Adult BMI Calculator
Enter units, height, weight, and an optional waist measurement on the left, then compare BMI under both adult references on the right.
Input
On bare skin, measure midway between the bottom ribs and top of the hips after breathing out naturally.
Results update automatically when the required fields are complete.
Comparison result
Adult reference comparison
KSSO Korean adult reference
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- BMI reference weight range
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- Distance to reference range
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WHO/CDC adult reference (CDC age 20+)
Category—
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- BMI reference weight range
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- Distance to reference range
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Primary BMI scale
WHO/CDC adult reference (CDC age 20+)
- NICE waist-to-height ratio (optional)
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- Formula
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- Input summary
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BMI and waist-to-height ratio are adult screening measures. NICE ratio bands apply to adults with BMI below 35 and are not a diagnosis or prescription.
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How to read adult BMI against two references
BMI divides weight in kilograms by height in meters squared. This calculator shows the primary WHO/CDC adult scale beside the explicitly Korean KSSO adult reference.
The classifications are screening context, not a diagnosis. Body composition, pregnancy, growth, high muscle mass, and medical treatment can change what the number means.
Formula and unit conversions
Metric and imperial entries convert to kilograms and meters before the same formula is applied. Display rounds to one decimal; classification uses the raw value.
- BMI = kg ÷ m²
- 1 in = 2.54 cm
- 1 lb = 0.45359237 kg
Required and optional inputs
Height and weight are required. Waist circumference is optional. Once required values are valid, both reference results update automatically.
KSSO Korean adult reference
KSSO uses 18.5, 23, 25, 30, and 35 to separate underweight through obesity class III for Korean adults.
- 18.5–22.9: normal weight
- 23.0–24.9: pre-obesity
- 25.0 or above: obesity class I or higher
WHO/CDC adult reference
WHO describes adults from age 18, while the CDC’s full adult categories apply from age 20. The comparison uses 18.5–24.9 for healthy weight and 25.0–29.9 for overweight.
- 18.5–24.9: healthy weight
- 25.0–29.9: overweight
- 30.0 or above: obesity classes
Why one BMI can receive two labels
A BMI of 24.2 is pre-obesity under KSSO Korean cutoffs and healthy weight under WHO/CDC. The formula is identical; population evidence and screening interpretation differ.
A reference weight range is not a prescription
Each card applies its normal BMI interval to the entered height. It is arithmetic context, not an individualized target or treatment recommendation.
How waist-to-height ratio complements BMI
Divide waist by height in the same unit. NICE advises using it alongside BMI for adults with BMI below 35.
- Below 0.4: outside the NICE bands
- 0.4–0.49: healthy central adiposity
- 0.5–0.59: increased
- 0.6 or above: high
Boundaries and display rounding
Cut points at 18.5, 23, 25, 30, 35, and 40 are applied to unrounded BMI. Near a boundary, do not reclassify from the one-decimal display alone.
When adult BMI is not enough
Children and teens, pregnancy, edema, rapid weight change, very muscular athletes, eating-disorder care, and chronic disease management require other assessment.
Worked example: 170 cm and 70 kg
BMI is about 24.2. The two cards make the KSSO pre-obesity and WHO/CDC healthy-weight interpretations visible together.
What to check after the result
The copy summary includes BMI and both classifications. Body-fat, basal-metabolic-rate, and protein tools answer different questions and do not replace clinical advice.
Common questions about adult BMI
What does this BMI calculator show?
It calculates adult BMI from height and weight and shows classifications under both the primary WHO/CDC adult reference and the KSSO Korean adult reference. It also shows reference weight ranges and an optional waist-to-height ratio, not a diagnosis.
Why do WHO/CDC and KSSO Korean ranges differ?
KSSO reflects Korean adult risk evidence and starts pre-obesity at BMI 23 and class I obesity at 25. WHO/CDC comparisons start overweight at 25 and obesity at 30, so one BMI can have different labels.
Why are BMI 23 and 25 different cut points?
BMI 23 starts pre-obesity in the KSSO Korean reference. BMI 25 starts KSSO class I obesity and WHO/CDC overweight. The formula is unchanged; the population evidence and public-health reference differ.
What is the BMI for 170 cm and 70 kg?
The BMI is about 24.2. Using the unrounded value, it is pre-obesity under the KSSO Korean reference and healthy weight under WHO/CDC.
Is the normal-weight range a target weight?
No. It applies each reference’s normal BMI interval to the entered height. A personal target also needs body composition, medical history, habits, and clinical advice.
Can children, pregnant people, or athletes use this result as-is?
This calculator is for general adult screening, and the CDC’s full adult categories apply from age 20. Growth, pregnancy, very high muscle mass, edema, rapid weight change, and ongoing treatment require other assessment.
Can BMI be normal while abdominal fat is higher?
Yes. BMI does not directly measure central adiposity. For adults with BMI below 35, NICE uses waist-to-height boundaries of 0.4, 0.5, and 0.6 alongside BMI.
Is waist circumference required?
No. Height and weight are enough to calculate BMI and both classifications. Waist is optional and only adds the waist-to-height ratio result.
Adult cut points and waist-to-height bands are described from the official sources below. Sources reviewed: 2026-08-20.