Health Calculator

BMI Calculator

Calculate your body mass index using height and weight, then read the result as a simple screening estimate.

Published by Calculator All-in-OneMethod: BMI = kg / m²Methods and limitations

What this calculator does

The BMI Calculator estimates Body Mass Index from height and weight. BMI is a screening number used to describe whether a person is broadly within underweight, normal, overweight, or obesity ranges. The calculator converts height from centimeters to meters and divides weight by height squared.

After you enter height and weight, the tool computes the index and places it on the standard adult scale: under 18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, and 30+ obesity. The number is a population-level screening statistic — it says how your weight compares for your height, not how healthy you are.

Who this is for

This tool uses the CDC adult BMI categories for ages 20 and older. It is a screening calculation, not a diagnosis or a target weight. Do not use these category labels for children or teenagers, who need BMI-for-age assessment. Pregnancy, muscle mass, health history and other clinical context affect interpretation.

CDC: adult BMI categories and limitations. A clinician can interpret BMI alongside other measurements and your health history.

Formula used

BMI = weight in kilograms / (height in meters x height in meters).

Because height is squared in the denominator, small height errors distort the result twice over: entering 165 cm instead of 170 cm shifts the BMI of a 70 kg person from 24.2 to 25.7 — across a category boundary. Measure height without shoes and enter weight in kilograms for the formula to work as designed.

Understanding each input

Height in centimetres — measured without footwear. The calculator converts to metres internally; if you know your height in feet and inches, convert first (5′7″ ≈ 170 cm).
Weight in kilograms — ideally measured at a consistent time of day, since normal daily fluctuation of 1–2 kg can move the result noticeably.

Example calculation

At 70 kg and 170 cm, height is 1.70 meters. BMI = 70 / (1.70 × 1.70) = 24.221..., displayed as 24.2. This falls in the standard adult healthy-weight category, 18.5 to below 25.

The category uses the unrounded value. A value just below 25 may display as 25.0 after rounding while remaining below the category boundary. Neither the number nor the label establishes a person's health.

Benefits

Get a quick screening number
Track broad changes over time
Use metric inputs easily
Understand BMI limits
Prepare for a health discussion

The most defensible use is tracking your own trend: because your height is constant, BMI changes map directly to weight changes, and a slow drift across months is easier to see in one number than in daily scale readings that bounce around.

Reading the result

Read the number as a zone, not a verdict. Within 1 point of a boundary, measurement noise alone could place you on either side. What deserves attention is a sustained move across categories over months, or a value far from the normal band — both are conversation starters with a clinician, alongside better measures like waist circumference and blood work.

Assumptions and limitations

BMI cannot distinguish fat from muscle, does not measure fat distribution (waist-centred fat carries different risk), and applies adult thresholds that are not valid for children or during pregnancy. Some populations use adjusted cut-offs — several Asian health bodies treat 23+ as overweight. A normal BMI does not rule out metabolic issues, and a high BMI in a strength athlete may be perfectly healthy.

Educational screening tool only — not medical advice, diagnosis, or treatment. Discuss weight and health decisions with a qualified clinician.

Common mistakes

Entering height in feet or inches into a centimetre field — a 5.7 entry produces an absurd result.
Weighing with shoes, heavy clothing, or right after a large meal and comparing against an old reading taken differently.
Treating a single category label as a diagnosis rather than a screening flag.
Applying adult cut-offs to a child or teenager.

FAQs

Is BMI accurate for everyone?

No. BMI is a screening estimate and may be less useful for athletes, children, pregnant people, and some older adults.

What BMI range is normal?

For many adults, 18.5 to 24.9 is commonly considered normal, but medical context matters.

Can BMI measure body fat directly?

No. BMI does not directly measure body fat, muscle, bone density, or body composition.

Should I make medical decisions from BMI alone?

No. Use it as a starting point and consult a qualified professional for personal advice.

Why use centimeters and kilograms?

Metric inputs make the standard BMI formula simple to apply.

A screening calculation, not a diagnosis

BMI is calculated from the standard weight divided by height-squared formula. The page avoids diagnosis claims and tells users to verify health decisions with qualified medical guidance, especially for children, pregnancy, athletes, or medical conditions.

Make the measurement more useful

Check units before interpreting a category

For height 175 centimetres and weight 70 kilograms, the arithmetic is 70 divided by 1.75 squared, approximately 22.86. The display rounds this to 22.9. If you mistakenly enter 1.75 into a field expecting centimetres, the converted height becomes 0.0175 metres and the result becomes implausibly large. That is a unit problem, not a meaningful health finding. Recheck measurements before acting on an unexpected result.

Use a measured height without shoes and a consistent weighing method where practical. The purpose is to reduce avoidable variation, not to make a home calculation a clinical assessment. Clothing, different scales, and a mistyped digit can alter the result. Record the measurements and their date if you plan to discuss them with a clinician; a detached category label provides much less context.

Understand rounding near a boundary

At 180 centimetres, 81 kilograms gives a BMI of exactly 25. At 80.9 kilograms the arithmetic is approximately 24.97, which can display as 25.0 when rounded to one decimal place. The category should be determined from the unrounded calculation. A displayed decimal close to a threshold is therefore not precise enough to reconstruct the classification on its own. Small changes in a home measurement also should not be treated as a sudden change in overall health.

Use the result in the right conversation

The CDC's adult categories apply to adults aged 20 and older and describe a screening measure. Assessment involves more than this number. This page has no information about symptoms, body composition, pregnancy, medications, or your clinical history. It cannot supply an individual diagnosis, calorie prescription, or target weight. Children and teenagers require age-specific interpretation rather than these adult cutoffs.

If an unexpected reading concerns you, bring the original measurements and relevant context to a qualified healthcare professional. Avoid repeatedly changing input values until the category feels reassuring. The tool is most useful as transparent arithmetic that supports a conversation, with its limitations visible beside the result.

Reference: CDC adult BMI categories and screening scope. Examples above use this calculator’s assumptions.