Free BMI Calculator - Check Your Body Mass Index

Based on peer-reviewed medical formulas. See our formulas & sources →

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Normal weight

Why Do We Even Use BMI?

A healthy BMI falls between 18.5 and 24.9. The Body Mass Index is calculated by dividing your weight in kilograms by your height in meters squared. A BMI below 18.5 indicates underweight, 25–29.9 indicates overweight, and 30 or above indicates obesity. BMI was developed by Adolphe Quetelet in the 1830s and is used worldwide by the World Health Organization as a screening tool for weight-related health risks.

Body Mass Index (BMI) is a simple screening tool that uses your weight and height to estimate body fat. Developed in the 1830s by Belgian mathematician Adolphe Quetelet, it is now widely used by the World Health Organization to assess population health. While BMI is not a diagnostic tool — it doesn't directly measure body fat or account for muscle mass — it serves as a useful starting point for identifying potential weight-related health risks.

Formula: BMI = weight (kg) ÷ height² (m²). Source: WHO, "Body mass index — BMI," 2000. Full methodology →

Where Your Number Sits on the Scale

Below 18.5

Underweight

18.5 – 24.9

Normal

25 – 29.9

Overweight

30 and above

Obese

These categories are defined by the World Health Organization. A BMI in the normal range is associated with the lowest risk of weight-related health conditions. However, BMI is just one measure — your overall health depends on many factors including diet, physical activity, and family history.

What BMI Misses About Muscle

While BMI is a helpful screening tool, it has important limitations. It does not distinguish between muscle and fat, so muscular individuals may be misclassified as overweight. It also doesn't account for where fat is stored in the body, which matters — abdominal fat poses higher health risks than fat stored elsewhere. Additionally, BMI categories were developed primarily from Western populations and may not be equally accurate for people of different ethnicities, older adults, or pregnant women.

Building Habits That Shift Your Number

Eat a balanced diet rich in fruits, vegetables, lean proteins, and whole grains while limiting processed foods and added sugars.
Stay physically active — aim for at least 150 minutes of moderate aerobic activity or 75 minutes of vigorous activity per week.
Get enough sleep — poor sleep is linked to weight gain and higher BMI. Adults should aim for 7–9 hours per night.
Stay hydrated — drinking enough water supports metabolism and can help control appetite. Use our Water Intake Calculator to find your needs.

BMI by Population: What the Numbers Miss

BMI's standard cutoffs (18.5–24.9 = normal) were developed from data on largely white, European-descent populations. Research over the past two decades has shown that different groups face health risks at different BMI thresholds:

Asian populations: The WHO recommends a lower overweight threshold of BMI 23 for Asian adults. Studies show that Chinese, Japanese, and South Asian populations face significantly higher risks of type 2 diabetes and cardiovascular disease at BMI levels that would be considered "normal" by Western standards. An Asian adult at BMI 25 may carry the same metabolic risk as a European-descent adult at BMI 28–30.
Athletes and muscular individuals: BMI does not distinguish between muscle and fat. A bodybuilder at 5'10" and 210 lbs may have a BMI of 30 (obese category) while carrying only 8% body fat. For this group, body fat percentage and waist circumference are far better indicators of health than BMI alone.
Older adults (65+): Slightly higher BMI (25–27) may actually be protective in older adults. Research published in the Journal of the American Geriatrics Society found that older adults in the "overweight" category had lower mortality rates than those in the "normal" category, likely because some extra body mass provides a buffer against muscle loss and frailty.
Pregnant women: BMI is not designed for use during pregnancy. Weight gain during pregnancy is expected and healthy; prenatal weight gain guidelines use a separate framework based on pre-pregnancy BMI.

BMI is a useful population-level screening tool, but it should never be the sole basis for individual health decisions. Pair it with waist circumference, body composition measurements, and blood markers for a complete picture.

Frequently Asked Questions

What is a good BMI?

A BMI between 18.5 and 24.9 is considered normal and healthy. This range is associated with the lowest risk of weight-related health issues. Below 18.5 is underweight, 25-29.9 is overweight, and 30+ is obese.

How is BMI calculated?

BMI = weight (kg) ÷ height² (m²). For example, if you weigh 70 kg and are 1.75 m tall, your BMI is 70 ÷ (1.75 × 1.75) = 22.9, which is in the normal range.

Is BMI accurate for athletes?

BMI may overestimate body fat in athletes because it doesn't distinguish between muscle and fat. Muscular individuals often have a high BMI despite having very low body fat percentages.

What BMI is considered obese?

A BMI of 30 or higher is classified as obese. This is further divided into Class 1 (30-34.9), Class 2 (35-39.9), and Class 3 (40+, also called severe obesity).

Can BMI be wrong?

BMI doesn't distinguish between muscle and fat, doesn't account for where fat is stored, and may not be accurate for different ethnicities, older adults, pregnant women, or very tall or short individuals.

What is the healthy BMI range?

The WHO defines normal weight as BMI 18.5-24.9. However, some research suggests that for Asian populations, a lower range of 18.5-23 may be more appropriate due to differences in body composition.

Does waist size matter more than BMI?

Waist circumference is often a better predictor of health risk than BMI because it measures abdominal fat, which is more metabolically dangerous than fat stored elsewhere. A waist circumference over 102 cm (40 inches) for men or 88 cm (35 inches) for women indicates elevated risk, regardless of BMI.

Is BMI different for children and teenagers?

Yes. Pediatric BMI is age- and sex-specific and is expressed as a percentile relative to other children of the same age and sex, not as a fixed number range. A BMI between the 5th and 85th percentile is considered normal for children. Your pediatrician can interpret your child's BMI percentile chart.

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