Starlight Tools

BMI Calculator - Body Mass Index and Healthy Weight Range

Enter height and weight in metric or US units to get your BMI, adult category, obesity class when applicable, and the healthy weight range for your height. Private by design - everything runs locally in your browser.

Inputs

BMI can be calculated without age. CDC adult interpretation focuses on 20+, while NHS adult BMI guidance starts at 18+.

Result

Your BMI
—

Enter height and weight to see your result.

Category
—
Category range
—
Obesity class
—
Entered height
—
Entered weight
—
Healthy weight range
—
BMI scale

Text equivalent: enter height and weight to place your BMI on the adult scale.

  • Underweight: below 18.5
  • Healthy: 18.5-24.9
  • Overweight: 25.0-29.9
  • Class 1: 30.0-34.9
  • Class 2: 35.0-39.9
  • Class 3: 40.0+

BMI is an adult screening measure. It is useful for a quick height-to-weight check, but it does not diagnose health or body fat.

Formula

Metric: BMI = weight(kg) / [height(m)]²
Imperial: BMI = weight(lbs) / [height(in)]² × 703

Worked examples

Metric example

70 kg and 175 cm: height is 1.75 m, so BMI = 70 / 1.75² = 22.9.

Answer: BMI 22.9, healthy weight range.

US example

155 lb and 5 ft 9 in: height is 69 in, so BMI = 155 / 69² × 703 = 22.9.

Answer: BMI about 22.9, healthy weight range.

Category example

80 kg and 180 cm: height is 1.8 m, so BMI = 80 / 1.8² = 24.7.

Answer: BMI 24.7, still in the healthy adult range of 18.5-24.9.

Adult BMI categories

CategoryBMI rangeNotes
UnderweightBelow 18.5May warrant nutrition or medical context.
Healthy weight18.5-24.9Common adult screening range.
Overweight25.0-29.9Risk depends on broader health context.
Obesity class 130.0-34.9First adult obesity class.
Obesity class 235.0-39.9Higher adult obesity class.
Obesity class 340.0 or higherHighest adult obesity class in this convention.

Who this adult BMI calculator is for

This tool is for quick adult BMI screening, not diagnosis. Do not use it as the main guide for children or teens, pregnancy, diagnosed or suspected eating disorders, conditions that affect height, very muscular athletic builds, or any situation that needs medical assessment.

What is BMI and how is it calculated?

Body Mass Index (BMI) is a quick way to relate weight to height and estimate weight status for adults. It is not a direct measure of body fat, but it is widely used by health professionals as a screening tool. This calculator helps you find your BMI and see which general category it falls into, so you can better understand your results at a glance.

The concept is simple: your weight is divided by the square of your height. In metric units, the formula is BMI = weight(kg) / [height(m)]². In imperial units, the formula is BMI = weight(lbs) / [height(in)]² × 703. The tool supports both systems and performs the calculation instantly in your browser.

How to use the BMI calculator:

  1. Select your unit system: metric or US (lb, ft/in).
  2. Enter your height and weight. BMI appears as soon as the required values are present.
  3. Add age only if you want adult interpretation context in the result.
  4. Use the table and scale above to interpret where your result sits.

People use a BMI calculator for many practical reasons: tracking changes over time, setting general wellness goals, or preparing for a doctor visit. It can also be useful when comparing health metrics alongside other measurements such as waist circumference or body fat percentage. If you are starting a fitness plan, checking your BMI can provide a baseline to measure progress.

Waist and ethnicity context

BMI is only one screening measure. Waist circumference can add useful risk context, especially when BMI is near or above the overweight range. NHS guidance also notes that people from Asian, Black African, African-Caribbean, and Middle Eastern backgrounds may have higher health risk at lower BMI values. If that applies to you, treat BMI as a starting point and consider professional guidance.

Important caveats: BMI does not distinguish between fat and muscle, so athletes and people with muscular builds can see a higher BMI without excess body fat. Health risks at a given BMI can also vary by age, sex, ethnicity, waist circumference, medical history, and pregnancy status. For children and teens, BMI should be interpreted using age- and sex-specific growth charts with a healthcare professional.

Disclaimer: This BMI calculator is for general information and adult screening only and is not medical advice.

BMI calculator FAQ

What is a healthy BMI?

For most adults, BMI 18.5 to 24.9 is commonly described as the healthy weight range. It is a screening range, not a personal target or diagnosis.

Is BMI different for men and women?

The standard adult formula and category cutoffs are the same for men and women. Individual body composition and health risk can still differ.

Does BMI depend on age?

The formula does not use age, so this calculator can return BMI from height and weight alone. Interpretation does vary: CDC focuses adult BMI guidance on age 20 and older, while NHS frames adult BMI from age 18.

Is BMI accurate for athletes?

BMI can be misleading for very muscular athletes because it cannot separate muscle mass from fat mass.

What BMI is obesity class 1, 2, or 3?

Obesity class 1 is BMI 30.0 to 34.9, class 2 is 35.0 to 39.9, and class 3 is BMI 40.0 or higher.

Can I use BMI while pregnant?

Pregnancy changes weight and risk context, so BMI during pregnancy should be interpreted with a clinician rather than a general adult calculator.

What should I do if my BMI is high or low?

Use it as a prompt to look at the bigger picture: waist circumference, symptoms, medical history, medicines, and professional advice. Seek medical guidance if you are concerned or your BMI is very high or very low.

Sources and calculation method

This page uses the standard adult BMI formulas and common adult screening categories. It is informational and is not a medical review or diagnosis.

  • Metric formula: BMI = weight(kg) / [height(m)]².
  • US formula: BMI = weight(lb) / [height(in)]² × 703.
  • Category conventions: CDC adult BMI guidance, NIH/NHLBI BMI guidance, and WHO adult BMI category conventions, including obesity classes.
  • Context notes: NHS adult BMI guidance on age framing, waist measurement, and ethnicity-related risk context.
  • Last updated: June 24, 2026.

5 Fun Facts about BMI

Invented for statistics

BMI was created in the 1830s by Lambert Adolphe Quetelet to study population trends—not to size up individual health.

Origins

Why “×703”?

The imperial formula multiplies by 703 (technically 703.0696) to convert inches/pounds into the metric-based equation.

Unit quirk

Space stretch

Astronauts can “grow” up to 2 inches in microgravity. Same weight, taller height—so their BMI would dip on orbit.

Zero-G twist

Same BMI, different body fat

Two people with the same BMI can have very different body fat levels; women often carry 7–10% more body fat than men at the same BMI.

Hidden differences

Global cutoffs vary

Some Asian guidelines flag “overweight” starting near BMI 23 (not 25) because risks rise at lower values in those populations.

Regional view

Explore more tools