Calculate adult body mass index from height and weight in metric, US or UK units. Review the standard BMI category, the weight interval corresponding to a healthy BMI, a selected target-BMI weight and each calculation step.
- Metric, US and UK units
- Six adult BMI ranges
- Healthy-BMI weight interval
- Formula and substituted steps
BMI Calculator with Category and Healthy Weight Range
Choose a unit system and enter current weight and height. Categories use the standard adult cutoffs for people aged 20 and older. The target-BMI field converts a selected BMI into a reference weight at the same height.
In the healthy weight range
Adult BMI category scale
Standard adult cutoffs- Underweight
- Healthy weight
- Overweight
- Class 1 obesity
- Class 2 obesity
- Class 3 obesity
Measurement and weight breakdown
| Measure | Result | Meaning |
|---|---|---|
| Height | 1.75 m | Height used after conversion |
| Weight | 70 kg | Weight used after conversion |
| BMI | 22.86 kg/m² | Healthy weight range |
| Selected target | BMI 22.5 = 68.9 kg | Reference only, not a prescription |
Standard adult BMI categories
| Adult category | BMI range | Screening label |
|---|---|---|
| Underweight | Below 18.5 | Below the standard healthy range |
| Healthy weight | 18.5 to below 25 | Standard healthy BMI range |
| Overweight | 25 to below 30 | Above the standard healthy range |
| Class 1 obesity | 30 to below 35 | Obesity range, class 1 |
| Class 2 obesity | 35 to below 40 | Obesity range, class 2 |
| Class 3 obesity | 40 or higher | Severe obesity range |
Calculation steps
- Convert height to metres: 175 cm ÷ 100 = 1.75 m.
- Square height: 1.75 × 1.75 = 3.0625 m².
- Divide weight by squared height: 70 ÷ 3.0625 = 22.8571.
- Classify the unrounded result: BMI 22.8571 is in the healthy weight range.
Recent calculations
Results stay in this page session. Nothing is sent by this calculator.
How to Use This BMI Calculator
- Select metric, US or UK units.
- Enter an adult’s current weight and height. Use kilograms and centimetres, pounds and feet/inches, or stones/pounds and feet/inches.
- Keep or change the selected target BMI. This converts a BMI value into a reference weight at the entered height.
- Select “Calculate BMI.” Review the unrounded category, healthy-BMI weight interval, gauge and formula steps.
- Read the limitations before using the result. BMI supports screening. It does not diagnose a health condition.
This page uses adult categories for people aged 20 and older. Children and teenagers need BMI-for-age percentiles based on age and sex, not the adult table.
What Is BMI?
Body mass index, or BMI, relates weight to height. It gives a standardized number that health services use as one part of adult weight-status screening. A BMI calculator does not inspect body composition. Two adults with the same BMI may have different proportions of body fat, muscle, bone and water.
BMI remains useful at a population level because height and weight are easy to collect and the result is consistently defined. For an individual, it works best as a starting point. A complete health assessment may also consider medical history, waist measurement, blood pressure, glucose, cholesterol, medicines, mobility, diet and physical examination.
Adult BMI Formula
The metric formula divides weight in kilograms by height in metres squared:
If height is entered in centimetres, divide it by 100 before squaring it. The US formula uses pounds and total inches:
The factor 703 is the conventional rounded conversion factor. This calculator converts measurements through exact SI unit factors, then applies the metric definition. It keeps full precision for classification and rounds only the displayed result.
Metric BMI example
An adult weighing 70 kg at 175 cm is 1.75 m tall. Squared height is 3.0625 m². The calculation is 70 ÷ 3.0625 = 22.8571, displayed as BMI 22.86. Because the unrounded result is at least 18.5 and below 25, it falls in the standard healthy weight range.
US BMI example
An adult weighing 185 lb at 5 ft 10 in has a total height of 70 in. The familiar US calculation is 185 ÷ 70² × 703, which is about 26.54. That value is in the standard overweight range. Minor last-decimal differences between tools usually come from using 703 versus a more precise conversion factor.
Adult BMI Categories
| Category | Adult BMI | How to read it |
|---|---|---|
| Underweight | Below 18.5 | Below the standard healthy BMI range |
| Healthy weight | 18.5 to below 25 | Within the standard healthy BMI range |
| Overweight | 25 to below 30 | Above the standard healthy BMI range |
| Class 1 obesity | 30 to below 35 | In the class 1 obesity range |
| Class 2 obesity | 35 to below 40 | In the class 2 obesity range |
| Class 3 obesity | 40 or higher | In the class 3, or severe obesity, range |
The calculator says a result is “in” a screening range. It does not label a person or diagnose obesity-related disease. Category boundaries are applied to the full calculated BMI before display rounding.
Healthy-BMI Weight Range
For the entered height, the tool reverses the BMI formula at 18.5 and 24.9. This gives the weights corresponding to the commonly displayed healthy adult BMI interval. It is a mathematical reference range, not an individual ideal-weight prescription.
At 1.75 m, BMI 18.5 corresponds to about 56.7 kg and BMI 24.9 corresponds to about 76.3 kg. A clinician may recommend a different focus based on current health, weight history, muscle mass, medicines, pregnancy status, disability, age or treatment needs.
The nearest-boundary figure reports the difference between current weight and the closest entry point into the standard healthy BMI range. It describes arithmetic only. It is not a recommendation to gain or lose that amount, or to change weight rapidly.
What Is BMI Prime?
