BMI calculator
Free BMI calculator for adults age 20 and older. Use kg/cm or lb/in, see the adult screening category, and understand the limits of body mass index.
By Calcr · Updated
Free BMI calculator for adults
Calculate adult body mass index from weight and height using kilograms and centimetres or pounds and total inches. The result includes the index and its CDC adult screening category. This form is for adults aged 20 and older. BMI is one screening measure; it cannot diagnose a condition, measure body fat directly or determine an individual's overall health.
How to enter your measurements
Choose the unit system before typing. In metric mode, enter weight in kilograms and height in centimetres: 70 and 175 are a valid example. In US mode, enter pounds and total inches. A height of 5 feet 9 inches is 69 inches, not 5.9. Enter age in completed years so the tool can reject use of adult categories below age 20.
Check the measurements rather than copying an old value without its unit. A weight in pounds entered into the kilograms field produces a different index even when the arithmetic is correct. If comparing results over time, keep the measurement method and units consistent. The calculator cannot tell whether an entered measurement was taken accurately or whether a short-term change has clinical meaning.
BMI formula and adult categories
BMI = weight in kilograms / height in metres squared. For 70 kg and 1.75 m, the calculation is 70 / (1.75 * 1.75), approximately 22.86 kg/m². The US-unit calculation first converts pounds to kilograms and inches to metres, then uses the same formula. This avoids changing the definition between unit systems.
The CDC adult BMI categories use the following boundaries for adults 20 and older. The category is chosen from the unrounded value, while the displayed index rounds to two decimal places.
| Adult screening category | BMI in kg/m² |
|---|---|
| Underweight | below 18.5 |
| Healthy weight | 18.5 to below 25 |
| Overweight | 25 to below 30 |
| Obesity, class 1 | 30 to below 35 |
| Obesity, class 2 | 35 to below 40 |
| Obesity, class 3 | 40 or above |
These labels summarize the adult classification system. They are not personal judgments or a substitute for a clinical assessment. A result close to a boundary should be read with the original measurements and the rounding note, rather than treated as a sudden change in health.
Worked BMI examples
Metric measurements
An adult weighing 70 kg at a height of 175 cm has a height of 1.75 m. Squaring it gives 3.0625 m². Dividing 70 by 3.0625 gives approximately 22.86, within the adult healthy-weight category. Enter age 30 to reproduce the example. The arithmetic describes this pair of measurements; it does not establish fitness, nutritional status or a recommended body weight.
US measurements
An adult weighing 180 lb and measuring 70 inches has a BMI of approximately 25.83. The tool converts 180 lb to 81.6466266 kg and 70 inches to 1.778 m before dividing. This falls in the adult overweight category. If you compare this with a metric calculation, use the converted measurements without rounding them heavily first; otherwise a small difference in the displayed index is expected.
A value at a classification boundary
At 100 kg and 200 cm, BMI is 100 / 2² = 25 exactly. That belongs to the category beginning at 25, rather than the range below 25. At 99.98 kg and the same height, BMI is 24.995, which may display as 25.00 after rounding but remains below 25 for classification. The category uses the unrounded calculation to avoid moving a value across a boundary solely because of display rounding.
When to use BMI, and when it is insufficient
Use the calculator to understand a screening number, check a stated formula or prepare questions for a health professional. Keep weight, height, units and the date of measurement with any result you record. A number detached from its measurements is harder to review and can give a false impression of precision.
Do not use adult cutoffs for children and teenagers. Their interpretation needs age- and sex-specific growth references; the CDC adult calculator directs ages 2–19 to a separate child and teen calculator. This tool also excludes pregnancy from its intended screening use. Seek an appropriate clinical assessment rather than trying to force a special situation into an adult table.
BMI cannot distinguish muscle, fat and bone or show where body fat is located. The CDC BMI FAQ explains why a muscular person may fall into a higher BMI category without the index identifying body composition. Medical history, physical findings and other information can change how a professional interprets the same number. This page does not prescribe weight change, medication, calorie restriction or treatment. A BMI category can be a starting point for a conversation, but it does not establish a personal goal. Keep your questions focused on your own circumstances rather than comparing one screening number with another person’s result.
FAQ
Is a healthy-weight BMI proof that I am healthy?
No. It describes the category for a weight-to-height ratio. Health depends on more than that relationship, and a screening table does not evaluate symptoms, laboratory findings or medical history. Discuss concerns with a qualified professional rather than relying on one index to confirm or dismiss them.
Why does the form require age when age is not in the formula?
The formula itself uses weight and height. Age is requested to prevent applying adult categories to people under 20. Once that adult scope is satisfied, changing age does not change the arithmetic result. This is different from child BMI interpretation, where age affects the relevant growth reference.
Can BMI estimate my body-fat percentage?
No. The result is measured in kilograms per square metre, not percent. Two adults with the same BMI can have different body composition. Do not label the index as a body-fat measurement or subtract it from 100 to derive another quantity. A different assessment is needed for that question.
Why does a rounded value appear beside a different category?
A value just below a boundary can round up in the two-decimal display. Classification uses the underlying value, so 24.995 remains below 25 even if displayed as 25.00. Check the input precision and avoid interpreting a tiny numerical difference as a clinical finding.
Do I need to upload a photo or create an account?
No. Only the measurements entered in the form are needed, and the arithmetic runs in your browser. No photograph, identity document or public profile is required. You can test the examples with invented measurements and leave optional site services disabled.
Related tools and guides
For reference only. BMI is a screening measure and this page does not provide medical advice, diagnosis or treatment.