What is a healthy BMI for adults?

The World Health Organization classifies adult BMI from 18.5 to 24.9 as the healthy range. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or higher is obese. These thresholds apply to non-pregnant adults aged 18 and older across most populations, though optimal cut-offs differ slightly for some Asian populations (the WHO public-health action point begins at 23 in those groups).

Is BMI a reliable measure of health?

BMI is a screening tool, not a diagnosis. It correlates well with body fat at the population level, but at the individual level it can misclassify muscular athletes as overweight and people with low muscle mass as healthy when they aren't. Use BMI alongside waist circumference, body fat percentage, and routine bloodwork for a fuller picture.

How does BMI differ for men and women?

The BMI formula and category thresholds are the same for both sexes. Body composition does differ — at the same BMI, women on average carry slightly more body fat than men — but the WHO classification doesn't adjust for this.

Why is my BMI different on different sites?

Most discrepancies come from rounding (your value rounded to one decimal vs the underlying float), unit conversions (some sites use 2.205 vs 2.20462 for lbs-to-kg), or inputs (you may have entered slightly different numbers). The math is identical: weight in kg divided by height in meters squared.

Should I use BMI during pregnancy?

No. BMI is not appropriate during pregnancy because weight gain reflects normal physiological changes. Your obstetric provider tracks pregnancy weight gain against guidelines based on your pre-pregnancy BMI.

Does BMI work for children?

Adult BMI cut-offs do not apply to children. For people under 20, growth charts that adjust for age and sex (CDC for the US, WHO for international) are used to convert BMI into a percentile.

What's the difference between BMI and body fat percentage?

BMI is a height-weight ratio with no information about composition. Body fat percentage measures actual fat mass relative to total body mass. A 25-year-old strength athlete might have a BMI of 27 (officially overweight) but 12% body fat (lean). Body fat percentage is more informative if you have access to a measurement method like DEXA, calipers, or US Navy circumference tape.

BMI Calculator — WHO classification

BMI Range	Category	Health Risk
< 18.5	Underweight	Increased risk of malnutrition
18.5 – 24.9	Normal weight	Lowest health risk
25.0 – 29.9	Overweight	Moderate health risk
30.0 – 34.9	Obese Class I	High health risk
35.0 – 39.9	Obese Class II	Very high health risk
≥ 40.0	Obese Class III	Extremely high risk

What BMI tells you (and what it doesn't)

Body Mass Index is a simple ratio of weight to height squared. It was never designed as a personal health metric — Adolphe Quetelet developed it in the 1830s as a population-level statistical tool. But because it's cheap to compute and correlates reasonably well with body fat at the population level, the World Health Organization adopted it in the 1990s as a global screening tool.

For most adults aged 18 to 65, a BMI between 18.5 and 24.9 is associated with the lowest all-cause mortality. Below 18.5, mortality risk rises modestly, mostly from underweight-associated conditions like osteoporosis and immune dysfunction. Above 25, cardiovascular and metabolic risks rise, with the slope steepening sharply above 30.

Where BMI gets it wrong

BMI ignores composition. A muscular athlete at 6 feet and 220 lbs has a BMI of 29.8 (officially overweight), but their body fat might be 11%. Conversely, a sedentary person at 5'7" and 145 lbs has a BMI of 22.7 (officially healthy) but might have 33% body fat — a phenomenon called normal-weight obesity that carries cardiovascular risks similar to overt obesity.

BMI also performs less well in some groups: tall people skew high, short people skew low, and the standard cut-offs were developed in mostly European populations. The WHO publishes additional public-health action points for some Asian populations (overweight starting at BMI 23), reflecting different body-composition averages.

When to act on a BMI result

If your BMI is in the overweight or obese range, the most useful next step is a conversation with your physician about your cardiometabolic risk — blood pressure, lipids, fasting glucose, and waist circumference. Those numbers tell you more about your actual health than BMI alone. If your BMI is in the underweight range, especially if it's recent, ask for evaluation: thyroid, absorption, mood, and disordered eating are all worth ruling out.

Sources: WHO Body Mass Index classification (updated 2024); CDC Adult BMI; Flegal KM, et al. Association of all-cause mortality with overweight and obesity using standard body mass index categories: a systematic review and meta-analysis. JAMA. 2013;309(1):71-82.

BMI Calculator

What BMI tells you (and what it doesn't)

Where BMI gets it wrong

When to act on a BMI result

Frequently asked questions