Body Mass Index (BMI) Calculator

Estimated BMI

{{ bmiCategory }} {{ bmiScaleLabel }} {{ healthyWeightSummary }} BMI' {{ formatNumber(bmiPrime, 2) }} WHtR {{ formatNumber(waistHeightRatio, 2) }} PI {{ formatNumber(ponderalIndex, 1) }}

Weight:

Examples: 70 kg or 154 lb; unit changes convert the current value.

Height:

Examples: 170 cm, 1.70 m, 67 in, or 5 ft 7 in.

BMI classification:

Choose WHO, Asia-Pacific, or NIH/CDC adult bands.

Age (years):

Optional; enter 20 or older for adult screening notes, or leave blank.

Sex:

Optional; choose female or male only if you want the waist-threshold comparison.

Waist circumference:

Optional; measure midway between lowest rib and hip after a normal exhale.

Target BMI:

Drag within the selected healthy BMI span; current target updates beside the slider.

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Metric	Value	Copy
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Guidance area	Status	Detail	Copy
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BMI range	Category	Weight at your height	Copy
{{ row.range }}	{{ row.category }} Current Target	{{ row.weightRange }}

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Body mass index turns two ordinary measurements, height and weight, into one adult screening number. It does not measure body fat directly. Its value is that it makes weight relative to height, so two adults with the same body weight are not treated the same when one is much taller than the other.

BMI is widely used because it is quick, inexpensive, and repeatable. Clinics can track it over time, public-health teams can compare broad population patterns, and individuals can use it as a starting point before looking at more personal health information. The tradeoff is bluntness. BMI cannot see whether weight comes from muscle, bone, fat, pregnancy, fluid retention, or a short-term medical change.

BMI-related measures and what each can and cannot show
Measure	What it adds	What it can miss
BMI	Weight adjusted for height squared.	Body composition, fat distribution, pregnancy, edema, and individual medical history.
BMI Prime	BMI divided by 25, giving a quick distance from the common adult overweight cutoff.	Population-specific cutoffs and personal risk factors.
Waist-to-height ratio	Central-adiposity context when waist size is known.	Full cardiometabolic risk, body composition, and tape-placement error.
Target BMI	An arithmetic way to estimate the weight linked to a chosen BMI.	Whether that target is healthy or realistic for a specific person.

Height and weight feeding a BMI formula and adult category band — BMI is a ratio first and a category second. The number stays the same when a different adult standard is selected.

Adult BMI categories are conventions, not diagnoses. WHO-style and NIH/CDC adult bands use familiar cutoffs at 18.5, 25, 30, 35, and 40. Asia-Pacific action bands use lower cutoffs because some populations show cardiometabolic risk at lower BMI. The formula does not change between standards; the label and healthy-weight span attached to the same BMI can change.

Waist measurement adds a different clue because fat location matters. A person can sit in a healthy BMI band and still have elevated central adiposity, while a muscular person can sit above a BMI cutoff without the same body-fat profile as another person at that number. Waist size, blood pressure, glucose, cholesterol, symptoms, medicines, physical function, and clinical history all affect interpretation.

Children and teens need BMI-for-age percentiles because growth changes the relationship between height, weight, age, and sex. Adult BMI bands are meant for people 20 and older, and personal health decisions should be made with broader context than the BMI category alone.

How to Use This Tool:

Start with accurate height and weight, then add optional context only when it improves interpretation.

Enter Weight and choose kg or lb. If the number was copied from a scale, confirm that the selected unit matches the scale display.
Enter Height in cm, m, in, or ft/in. Height is squared in the BMI formula, so a height-unit mistake can move the result far across the category bands.
Choose BMI classification. WHO, Asia-Pacific, and NIH/CDC use the same BMI formula but different adult category ranges.
Open Advanced for extra context. Age adds an under-20 warning, Waist circumference adds waist-to-height ratio, and Sex enables the common female or male waist-threshold note.
Adjust Target BMI only as an arithmetic comparison. The slider stays within the selected standard's healthy BMI span and updates the matching target weight.
Read the summary before the tables. Warnings call out under-20 age, unusual adult height, weight, or waist values, BMI 35 or higher with waist entered, and missing sex when a sex-specific waist note would otherwise appear.
Use BMI Metrics for the main numbers, Guidance & Risk for the plain-language readout, BMI Categories for weight ranges at your height, and the chart tabs when a visual comparison is easier to review.

