Body Mass Index Calculator
Calculate your BMI, body fat percentage, ideal weight range, and waist-to-height ratio. Get comprehensive body composition analysis with multiple health
Medical disclaimer: This calculator is provided for educational and informational purposes only and does not constitute medical advice, diagnosis, or treatment. Results are general estimates and may not reflect your individual circumstances. Always consult a qualified healthcare professional before making decisions about your health.
Body Mass Index Calculator
Calculator
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Formula: BMI = weight (kg) / height (m)^2
Worked example — BMI = 22.5 (Normal Weight, low health risk)
Formula
BMI = weight (kg) / height (m)^2
BMI is calculated by dividing body weight in kilograms by the square of height in meters. WHO categories: Underweight < 18.5, Normal 18.5-24.9, Overweight 25-29.9, Obese I 30-34.9, Obese II 35-39.9, Obese III 40+. BMI Prime = BMI / 25 (values above 1.0 indicate overweight).
Worked Examples
Example 1: Normal Weight BMI Calculation
Problem:Calculate BMI for a 65 kg person who is 170 cm tall.
Solution:BMI = weight / height^2 BMI = 65 / (1.70)^2 BMI = 65 / 2.89 BMI = 22.5 Category: Normal Weight (18.5-24.9)
Result:BMI = 22.5 (Normal Weight, low health risk)
Example 2: Overweight BMI with Ideal Weight
Problem:A 90 kg person is 175 cm tall. Calculate BMI and how much weight to reach normal range.
Solution:BMI = 90 / (1.75)^2 = 90 / 3.0625 = 29.4 Category: Overweight (25-29.9) Ideal weight upper limit: 24.9 x (1.75)^2 = 24.9 x 3.0625 = 76.3 kg Weight to lose: 90 - 76.3 = 13.7 kg
Result:BMI = 29.4 (Overweight), need to lose 13.7 kg to reach normal range
Example 3: Normal-range BMI example
Problem:A person weighs 65 kg and is 170 cm tall.
Solution:BMI = 65 / 1.70^2 = 22.5. That falls in the normal-weight range and supports a healthy-weight estimate for this height.
Result:BMI: 22.5 | Category: Normal Weight
Frequently Asked Questions
What is BMI?
BMI, or Body Mass Index, compares body weight with height. It is a screening tool, not a direct body-fat measurement, so it works best alongside other context such as waist size, health markers, and training status.
What are the main limitations of BMI?
BMI does not separate muscle from fat, does not show where fat is stored, and can misclassify very muscular or older users. It is helpful for broad screening, but not perfect for every individual body type.
Why include waist-to-height ratio?
Waist-to-height ratio adds information about central fat distribution. That is useful because abdominal fat often correlates more strongly with health risk than body weight alone.
What is a practical healthy-weight range?
A BMI-based range of 18.5 to 24.9 is a common starting point, but the best target still depends on body composition, age, training background, and overall health markers.
How is waist-to-height ratio calculated?
Divide waist circumference by height in the same units, so a 34 inch waist on a 68 inch frame gives 0.5. Because both terms are lengths the ratio is unit-free and needs no conversion. The widely repeated rule of thumb is to keep the waist under half of height, which makes 0.5 an easy reference point. Like any single cut-off it is a screening heuristic rather than a clinical decision rule.
What is BMI Prime and how do you read it?
BMI Prime is your BMI divided by 25, the upper edge of the normal range, so it expresses the result as a proportion of that limit. A BMI of 22.5 gives a Prime of 0.90, and a BMI of 29.4 gives about 1.18, meaning roughly 18 percent above the threshold. Because it is dimensionless it makes comparisons across people easy to read at a glance, and any value above 1.0 signals the overweight range without needing the category table.
What is the difference between this index and body-fat percentage?
The index is a ratio of mass to stature and makes no attempt to say what that mass consists of. Body-fat percentage states what share of total mass is adipose tissue, which requires an actual measurement method such as skinfolds, bioelectrical impedance or a scan. Two people at the same index can differ by ten or more percentage points of body fat, so the two numbers answer different questions and are best read together.
What is the Ponderal Index and when is it used instead?
The Ponderal Index divides mass by height cubed rather than squared, which follows pure geometric scaling. It is used mainly in neonatal assessment, where body proportions differ sharply from adults, and occasionally for adults at the extremes of stature, where the squared form leaves a residual dependence on height. For most adults the two rank people similarly, so the squared index remains the conventional screening measure and the cubed form is a specialist alternative rather than a replacement.
Why do children use BMI-for-age percentiles instead of fixed cut-offs?
Body composition changes continuously through growth, so a value that is unremarkable at fifteen may be high at seven. Paediatric assessment therefore plots the same mass-over-height-squared value on a sex-specific growth reference and reports where it falls as a percentile for that age. Fixed adult categories such as the 25 overweight threshold have no meaning in childhood, and any concern about a child should be taken to a clinician who can read the growth trajectory rather than one point.
Why do some health authorities apply lower action points for certain populations?
At the same value, average body fat and its distribution differ between ancestry groups, and some populations show more visceral fat and metabolic risk at a lower reading than the original European-derived cut-offs assume. Several national bodies therefore set lower action points for screening in those groups. The mechanism is a shift in the value-to-body-fat relationship, not a different formula, and the applicable thresholds should be taken from current local public health guidance.
References
Reviewed for accuracy by Rahul Singh, Health & Wellness Specialist · Editorial policy
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