BMI Calculator — Adult & Child BMI with Age-Specific Norms (2026)

What Is BMI and Why Does It Matter?

Body Mass Index (BMI) is a simple ratio of weight to height used worldwide to screen for weight-related health risks. The formula is: weight in kilograms divided by height in meters squared (kg/m²). Despite its simplicity, BMI correlates reasonably well with more precise measures of body fat and is strongly associated with risks for type 2 diabetes, cardiovascular disease, and other chronic conditions at the population level.

Adult vs. Child BMI Interpretation

For adults 20 and older, WHO cutoffs are fixed: underweight below 18.5, normal 18.5–24.9, overweight 25–29.9, and obese at 30 or above. For children and adolescents aged 2–19, BMI is always interpreted as a percentile relative to age and sex using CDC growth charts, because body fatness changes substantially with age and differs between boys and girls.

Limitations of BMI

BMI is a screening tool, not a diagnostic measure. It cannot distinguish between muscle mass and fat mass, which means a highly muscular athlete may register as "overweight" despite having low body fat. Conversely, an older adult with low muscle mass may have a "normal" BMI while carrying excess visceral fat. Ethnicity also matters: research shows that South Asian and East Asian populations face elevated metabolic risks at lower BMI thresholds than Western populations, leading some health authorities to use adjusted cutoffs.

How to Move Toward a Healthy BMI

Understand your calorie balance. Weight is primarily driven by the difference between calories consumed and calories burned. Tools like Caloria can help you track both sides of that equation without guesswork.
Prioritize protein. Higher protein intake supports muscle retention during weight loss and increases satiety, making it easier to maintain a calorie deficit. Aim for 1.6–2.2 g per kg of body weight if you are active.
Add resistance training. Building lean muscle raises your resting metabolic rate and improves body composition even if the number on the scale changes slowly. Muscle gain can push BMI up temporarily while improving overall health markers.
Set realistic, gradual goals. A rate of 0.5–1 kg (1–2 lbs) per week of weight loss is sustainable for most people. Rapid weight loss increases the risk of muscle loss and nutritional deficiencies. Small, consistent changes compound significantly over months.

Frequently Asked Questions

What is BMI and how is it calculated?

BMI (Body Mass Index) is a screening measure calculated by dividing your weight in kilograms by your height in meters squared (kg/m²). It was developed in the 1830s by Adolphe Quetelet as a population-level tool. While imperfect for individuals, it remains widely used because it requires no special equipment and correlates well with body fat at a population level.

What is a healthy BMI for adults?

For adults 20 and older, the WHO defines a healthy BMI as 18.5–24.9. A BMI below 18.5 is considered underweight; 25–29.9 is overweight; and 30 or above is obese. These thresholds are the same for all adult ages and both sexes, though health risks at any given BMI can vary by ethnicity and body composition.

How is BMI interpreted differently for children?

For children and teens aged 2–19, a single BMI number is not enough — it must be interpreted relative to age and sex using CDC growth charts. The CDC defines healthy weight as BMI-for-age at the 5th–84th percentile, overweight as 85th–94th percentile, and obese as at or above the 95th percentile. Below the 5th percentile is considered underweight.

Can BMI be misleading?

Yes. BMI does not distinguish between muscle and fat, so muscular athletes can appear overweight by BMI standards despite having very low body fat. Conversely, older adults can have a "normal" BMI while carrying too much fat relative to muscle (sarcopenic obesity). BMI is best used as a starting screening tool alongside waist circumference, blood tests, and clinical judgment.

What should I do if my BMI is outside the healthy range?

Consult a healthcare provider for personalized guidance. A BMI outside the normal range is a signal worth discussing, not a diagnosis. Lifestyle factors — particularly nutrition and physical activity — are the first line of intervention. Tracking your food intake and calories with an app like Caloria can help you understand your current habits and make evidence-based adjustments.

Is this BMI calculator free?

Yes, completely free with no sign-up required. Switch between metric and imperial, enter your age to get child or adult interpretation, and results update instantly. Use it as often as you need.