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BMI Calculator

Calculate your Body Mass Index (BMI) in metric or imperial units. Get your BMI category, ideal weight range, and health risk assessment. Free, instant results.

Your BMI
24.2
Normal weight
Health Risk
Low health risk
Ideal Weight Range
53.5 kg, 72.0 kg

Understanding BMI

Body Mass Index (BMI) is a standardized measure of weight-to-height ratio, widely used by healthcare providers as a screening tool. While not perfect, BMI correlates with body fat and health risks for most people. Use it as one indicator alongside other health metrics.

How BMI Is Calculated

Metric formula

BMI = weight in kilograms divided by height in meters squared: kg / m². This calculator takes your height in centimeters and converts it to meters before applying the formula, then rounds the result to one decimal place.

Imperial formula

For pounds and feet/inches, the tool first converts weight to kilograms (multiplying by 0.453592) and height to meters (feet and inches combined, then multiplied by 0.0254), so the underlying calculation is always the same metric formula. This avoids the common imperial shortcut of multiplying by 703, though the two methods produce equivalent results.

Ideal weight range

The ideal weight range shown is the weight that would put you at the boundaries of the normal category, calculated as 18.5 × height² (minimum) and 24.9 × height² (maximum). It scales with your actual height, so taller and shorter people get different target ranges.

Worked Example

Metric: 170 cm, 65 kg

Height in meters is 1.70, so height squared is 2.89. BMI = 65 / 2.89 = 22.5, which falls in the normal range (18.5-24.9).

Imperial: 5'6", 150 lbs

150 lbs converts to 68.04 kg, and 5'6" (66 inches) converts to 1.6764 m, so height squared is 2.81. BMI = 68.04 / 2.81 = 24.2, still in the normal range but close to the overweight threshold.

A useful related figure is BMI Prime, the ratio of your BMI to the upper bound of the normal range (25): BMI Prime = BMI / 25. A BMI Prime under 0.74 is underweight, 0.74 to 1.0 is normal, and above 1.0 is overweight, so it tells you at a glance how far your BMI sits from the healthy-weight ceiling regardless of which unit system you used to get there.

BMI Categories Explained

Underweight: below 18.5

May indicate a nutritional deficiency or increased osteoporosis risk. Consider consulting a healthcare provider if this is unintentional.

Normal weight: 18.5 to 24.9

Associated with the lowest average health risk in most population studies. This is the range the "ideal weight" box targets.

Overweight: 25 to 29.9

Linked to increased risk of cardiovascular disease and type 2 diabetes. Small, sustained changes in diet and activity typically move BMI back toward normal.

Obese Class I: 30 to 34.9

High risk of cardiovascular disease and diabetes. Clinical guidelines generally recommend structured weight management at this stage.

Obese Class II: 35 to 39.9

Very high risk of related health problems. Medical supervision is commonly recommended for weight management planning.

Obese Class III: 40 and above

Extremely high risk category, sometimes called severe or morbid obesity in clinical literature. Consult a healthcare provider for a full assessment.

Common Use Cases

Routine health screening: Many annual checkups and insurance health assessments record BMI as a quick, low-cost screening figure alongside blood pressure and cholesterol.
Fitness and weight-management tracking: Pair this calculator with the Calorie Calculator to set a realistic target weight range and matching daily calorie goal.
Pre-appointment preparation: Knowing your BMI and category in advance can make conversations with a doctor or dietitian more focused and specific.
Population and research reference: BMI thresholds used here match the World Health Organization's standard adult categories, the same ones used in most public health studies.

Limitations to Keep in Mind

BMI only uses weight and height, so it cannot distinguish muscle mass from fat mass and does not account for where fat is stored on the body.

  • Athletes and highly muscular people may show as overweight or obese despite low body fat.
  • Older adults can lose muscle mass, so a "normal" BMI may still mask a high body fat percentage.
  • Fat distribution (waist circumference and waist-to-hip ratio) is a stronger predictor of cardiovascular risk than BMI alone.

For a deeper look at what a BMI number can and cannot tell you, read our guide on what BMI does and does not tell you about your health.

How this is calculated

Method: Body Mass Index, weight in kilograms divided by height in meters squared

BMI is your weight in kilograms divided by the square of your height in meters. With imperial units the same figure comes from weight in pounds divided by height in inches squared, multiplied by 703.

The result is classified using the standard World Health Organization ranges: below 18.5 is underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 is overweight, and 30 or above is obese. BMI is a population-level screening measure. It does not distinguish muscle from fat, so very muscular people can read as overweight despite low body fat.

Sources and references

This tool provides estimates for general information only, not professional advice. See our Disclaimer. Last reviewed: July 2026.

Frequently Asked Questions

Is body mass the same as body weight?▾
In everyday use people say body mass when they mean weight, and this calculator accepts weight in kilograms or pounds either way. Strictly they differ: mass is how much matter you are made of, weight is the force gravity applies to it, which is why an astronaut has the same mass and almost no weight. BMI itself is a ratio of weight to height squared, and it says nothing about what the weight is made of, so a muscular person can land in the overweight band while carrying very little fat.
What is BMI?▾
BMI (Body Mass Index) is a screening tool for weight categories. It calculates weight-to-height ratio. Formula: weight (kg) / height² (m). Categories: underweight (<18.5), normal (18.5-24.9), overweight (25-29.9), obese (30+).
Is BMI accurate?▾
BMI is a general screening tool, not a diagnostic. It doesn't distinguish muscle from fat. Athletes may have high BMI but low body fat. For most people, BMI correlates with body fat and health risks. Consult healthcare providers for complete assessment.
What is a healthy BMI?▾
Normal BMI range is 18.5-24.9. This range correlates with lowest health risks. BMI 25-29.9 is overweight (increased risk). BMI 30+ is obese (high risk). Below 18.5 is underweight (may indicate malnutrition).
How do I calculate BMI in imperial units?▾
For imperial: BMI = (weight in pounds / (height in inches)²) × 703. Example: 150 lbs, 5'6" (66 inches): BMI = (150 / 66²) × 703 = 24.2 (normal). This calculator handles both metric and imperial automatically.
What is my ideal weight?▾
Ideal weight is the range for normal BMI (18.5-24.9) at your height. For 5'6" (168 cm), ideal is 115-154 lbs (52-70 kg). This calculator shows your personalized ideal range based on current height.
Does age affect BMI?▾
BMI formula doesn't account for age. However, older adults may have higher healthy BMI due to muscle loss. Some organizations recommend slightly higher BMI targets for seniors. Consult healthcare providers for age-appropriate guidance.
What are the limitations of BMI?▾
BMI doesn't measure body fat directly. It may misclassify athletes (high muscle) or elderly (low muscle). It doesn't show fat distribution (belly fat is riskier). For complete health assessment, consider waist circumference, body fat percentage, and overall fitness.
What is BMI Prime?▾
BMI Prime is your BMI divided by 25, the upper limit of the normal range. It is a dimensionless ratio: below 0.74 is underweight, 0.74 to 1.0 is normal, above 1.0 is overweight, and above 1.2 is generally considered obese. It shows how close a result sits to the healthy-weight ceiling without needing to remember the category cutoffs.
Does BMI work the same way for children and teenagers?▾
No. This calculator uses the WHO adult categories, which apply to people 20 and older. For children and teens, BMI is plotted against age- and sex-specific growth charts and expressed as a percentile rather than a fixed number, because healthy body composition changes as kids grow. A pediatrician or a dedicated child BMI percentile tool is the right way to interpret BMI in that age group.

By Toolember · Updated September 2026