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.

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.

BMI classification:

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

Target BMI:

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

{{ warning }}

Metric	Value	Copy
{{ row.label }}	{{ row.value }}

Guidance area	Status	Detail	Copy
{{ row.title }}	{{ row.badge }}	{{ row.detail }}

BMI range	Category	Weight at your height	Copy
{{ row.range }}	{{ row.category }} Current Target	{{ row.weightRange }}

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Body mass index, or BMI, compares body weight with height and turns those two measurements into a single screening number. Health services use it because it is quick to calculate and easy to compare across visits, but it is still only a screening measure. BMI does not directly measure body fat, and it cannot show where fat is carried.

This calculator builds on that BMI value by pairing it with three adult standards, a healthy-weight range for the entered height, and a target BMI setting that converts a chosen goal into a target weight. Add a waist measurement and the result also shows waist-to-height ratio, which helps flag abdominal fat distribution that BMI can miss. Leave waist blank and the result shows ponderal index instead, giving you another size measure without pretending it replaces waist data.

Five linked cards showing weight and height, BMI, adult standards, waist check, and weight targets.

Adult interpretation is the critical boundary. CDC uses adult BMI categories for ages 20 and older, while children and teens need BMI-for-age percentiles that are matched to age and sex. This calculator warns when the optional age field is under 20, but it still cannot replace a pediatric growth-chart assessment.

Treat the output as an informational estimate, not a diagnosis or treatment plan. A muscular build, pregnancy, edema or other fluid-retention states, older age, and some medical conditions can make BMI less representative of body-fat risk than the number first suggests. When that matters, read the BMI, the selected adult standard, the waist signal, and the warning banner together.

Technical Details:

Body mass index is the ratio of body mass to height squared. That scaling makes a 70 kg body at 170 cm read very differently from the same weight at 150 cm, which is why BMI is widely used as a quick adult screening measure. The BMI value itself is fixed once weight and height are known. What changes afterward is the classification standard used to label that value.

A selected healthy band can be turned back into kilograms or pounds at the current height by multiplying the chosen BMI cutoffs by height squared. That produces the healthy floor, healthy ceiling, and target weight. Waist-to-height ratio answers a separate question: how large the waist is relative to height. BMI Prime compares BMI with 25, while ponderal index uses height cubed and remains available when no waist value is entered.

The core formulas below explain every numeric output that appears in the summary, metrics table, category table, and charts.

\begin{array}{lcl} BMI & = & \frac{W}{H^{2}} \\ BMI Prime & = & \frac{BMI}{25} \\ WHtR & = & \frac{WC}{H} \\ PI & = & \frac{W}{H^{3}} \\ W_{target} & = & {BMI}_{target} H^{2} \end{array}

In these formulas, W is body weight in kilograms, H is height in meters for BMI, BMI Prime, ponderal index, and target-weight math, and WC is waist circumference in the same unit as height for waist-to-height ratio.

Adult BMI standards used by the calculator
Preset	Band	Lower (≥)	Upper (<)	Meaning
WHO	Severely Underweight	0.0	16.0	Very low adult BMI.
WHO	Underweight	16.0	18.5	Below the common adult healthy floor.
WHO	Normal	18.5	25.0	Common international healthy-weight band.
WHO	Overweight	25.0	30.0	Above the healthy band, below obesity classes.
WHO	Obese I	30.0	35.0	First obesity class in this preset.
WHO	Obese II	35.0	40.0	Second obesity class in this preset.
WHO	Obese III	40.0	—	Highest open-ended obesity band in this preset.
Asia-Pacific	Underweight	0.0	18.5	Below the adult healthy floor.
Asia-Pacific	Normal	18.5	23.0	Lower-threshold healthy band.
Asia-Pacific	At Risk	23.0	25.0	Early caution band before obesity classes.
Asia-Pacific	Obese I	25.0	30.0	First obesity band in this preset.
Asia-Pacific	Obese II	30.0	—	Open-ended obesity band in this preset.
NIH/CDC	Underweight	0.0	18.5	Below the adult healthy floor.
NIH/CDC	Normal	18.5	25.0	Common U.S. healthy-weight band.
NIH/CDC	Overweight	25.0	30.0	Above healthy weight, below obesity classes.
NIH/CDC	Obesity Class I	30.0	35.0	First obesity class in the U.S. preset.
NIH/CDC	Obesity Class II	35.0	40.0	Second obesity class in the U.S. preset.
NIH/CDC	Obesity Class III	40.0	—	Highest open-ended obesity band in the U.S. preset.

