying to change your weight, the useful next calculation is energy balance. The Calorie Calculator estimates your maintenance intake from height, weight, age and activity, which is the number a target weight actually depends on.

How to use BMI sensibly

1. Calculate it once with the BMI Calculator and note the category, using the Asian thresholds if they apply to you.
2. Measure your waist and work out waist-to-height.
3. If BMI and waist both sit in the higher-risk ranges, that is a reliable signal worth a conversation with a doctor. If they disagree, the waist measurement is usually the one to trust.
4. Re-check every few months rather than every day. Weight fluctuates 1–2 kg with water and food; BMI inherits that noise.

BMI is a screening tool, not a verdict. Used alongside one other measurement and a bit of context about your body, it earns its place; used alone, it will mislead a meaningful share of people in both directions.

Frequently asked questions

What is a healthy BMI?

18.5 to 24.9 for most adults under WHO categories; 18.5 to 22.9 under the thresholds used for South and East Asian populations.

Is BMI accurate?

Accurate as a formula, limited as a health measure. It works reasonably for large groups and for people of average build, and poorly for athletes, older adults and the very tall or short.

Does BMI differ for men and women?

The adult formula and categories are the same for both. Women naturally carry more body fat at the same BMI, which is one of its known limitations.

How do I calculate BMI for a child?

Use the same formula, then look the result up on the CDC or WHO growth chart for the child’s age and sex. The percentile, not the raw number, gives the category.