BMI Prime divides BMI by 25, the upper boundary of the standard healthy adult range. A BMI Prime of 1 means BMI 25. A value of 0.80 means BMI 20, while 1.20 means BMI 30. It is another way to express position relative to that cutoff, not a separate health diagnosis.
BMI Limits and Important Exceptions
Muscular adults and athletes
BMI cannot tell whether weight comes from fat, muscle or bone. A strength athlete may have a high BMI with relatively low body fat. The number may still be recorded, but body composition, waist measures and clinical context become more important.
Pregnancy
Do not use current pregnancy weight in this general adult BMI calculator. Pregnancy changes weight for reasons the standard category table does not model. Use maternity guidance based on pre-pregnancy BMI and advice from the professional managing the pregnancy.
Children and teenagers
Adult cutoffs do not apply to people aged 2 through 19 under the CDC framework. Their BMI is compared with sex-specific BMI-for-age growth charts. The percentile, not the adult category table, determines the screening category. Infants under age 2 require other growth measures.
Older adults, illness and height changes
Age-related muscle loss, fluid changes, amputation, spinal curvature and conditions that affect height or body composition may reduce the meaning of a standard BMI. Ask a qualified health professional which measurements fit the situation.
Ethnicity and Lower Risk-Assessment Thresholds
The main result uses the standard WHO and CDC adult cutoffs: overweight starts at BMI 25 and obesity at BMI 30. Some health systems begin additional risk assessment at lower values for certain ethnic backgrounds. The NHS, for example, uses lower adult thresholds for people from Asian, Middle Eastern, Black African and African-Caribbean backgrounds. NIDDK notes that Asian American adults may face type 2 diabetes risk at BMI 23 or higher.
How to Measure Height and Weight More Consistently
- Measure height without shoes on a firm, level floor.
- Stand upright with heels against a wall when practical and keep the measuring tool level.
- Use a calibrated scale on a hard surface.
- Weigh under similar conditions when comparing results over time.
- Enter total height correctly. For example, 5 ft 10 in is 70 in, not 5.10 ft.
- In UK mode, keep additional pounds below 14. Fourteen pounds equals one stone.
- Avoid rounding height too early because height is squared in the formula.
Common BMI Calculator Mistakes
- Entering centimetres in a field that expects metres, or kilograms in a pounds field.
- Treating 5 ft 8 in as 5.8 feet. The correct decimal-foot value is about 5.667 feet.
- Classifying from a rounded BMI instead of the full value near 18.5, 25, 30, 35 or 40.
- Calling the healthy-BMI interval a guaranteed ideal weight.
- Using adult categories for a child or teenager.
- Using current pregnancy weight with the general adult category table.
- Assuming BMI directly measures body-fat percentage or fat distribution.
- Ignoring high muscle mass, medical conditions or ethnicity-specific risk guidance.
Calculation Method and Review Sources
The formula, adult age scope, category boundaries and screening limitations were reviewed against the CDC BMI overview, CDC BMI formula guidance and CDC adult BMI categories. The global overweight and obesity cutoffs were checked against the WHO obesity and overweight fact sheet.
Child and teen distinctions were checked against the CDC BMI-for-age categories. Pregnancy, high-muscle and lower ethnicity-threshold cautions were reviewed against the NHS adult BMI guidance and NIDDK type 2 diabetes risk guidance.
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Frequently Asked Questions
How do I calculate BMI?
Divide weight in kilograms by height in metres squared. With US units, divide pounds by total inches squared and multiply by 703. This tool converts all supported units before calculating.
What is a healthy BMI for adults?
The standard CDC adult healthy weight range is BMI 18.5 to below 25. BMI is a screening measure, so health cannot be judged from this interval alone.
Does BMI measure body fat?
No. BMI uses only height and weight. It does not directly measure body fat, separate fat from muscle and bone, or show where fat is stored.
Why does the calculator use age 20 and older?
The CDC adult category table applies from age 20. People aged 2 through 19 need sex-specific BMI-for-age percentiles rather than adult cutoffs.
Can I use this BMI calculator during pregnancy?
No. Current pregnancy weight should not be classified with this general adult tool. Use pregnancy guidance based on pre-pregnancy BMI and advice from your maternity-care professional.
Is BMI accurate for athletes?
BMI calculates correctly from height and weight, but its health meaning may be limited for muscular athletes because it cannot distinguish muscle from body fat.
What weight corresponds to a healthy BMI?
The calculator multiplies BMI 18.5 and 24.9 by height squared. The resulting interval is a general healthy-BMI reference, not a personalized ideal-weight prescription.
Why do some ethnicity guidelines use BMI 23?
Some groups, including people of Asian background, may develop metabolic risks at lower BMI values. Some health systems begin added risk assessment at BMI 23 while retaining the standard global categories for comparison.
What is BMI Prime?
BMI Prime is BMI divided by 25. It expresses the result relative to the upper boundary of the standard healthy adult BMI range and does not create a new diagnosis.
Why can two BMI calculators differ slightly?
Small differences usually come from rounded height, weight or the US conversion factor 703. This tool converts with exact unit factors, keeps calculation precision and rounds only for display.
Health and Calculation Disclaimer
BMI is a screening measure, not a diagnosis or a direct measurement of body fat. It does not distinguish fat from muscle or bone and does not show where body fat is stored. Consider the result with medical history, physical examination, blood pressure and laboratory results. This adult calculator is not intended for pregnancy or for children and teens. Speak with a qualified health professional if you have concerns about weight, nutrition or health.