Waist-to-height ratio bands used by the calculator
Band	Lower (≥)	Upper (<)	Meaning
Lower waist risk	0.00	0.50	Below the main action threshold.
Elevated waist risk	0.50	0.60	Higher abdominal-fat risk signal.
High waist risk	0.60	—	Strong abdominal-fat warning band.

These boundaries explain most disagreements a reader will see. A BMI close to 23.0 or 25.0 can cross into a different category when the preset changes even though the BMI value itself stays fixed. A waist-to-height ratio can also push the risk reading upward when BMI still falls in a normal band, which is why the waist result is worth reading separately instead of treating it as a cosmetic extra.

Everyday Use & Decision Guide:

The best first pass is simple: enter weight and height, leave the advanced controls alone, and read the summary box before opening any chart. That top row tells you the BMI value, the current BMI classification, the healthy-weight span at your height, and either waist-to-height ratio or ponderal index. For a quick adult check, that is usually enough to tell you whether you need a closer look.

If you do need more detail, use the advanced controls with a specific reason in mind.

Stay with the WHO or NIH/CDC preset when you want the familiar adult healthy band of 18.5 to less than 25.0.
Switch to the Asia-Pacific preset when you want to compare your BMI against lower adult cutoffs used in some Asian-population guidance.
Add waist circumference when abdominal fat distribution matters or when a healthy-looking BMI might still be misleading.
Move the Target BMI slider only after choosing the preset, because the allowed target range is clamped to that preset's healthy band.

The tabs are arranged by job. BMI Metrics Overview is the fastest place to copy numbers into notes. BMI Categories shows the weight span for each band at your current height. Guidance & Risk condenses the main findings into short text cards. The four chart tabs are useful when you want a visual check or an image/CSV export, and the JSON tab is the cleanest way to capture the whole current state for record-keeping.

Do not switch presets to hunt for the most flattering label, and do not trust a polished chart if the warning banner is already telling you something is off. If the alert mentions age under 20 or an unusual height, weight, or waist measurement, correct that first. A wrong unit will travel into every table, chart, export, and shared result.

Step-by-Step Guide:

Use this order when you want the result to stay consistent from the first number you enter to the final export.

Enter Weight and Height, then confirm the units immediately. If you want feet and inches, switch the height unit to ft/in before typing the split values.
Open Advanced only if you need extra interpretation. Add Age, Sex, Waist circumference, a different BMI classification preset, or a Target BMI.
Read the summary box first. You should see the BMI value, the active BMI classification badge, the healthy-weight badge, and either a WHtR badge or a PI badge.
If the yellow warning alert appears, stop there. Recheck the unit or measurement named in the warning, or move to BMI-for-age assessment if the age entry is under 20.
Open BMI Metrics Overview for the exact rows such as Healthy weight range, Weight change to healthy range, Target weight, and Change needed for target.
Open BMI Categories when you want to compare every band at your height, then use Guidance & Risk, Gauge Overview, Healthy Weight Span, BMI Band Visual, or BMI Zone Map if you want a visual explanation before exporting CSV, DOCX, image, or JSON output.

That sequence keeps the copied or exported result aligned with the standard and units you actually meant to use.

Interpreting Results:

The first number to trust is the BMI value itself. A BMI of 23.5 stays 23.5 no matter which preset you pick. If the label changes after switching from WHO to Asia-Pacific or NIH/CDC, that means the cutoff lines moved, not the body.

BMI classification tells you the current band under the selected adult standard.
Healthy weight range shows the body-weight span attached to that standard at the current height.
Weight change to healthy range points to the nearest healthy boundary, not to an arbitrary goal weight.
Waist-to-height ratio should stay below 0.50 in this calculator. A value from 0.50 to less than 0.60 is elevated, and 0.60 or more is high.

A Normal badge should not be treated as an all-clear signal on its own. BMI can sit inside a healthy adult band while Waist-to-height ratio still reads elevated or high. When that happens, the waist result is the better warning about central adiposity.

Ponderal index is only a fallback size check when waist is blank, not a replacement for waist distribution. If BMI is 35 or higher, the tool warns that waist-to-height ratio adds less screening value. If any result looks strange, verify Height, Waist circumference, and the selected unit before you trust the charts or exports.

