What Is a Good BMI? Healthy Ranges, How to Calculate It, and What BMI Misses

For most adults, a BMI between 18.5 and 24.9 is classified as the "normal" or healthy-weight range by the World Health Organization. Below 18.5 is classified as underweight, 25 to 29.9 as overweight, and 30 or above as obese. These cut-offs are population-level screening thresholds, not a diagnosis. A single BMI number cannot tell you whether you are healthy.

BMI (Body Mass Index) is simply your weight divided by the square of your height. It takes seconds to compute by hand, or you can use the BMI calculator to get your number and category instantly. This guide covers the standard categories, the exact formulas for metric and imperial units with worked examples, and the well-documented situations where BMI gives a misleading picture.

The WHO BMI Categories

BMI (kg/m²)	Category
Below 18.5	Underweight
18.5 to 24.9	Normal weight
25.0 to 29.9	Overweight
30.0 and above	Obese

The WHO further divides obesity into class I (30 to 34.9), class II (35 to 39.9), and class III (40+). These ranges were derived from population studies linking BMI to health outcomes, and they apply to adults aged 20 and over. Children and teenagers use age- and sex-specific percentile charts instead, because healthy body composition changes substantially during growth.

How to Calculate BMI

Metric formula

BMI = weight (kg) ÷ height (m)²

Worked example: A person who weighs 70 kg and is 1.75 m tall.

1. Square the height: 1.75 × 1.75 = 3.0625
2. Divide weight by that: 70 ÷ 3.0625 ≈ 22.9

A BMI of 22.9 falls in the normal range (18.5 to 24.9).

Imperial formula

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

The factor 703 converts pounds and inches into the same kg/m² scale.

Worked example: A person who weighs 160 lb and is 5'8" (68 inches) tall.

1. Square the height: 68 × 68 = 4,624
2. Multiply weight by 703: 160 × 703 = 112,480
3. Divide: 112,480 ÷ 4,624 ≈ 24.3

A BMI of 24.3 is also in the normal range, near its upper end.

If you'd rather skip the arithmetic, the BMI calculator accepts both unit systems and shows where you land in the categories.

What BMI Doesn't Tell You

BMI is popular because it's cheap and fast, but it measures mass, not composition. Several well-documented limitations are worth knowing.

Muscle vs. fat

Muscle is denser than fat, so muscular people (athletes, weightlifters, people with physically demanding jobs) can register as "overweight" or even "obese" while carrying little body fat. BMI cannot distinguish a rugby player from someone of the same height and weight with much higher body fat.

Age

Older adults tend to lose muscle mass and may gain fat without much change in weight, so the same BMI can correspond to a higher body-fat percentage at 70 than at 30. Some research also suggests slightly higher BMI ranges are associated with better outcomes in older adults, though the evidence is mixed.

Ethnicity

The standard cut-offs were derived largely from European populations. The WHO has noted that people of Asian descent may face elevated metabolic risk at lower BMI values, and some health bodies use lower action thresholds (for example, 23 instead of 25) for these populations. Conversely, the standard thresholds may overstate risk for some other groups.

Where fat is stored

BMI ignores fat distribution. Abdominal (visceral) fat carries different health implications than fat stored elsewhere, which is why measurements like waist circumference are often used alongside BMI.

Disclaimer: This article is for general informational purposes only and is not medical advice. BMI is a screening tool, not a diagnostic one. For an assessment of your individual health, consult a qualified healthcare professional.

Beyond BMI: Related Numbers Worth Knowing

If you're using BMI as a starting point for health or fitness goals, a few companion figures add useful context. Your BMR (basal metabolic rate) estimates the calories your body burns at rest, which feeds directly into the calorie calculator for estimating daily energy needs at different activity levels. And if you're curious how formula-based weight estimates compare across methods, the ideal weight calculator shows results from several published equations side by side.

FAQ

What is a good BMI for adults?

A BMI between 18.5 and 24.9 is classified as normal weight by the WHO for adults aged 20 and over. This is a population-level screening range, not an individual health verdict.

Is a BMI of 26 bad?

A BMI of 26 falls in the overweight category (25 to 29.9), but BMI alone can't determine health. Muscle mass, fat distribution, age, and other factors all matter. A muscular person at 26 may be perfectly lean. If you're concerned, discuss it with a healthcare professional.

Is BMI calculated differently for men and women?

No. The formula and the adult categories are identical for both sexes. However, women typically carry a higher body-fat percentage than men at the same BMI, which is one of the metric's known blind spots.

Why is the imperial formula multiplied by 703?

BMI is defined in kilograms and meters. The constant 703 converts pounds-per-square-inch into kg/m² (since 1 kg ≈ 2.2046 lb and 1 m ≈ 39.37 in, the conversion factor works out to approximately 703).