Interpreting Results:

The headline BMI is the main screening number. Read it with the selected classification label, and treat a value near a cutoff as fragile because small height or weight corrections can change the category. Changing from WHO or NIH/CDC to Asia-Pacific changes the band labels and healthy-weight span, not the BMI itself.

BMI Prime shows distance from the common adult BMI 25 cutoff. A value below 1.000 is below that cutoff, and a value above 1.000 is above it. Ponderal index uses height cubed instead of height squared, which can be useful for comparison across larger height differences, but it is still based only on height and weight.

Waist-to-height ratio adds central-adiposity context when waist size is entered. The tool uses less than 0.50 as lower waist risk, 0.50 to less than 0.60 as elevated, and 0.60 or higher as high. If BMI is 35 or higher, a warning explains that waist-to-height ratio adds less screening value because BMI is already in a high adult category.

Healthy weight span and target weight are arithmetic estimates from the selected BMI band. They can help compare numbers, but they are not a treatment plan. Use broader clinical advice when BMI conflicts with waist measures, athletic build, pregnancy status, illness, rapid weight change, or a clinician's recommendation.

Technical Details:

Body mass index normalizes weight by height squared. Squaring height gives a simple adult size adjustment, but it also amplifies height-entry errors. A weight entry can be plausible while a height unit is wrong, and the resulting BMI will still be mathematically consistent but clinically useless.

All calculations convert weight to kilograms and height to meters before deriving BMI, BMI Prime, ponderal index, healthy-weight range, and target weight. Pounds use 1 kg = 2.2046226218 lb, inches use 1 in = 0.0254 m, and centimeters divide by 100. Displayed values are rounded for readability, so borderline results should be checked against the original measurements.

Formula Core:

The main equations use metric units after conversion.

BMI = \frac{Weight (kg)}{{Height (m)}^{2}}

BMIPrime = \frac{BMI}{25}

PonderalIndex = \frac{Weight (kg)}{{Height (m)}^{3}}

WaistToHeightRatio = \frac{Waist (cm)}{Height (cm)}

TargetWeight = TargetBMI \times {Height (m)}^{2}

A 72 kg adult at 1.75 m has BMI 72 / 1.75² = 23.51, displayed as 23.5. BMI Prime is 23.51 / 25 = 0.940. A target BMI of 22.0 at the same height corresponds to 22.0 x 1.75² = 67.4 kg.

Adult Category Rules:

Category checks include the lower bound and exclude the upper bound. The final category in each preset has no upper limit.

Adult BMI category thresholds used by each classification preset
Preset	Band	Lower BMI	Upper BMI
WHO	Severely Underweight	≥ 0	< 16
WHO	Underweight	≥ 16	< 18.5
WHO	Normal	≥ 18.5	< 25
WHO	Overweight	≥ 25	< 30
WHO	Obese I	≥ 30	< 35
WHO	Obese II	≥ 35	< 40
WHO	Obese III	≥ 40	No upper limit
Asia-Pacific	Underweight	≥ 0	< 18.5
Asia-Pacific	Normal	≥ 18.5	< 23
Asia-Pacific	At Risk	≥ 23	< 25
Asia-Pacific	Obese I	≥ 25	< 30
Asia-Pacific	Obese II	≥ 30	No upper limit
NIH/CDC	Underweight	≥ 0	< 18.5
NIH/CDC	Normal	≥ 18.5	< 25
NIH/CDC	Overweight	≥ 25	< 30
NIH/CDC	Obesity Class I	≥ 30	< 35
NIH/CDC	Obesity Class II	≥ 35	< 40
NIH/CDC	Obesity Class III	≥ 40	No upper limit

Waist and Warning Rules:

Waist, age, and measurement warning rules used with BMI results
Signal	Rule	Meaning
Waist-to-height ratio	< 0.50, 0.50 to < 0.60, or ≥ 0.60	Lower, elevated, or high central-adiposity context.
Sex-specific waist note	Female ≥ 88 cm or male ≥ 102 cm	Flags the common high-waist threshold when sex and waist are both entered.
Age warning	Age < 20	Adult BMI bands are not intended for children or teens.
Measurement warning	Weight < 30 kg or > 300 kg, height < 120 cm or > 230 cm, or waist < 45 cm or > 190 cm	Prompts a unit or tape-placement check before interpreting the result.
High-BMI waist note	BMI ≥ 35 with waist entered	Explains that waist-to-height ratio adds less screening value at that BMI level.