Worked Examples:

1. A routine adult check with target planning

Enter 70 kg and 170 cm with the WHO preset and a target BMI of 22. The summary shows BMI 24.2 and BMI classification Normal. In BMI Metrics Overview, the Healthy weight range (kg) is 53.5 to 72.2 kg and Target weight is 63.6 kg, so Change needed for target reads Lose 6.4 kg. The result means the current weight is still inside the WHO healthy band, but the chosen target sits noticeably lower inside that same band.

2. The same BMI can cross a boundary under a different standard

Enter 68 kg, 170 cm, and 88 cm waist. The BMI is 23.5. Under WHO or NIH/CDC, BMI classification stays Normal. Switch to the Asia-Pacific preset and the same BMI moves to At Risk. The Healthy weight range (kg) narrows from 53.5 to 72.2 kg down to 53.5 to 66.5 kg, and Weight change to healthy range becomes Lose 1.5 kg. The added waist entry also produces Waist-to-height ratio 0.518, which lands in the elevated band. That combination shows why a boundary value deserves more than a single badge glance.

3. A waist-unit mistake creates a false alarm

Keep the same 68 kg and 170 cm, but enter waist as 84 while the unit is set to inches instead of centimeters. The calculator converts that to Waist circumference (cm) 213.4, raises a warning about an unusual waist measurement, and shows Waist-to-height ratio 1.255 with a high waist-risk band. Change the waist unit back to centimeters and the same number becomes Waist-to-height ratio 0.494 with a lower waist-risk band. This is a good example of why the warning banner should be cleared before you interpret the result.

4. Age under 20 needs a different reference

Enter 58 kg, 165 cm, age 16, and a 72 cm waist. The calculator can still show BMI 21.3, but the summary and warning text tell you that adult BMI cutoffs are not intended for children or teens. The correct follow-up is not to argue about whether 21.3 looks healthy on an adult table. It is to move to BMI-for-age percentiles, because growth and sex-specific reference curves matter at that age.

FAQ:

Why did my healthy label disappear when I switched presets?

Because the BMI value stays fixed while the adult cutoff lines move. A result near 23.0 or 25.0 can stay Normal under one preset and move to At Risk or Overweight under another without any change in body size.

Does sex change the BMI number in this calculator?

No. Sex does not change BMI or BMI classification here. It only turns on the extra waist note that compares the entered waist with the common adult high-waist thresholds of 88 cm for female entries and 102 cm for male entries.

If my BMI is normal, am I finished?

Not always. A normal BMI can still sit beside an elevated or high Waist-to-height ratio. If the BMI badge looks reassuring but the waist badge does not, read the waist result as the stronger warning about abdominal fat distribution.

Why does the calculator warn me when age is under 20?

Because adult BMI categories are not the right reference for children and teens. Growth changes the meaning of a BMI value, so the correct comparison is a BMI-for-age percentile matched to age and sex.

What should I check first if the result looks impossible?

Start with units. Review Height, Weight, and Waist circumference, then look at the warning alert. A centimeters-versus-inches mix-up can make every table and chart look dramatic even when the underlying measurement was entered correctly in the wrong unit.

Are my measurements sent to a server for the calculation?

The calculation itself runs in the browser and the JSON tab is built from the same local state. After you edit a field, the page also syncs the current inputs into the URL query string, so avoid sharing the link if you do not want those measurements exposed.

Glossary:

BMI: Body mass index, calculated from weight divided by height squared.
BMI Prime: A ratio that compares BMI with the reference value of 25.
Waist-to-height ratio: Waist circumference divided by height after both are expressed in the same unit.
Ponderal index: A size measure that uses height cubed instead of height squared.
Healthy-weight range: The body-weight span created by the selected healthy BMI band at the current height.
Target BMI: The chosen BMI goal used to calculate target weight and the required weight change.

References:

BMI Frequently Asked Questions, Centers for Disease Control and Prevention, June 28, 2024.
Child and Teen BMI Categories, Centers for Disease Control and Prevention, June 28, 2024.
Obesity and overweight, World Health Organization, December 8, 2025.
Overweight and obesity management quality statement 5, National Institute for Health and Care Excellence, 2025.
Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies, The Lancet, 2004.