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BMI Calculator

Body mass index calculator

By UtilityKit Team · Reviewed for accuracy by UtilityKit Editors · Updated May 2026

About BMI Calculator

Body Mass Index (BMI) is a simple screening number that tells you whether your weight falls within a healthy range for your height. Calculated as weight in kilograms divided by height in metres squared — or using an equivalent imperial formula — a BMI between 18.5 and 24.9 is considered normal by the World Health Organization. Values below 18.5 indicate underweight, 25–29.9 is overweight, and 30 or above falls into obesity classes I, II, or III. This free BMI calculator supports both metric (kg and cm) and imperial (lb, feet, and inches) inputs, so you can skip any manual conversion. Beyond the number itself, the tool shows you the healthy weight range for your exact height — a practical target band rather than just a category label. A colour-coded indicator makes it instantly clear where you stand.

Why use BMI Calculator

WHO-standard Categories

Results map directly to the four World Health Organization weight tiers — Underweight, Normal, Overweight, and Obese Classes I to III — so the output matches what doctors, insurers, and screening forms reference.

Metric and Imperial in One Tool

Switch between kilograms and centimetres or pounds and feet-inches without doing any pre-conversion. Both unit modes use the correct formula variants so precision is maintained regardless of the system you choose.

Healthy Weight Band for Your Height

Instead of just showing a number, the calculator displays the actual weight range that corresponds to a normal BMI for your specific height, giving you a concrete, actionable target rather than an abstract score.

Visual Category Indicator

A colour-coded progress bar shows exactly where your BMI sits across the full spectrum from underweight to obese, making the result far easier to interpret at a glance than reading a raw decimal number.

Complete Privacy

Your height and weight are processed entirely in your browser using JavaScript. No data is sent to any server, stored in a database, or linked to your identity — health details stay on your own device.

Mobile-friendly Layout

Large input fields, clear labels, and touch-optimised controls ensure the calculator is fully usable on a smartphone screen, so you can check your BMI quickly at a gym, clinic, or pharmacy.

How to use BMI Calculator

  1. Select your preferred unit system: metric (kg and cm) or imperial (lb, feet, and inches)
  2. Enter your current body weight in the weight field
  3. Enter your height — for imperial, use the feet and inches fields separately
  4. Click Calculate or watch the result update live as you type
  5. Read your BMI value and the WHO category it falls into (Underweight, Normal, Overweight, or Obese)
  6. Check the healthy weight range displayed for your height to see a concrete target band

When to use BMI Calculator

  • When you want a quick, no-equipment snapshot of whether your weight is in the healthy range for your height
  • When filling in health insurance, visa medical, or school admission forms that ask for your BMI or weight category
  • When a doctor asks you to track BMI changes over weeks or months during a diet or fitness programme
  • When helping a family member or friend understand their weight category without clinical equipment
  • When comparing your BMI before and after a period of weight gain or loss to measure progress
  • When you want to know the exact weight range you should aim for given your current height

Examples

Adult metric

Input: Weight: 70 kg, Height: 175 cm

Output: BMI 22.9 — Normal weight (WHO range 18.5–24.9)

Adult imperial overweight

Input: Weight: 180 lb, Height: 5 ft 9 in

Output: BMI 26.6 — Overweight (WHO range 25–29.9)

Healthy range target lookup

Input: Height: 168 cm (no weight entered)

Output: Healthy weight band: 52.2 kg to 70.3 kg

Tips

  • BMI is a screening number, not a medical diagnosis — pair it with waist circumference or body-fat percentage for a fuller picture
  • Athletes and strength trainers often score 'Overweight' on the BMI scale because dense muscle pushes the number up without any health risk
  • If you are of South or East Asian descent, the locally recommended overweight threshold is 23 rather than 25 — worth knowing when interpreting your result
  • Re-measure monthly rather than daily; normal daily weight fluctuations of 1–2 kg can make BMI appear to change when nothing meaningful has shifted
  • If your BMI sits outside the normal range, consult a clinician before making dramatic dietary or exercise changes — context matters more than a single number

Frequently Asked Questions

Is BMI accurate for muscular athletes?▾
No — BMI cannot distinguish between muscle and fat. A bodybuilder or rugby player may score 'Overweight' or even 'Obese' purely because muscle is denser than fat. For athletic populations, waist circumference or body-fat percentage measurements are more informative than BMI alone.
What BMI is considered healthy by the WHO?▾
The World Health Organization defines the normal or healthy range as 18.5 to 24.9. A BMI below 18.5 is underweight, 25–29.9 is overweight, and 30 or above is obese. These thresholds apply to adults and are not age-adjusted.
Should children use the adult BMI formula?▾
No — children and teenagers require age-and sex-specific BMI percentile charts rather than fixed adult cut-offs. The standard adult BMI scale is only appropriate for individuals aged 18 and over. Most paediatric guidelines use growth reference charts instead.
Why does BMI ignore body-fat percentage?▾
BMI was designed as a simple population-screening tool that requires only two easy-to-measure inputs: weight and height. Measuring actual body-fat percentage requires methods like DEXA scanning, hydrostatic weighing, or skinfold calipers. BMI deliberately sacrifices precision for accessibility and speed.
How is BMI calculated in metric versus imperial?▾
In metric, BMI equals weight in kilograms divided by height in metres squared: BMI = kg / m². In imperial, the equivalent formula is BMI = (weight in pounds × 703) divided by height in inches squared. Both give identical results when the inputs represent the same person.
What BMI value indicates obesity?▾
A BMI of 30 or above is classified as obesity. The WHO further divides obesity into Class I (30–34.9), Class II (35–39.9), and Class III, sometimes called severe or morbid obesity, at 40 and above. Each class is associated with progressively higher health risk.
Does BMI vary by ethnicity or age?▾
Research shows that people of Asian descent tend to carry higher metabolic risk at lower BMI values. Several Asian health authorities use an overweight threshold of 23 rather than 25. Older adults may also retain more fat at the same BMI as younger adults. If these factors apply to you, discuss adjusted thresholds with a clinician.
Should I see a doctor based on my BMI alone?▾
BMI is a screening signal, not a diagnosis. If your result falls outside the normal range, it is a reasonable prompt to have a conversation with a healthcare professional, but a single BMI number is not sufficient grounds for alarm or for starting any medical intervention on its own.

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Glossary

Body Mass Index (BMI)
A numerical value derived from a person's weight and height, calculated as weight in kg divided by height in metres squared. It is used as a population-level screening proxy for body fatness.
WHO Classification
The World Health Organization's four-tier system that labels BMI ranges as Underweight (below 18.5), Normal (18.5–24.9), Overweight (25–29.9), and Obese (30 and above).
Underweight
A BMI below 18.5, indicating that a person's body weight may be too low relative to their height. It is associated with risks such as nutrient deficiency, low bone density, and weakened immune function.
Overweight
A BMI between 25 and 29.9. It indicates a weight above the normal range but below the obesity threshold and is associated with increased risk of metabolic and cardiovascular conditions.
Obesity Class I / II / III
Sub-categories of obesity where Class I is BMI 30–34.9, Class II is 35–39.9, and Class III (severe or morbid obesity) is 40 and above. Higher classes carry progressively greater health risk.
Body Composition
The proportion of fat mass versus lean mass (muscle, bone, water, and organs) in the body. BMI does not measure body composition directly; methods like DEXA or skinfold calipers are required for that.