Limitations and Safety Notes:

BMI is an informational adult screening estimate, not medical advice. It can support a health conversation, but it cannot diagnose obesity, prescribe a weight goal, or decide whether weight loss, weight gain, medication, or nutrition changes are safe.

BMI does not directly measure body fat, muscle mass, bone density, pregnancy status, fluid retention, or fat distribution.
Adult bands are not suitable for children and teens; use BMI-for-age percentiles instead.
Athletes, older adults, pregnant people, people with edema, and people with chronic conditions may need different interpretation.
Waist-to-height ratio and sex-specific waist thresholds add context, but they still do not replace clinical assessment.
People with a history of eating disorders, rapid unexplained weight change, or weight-related medical treatment should avoid using BMI targets without professional support.

Worked Examples:

Typical adult entry

A weight of 72 kg and height of 1.75 m gives BMI 23.5. With the WHO preset, BMI Metrics shows a Normal classification, BMI Prime 0.940, and a healthy weight range of about 56.7 kg to 76.6 kg. A target BMI of 22.0 at the same height corresponds to about 67.4 kg.

Same BMI, different standard

A weight of 68 kg at 170 cm gives BMI 23.5. Under WHO or NIH/CDC bands, that sits inside the Normal range. Under the Asia-Pacific preset, the same BMI is in the At Risk band because that preset's Normal range stops at 23.

Waist context

A 90 cm waist at 170 cm height gives waist-to-height ratio 0.529. Guidance & Risk reports elevated waist risk because the ratio is at least 0.50 but below 0.60. If Sex is blank, the warning explains that sex-specific waist-threshold notes are hidden.

Unit troubleshooting

Entering 700 lb instead of 70 kg at 170 cm produces a very high BMI and an unusual adult weight warning. Fix the Weight value or unit before using the category, healthy weight span, target-weight estimate, or charts.

FAQ:

Why does my category change when my BMI stays the same?

The formula does not change, but the selected BMI classification preset changes the cutoff bands. A BMI of 23.5 is treated differently by the Asia-Pacific preset than by WHO or NIH/CDC.

Can I use this for a child or teen?

Adult BMI categories are not intended for people under 20. If Age is below 20, the warning directs you toward BMI-for-age percentiles instead.

Does sex change the BMI result?

No. Sex does not change BMI, BMI Prime, ponderal index, healthy weight range, or target weight. It only enables the common female or male waist-threshold note when waist circumference is entered.

Why did I get an unusual measurement warning?

The warning appears when adult weight, height, or waist values fall outside the tool's typical check range. Recheck the number, unit, and waist tape placement before interpreting the category or chart.

Is target BMI a recommended goal?

No. Target BMI is an arithmetic setting used to estimate a matching target weight inside the selected healthy BMI span. A personal goal should account for medical history, body composition, waist measures, and clinician guidance.

Glossary:

Body mass index: Weight in kilograms divided by height in meters squared.
BMI Prime: BMI divided by 25, used here as a quick comparison with the common adult overweight cutoff.
Ponderal index: Weight in kilograms divided by height in meters cubed.
Waist-to-height ratio: Waist circumference divided by height using the same unit family for both measurements.
Healthy weight span: The weight range that corresponds to the selected standard's healthy BMI band at the entered height.
Target BMI: The selected BMI used to estimate a matching target weight.

References:

Adult BMI Categories, Centers for Disease Control and Prevention, March 19, 2024.
Obesity and overweight, World Health Organization.
Identifying and assessing overweight, obesity and central adiposity, NICE guideline NG246.
Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies, WHO Expert Consultation via PubMed, 2004.