What Does Your BMI Tell You, and What Does It Miss?

Body mass index (BMI) is your weight in kilograms divided by the square of your height in metres. It’s quick, needs only a scale and a tape, and across large populations it tracks the health risks of carrying too much weight. For one person, it’s a rough first look: it can’t see how much of your weight is fat, or where that fat sits.

This article is general information, not medical advice.

The Categories

The World Health Organization (WHO) uses these cut-offs for adults:

BMI (kg/m²)	Category
Below 18.5	Underweight
18.5 to 24.9	Normal weight
25.0 to 29.9	Overweight (pre-obesity)
30.0 to 34.9	Obesity class I
35.0 to 39.9	Obesity class II
40.0 and above	Obesity class III

Risk doesn’t jump at a line; it rises gradually. In a meta-analysis of 239 studies and over 10 million adults, the Global BMI Mortality Collaboration found the lowest risk of death among never-smokers without chronic disease at a BMI of about 20 to 25, with risk rising steadily above 25.

Lower Cut-Offs for Many Populations

People of different ancestries develop type 2 diabetes and heart disease at different BMIs. A 2004 WHO expert consultation found that many Asian populations face higher risk at lower BMIs and proposed action points of 23 and 27.5. NICE, England’s guideline body, now advises using 23 for overweight and 27.5 for obesity for people of South Asian, Chinese, other Asian, Middle Eastern, Black African, or African-Caribbean family background.

What BMI Can’t See

• Fat versus muscle. A muscular athlete can have a BMI of 28 with little body fat; an older adult who has lost muscle can have a “normal” BMI with a lot of it.
• Where fat sits. Fat around the organs in the abdomen carries more risk than fat on the hips and thighs, and BMI doesn’t distinguish them. Waist-based measures do; see Why Waist-to-Height Ratio Is a Better Quick Check Than BMI.
• Health itself. A 2025 commission in The Lancet Diabetes & Endocrinology argued that BMI alone shouldn’t be used to diagnose obesity: excess fat should be confirmed with a waist measure or a direct body fat measurement, and obesity judged by its effects on the body’s organs and on daily life.

How to Read Your Own BMI

• Use the trend. Height barely changes in adulthood, so your BMI moves with your weight. A weekly average smooths out day-to-day water changes; see Why Does Your Weight Change From Day to Day?
• Pair it with your waist. BMI plus waist circumference says much more than either alone.
• Measure height properly once. Stand barefoot against a wall, heels together, looking straight ahead. An error of 2 cm in height shifts BMI by about 0.5.

Following It in Kaizora

In Kaizora, Body Mass Index is a derived metric on the Body page: you log Weight and Height (or import them, such as from a Withings scale or Apple Health), and BMI is calculated from them at each new weight, with your last height carried forward. Editing a weight changes its BMI at once. The WHO’s categories come as default reference ranges, so a value outside 18.5 to 24.9 is flagged with its category; you can hide a default and add your own range, such as NICE’s lower cut-offs if they apply to you. See Reference Ranges.

The Short Version

• BMI is weight divided by height squared; 18.5 to 24.9 is the WHO’s normal range.
• Many populations face higher risk at lower BMIs, from 23 and 27.5.
• It can’t tell fat from muscle, or where fat sits, so pair it with your waist.
• Follow its trend rather than any single value.

Sources

• World Health Organization, Obesity and overweight (fact sheet).
• WHO Expert Consultation, “Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies,” The Lancet 363 (2004): 157–163. doi:10.1016/S0140-6736(03)15268-3
• National Institute for Health and Care Excellence, Overweight and obesity management (NG246).
• Global BMI Mortality Collaboration, “Body-mass index and all-cause mortality: individual-participant-data meta-analysis of 239 prospective studies in four continents,” The Lancet 388 (2016): 776–786. doi:10.1016/S0140-6736(16)30175-1
• Rubino F et al., “Definition and diagnostic criteria of clinical obesity,” The Lancet Diabetes & Endocrinology 13, no. 3 (2025): 221–262. doi:10.1016/S2213-8587(24)00316-4