BMI Calculator

What It Does

Calculate your Body Mass Index from your height and weight. BMI is a widely-used screening tool that estimates body fat and categorizes results as underweight, normal weight, overweight, or obese using standard WHO thresholds. While useful for population-level health trends, BMI does not distinguish between muscle and fat mass, so athletes or muscular individuals may show a high BMI despite low body fat. It does not account for age, sex, bone density, or fat distribution either. Use BMI as a starting point for health awareness, not as a standalone medical diagnosis. For a complete picture, pair it with waist circumference, body fat percentage, and blood work.

Real-World Usage

Use BMI for general health screening at annual checkups — most doctors calculate it as part of a routine physical. Track changes over time: a BMI trending upward may signal gradual weight gain before it becomes a health concern. Fitness professionals use BMI as one of several metrics alongside body fat calipers and bioelectrical impedance. Life insurance companies often reference BMI ranges when setting premiums. Public health organizations use population BMI data to track obesity trends and allocate resources. For pregnant women, different BMI charts apply — standard BMI thresholds do not account for gestational weight gain. Athletes should interpret BMI cautiously and prefer body composition testing.

Local Context

WHO thresholds apply worldwide: below 18.5 is underweight, 18.5–24.9 is normal, 25–29.9 is overweight, and 30+ is obese. In the United States, about 42% of adults are obese (BMI 30+), making it one of the highest rates globally. The NHS in the UK uses the same thresholds but combines BMI with waist-to-height ratio for a more accurate assessment starting in 2025. Canada's clinical guidelines additionally recommend measuring waist circumference for anyone with a BMI over 25. While BMI is standardized globally, some Asian countries (Japan, South Korea, China) have adopted lower overweight thresholds (BMI 23+) because research shows higher metabolic risk at lower BMIs in these populations.

Health Risks by BMI Category

Every BMI band carries its own risk profile. Underweight (below 18.5) is linked to a weaker immune response, bone loss and fertility problems. The normal band (18.5–24.9) carries the lowest risk of type 2 diabetes, high blood pressure and cardiovascular disease. Overweight (25–29.9) raises the risk of type 2 diabetes, hypertension and joint strain. Obesity (30 and above) splits into Class I (30–34.9), Class II (35–39.9) and Class III (40+), and the risk of sleep apnoea, fatty liver disease, stroke and premature death climbs steeply with each class. These are associations across a population: an individual risk also depends on waist circumference, blood pressure, blood sugar and family history.

Where BMI Falls Short

BMI compares weight with height alone, so it cannot tell muscle from fat, where that fat sits, or how old the person is. A rugby player at 29 may carry more muscle and less fat than an office worker at 24. Older adults lose muscle and gain fat without their BMI moving, and athletes, pregnant women and children need different measures entirely. Body-fat percentage, waist circumference and the waist-to-height ratio all catch risk that BMI misses, and a waist-to-height above 0.5 flags abdominal fat at any BMI. Read the number as one screening signal, not as a verdict on health.

What BMI ranges indicate a healthy weight?

According to the WHO: below 18.5 is underweight, 18.5–24.9 is normal weight, 25–29.9 is overweight, and 30+ is obese.

Is BMI accurate for everyone?

BMI does not distinguish between muscle and fat, so very muscular individuals may be classified as overweight or obese despite having low body fat. It is one screening tool — not a diagnosis.

Should children use this BMI calculator?

No. This calculator is designed for adults aged 20 and over. Children and teens should use age- and sex-specific BMI percentiles, as their bodies are still developing.