BMI Calculator
Body Mass Index and healthy weight range.
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Body Mass Index is a quick screening measure that relates your weight to your height. Enter your details and this calculator gives your BMI, the category it falls into, and the healthy weight range for someone of your height.
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BMI — The Quick Answer
BMI is your weight in kilograms divided by your height in metres squared. A BMI of 18.5–24.9 is classed as a healthy weight by the WHO.
Shortcut
BMI = weight (kg) ÷ height (m)²
WHO BMI categories
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity (class I) |
| 35.0 – 39.9 | Obesity (class II) |
| 40.0 and above | Obesity (class III) |
BMI doesn't measure body fat directly or tell muscle from fat. A muscular athlete can have a 'high' BMI while being very lean, so it's a screening tool, not a diagnosis.
How It Works
The Formula
Weight over height squared
Formula
BMI = weight (kg) ÷ height (m)² Imperial: BMI = 703 × weight (lb) ÷ height (in)²
Variables
Body weight
In kilograms (or pounds, converted automatically).
Body height
In metres, squared. The squaring is why tall people aren't unfairly penalised.
Imperial factor
The constant that makes the pounds-and-inches version give the same number as the metric formula.
Note: BMI was designed for populations, not individuals. It's useful for spotting trends and risk at scale, but for a personal picture it's best combined with waist measurement, body-fat percentage, and a clinician's assessment.
Worked Example: BMI for a 175 cm, 70 kg Adult
The metric formula, step by step
Convert height to metres
175 cm ÷ 100 = 1.75 m
Square the height
1.75 × 1.75 = 3.0625 m²
Divide weight by height squared
70 ÷ 3.0625 = 22.9
Read the category
22.9 sits inside 18.5–24.9 → healthy weight range
Reference Guide
| BMI range | Category | What it suggests |
|---|---|---|
| Below 18.5 | Underweight | May signal undernutrition or an underlying issue |
| 18.5 – 24.9 | Healthy weight | Lowest weight-associated risk for most adults |
| 25.0 – 29.9 | Overweight | Raised risk — but build and waist size matter |
| 30.0 – 34.9 | Obesity (class I) | Moderately increased health risk |
| 35.0 – 39.9 | Obesity (class II) | High health risk |
| 40.0 and above | Obesity (class III) | Very high health risk |
What Your BMI Result Actually Means
How to read your number — and when it can mislead
A BMI under 18.5 can indicate you're carrying too little weight for your height, which may relate to nutrition, illness, or simply a naturally slight build. If it's unexpected or you're losing weight without trying, it's worth discussing with a doctor.
Best for: Checking in with a clinician if the result is unexpected
This is the band the WHO associates with the lowest weight-related health risk for most adults. It's a reassuring sign, but remember BMI says nothing about fitness, diet quality, or where you carry fat — a 'healthy' BMI with a large waist still warrants attention.
Best for: A general green light, best confirmed with waist and activity
Statistically linked to somewhat higher risk, but this band is where BMI most often misreads individuals. Muscular people frequently land here while being lean. Pair it with waist circumference and body-fat percentage before drawing conclusions.
Best for: Looking at the fuller picture rather than the number alone
Associated with meaningfully higher risk of conditions like type 2 diabetes and heart disease at a population level. It's a prompt to speak with a healthcare professional about a personalised assessment — not a diagnosis in itself.
Best for: A conversation with a doctor about individual risk and next steps
BMI can't tell muscle from fat, ignores where fat sits, and uses the same thresholds regardless of age, sex, or ethnicity. Athletes, older adults who've lost muscle, and some ethnic groups are the most commonly misclassified. It's a screening tool, not a verdict.
Best for: Combining BMI with waist-to-hip ratio and body-fat percentage
What Is BMI?
Body Mass Index (BMI) is a simple screening number that relates your weight to your height — your weight in kilograms divided by your height in metres squared. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet to describe populations, and was adopted as a quick weight screen for individuals in the 1970s. Because it uses only height and weight, BMI is fast and free to calculate but cannot distinguish muscle from fat or show where fat is stored. That makes it a useful first indicator rather than a diagnosis — most accurate for groups, and best paired with other measures for any one person. The 'Learn more' guide beside this tool traces where BMI came from and why it both works and misleads.
Key Features
💡 Pro Tips
- →Measure your height without shoes and weigh yourself at a consistent time of day (first thing in the morning is ideal) so readings are comparable over time.
- →If you're muscular or train with weights, don't rely on BMI alone — a lean, muscular build often reads as 'overweight'. Check your waist-to-hip ratio and body-fat percentage too.
- →Watch the trend rather than a single figure — normal day-to-day swings in water and food can move your BMI by a few tenths with no real change in body composition.
- →For children and teenagers, the adult categories don't apply; paediatric assessment uses BMI-for-age percentiles instead.
Common Mistakes
Treating BMI as a body-fat percentage
BMI only compares weight to height. It can't separate fat from muscle or bone, so a very muscular person and a higher-fat person at the same BMI are scored identically. Body-fat percentage measures what BMI cannot.
Applying adult BMI categories to children
Children and teens are assessed using age- and sex-specific BMI-for-age percentiles, not the fixed adult 18.5–24.9 bands. Using adult categories for a child gives a meaningless result.
Mixing measurement systems
Entering height in centimetres but weight in pounds (or any mix) produces a nonsense figure. Stay within one system — the calculator handles the conversions inside metric or imperial for you.
Reading one number as the final word on your health
BMI was built as a population screening tool. For a personal picture it should sit alongside waist measurement, body-fat percentage, activity level, and ideally a clinician's assessment — not stand alone.
Research & Citations
Sources and standards referenced for the factual claims on this page
- [1]
World Health Organization (2000). Obesity: Preventing and Managing the Global Epidemic. WHO Technical Report Series 894.
Source of the adult BMI classification thresholds used here.
View source - [2]
National Heart, Lung, and Blood Institute (NHLBI) (1998). Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults. National Institutes of Health.
Adult overweight and obesity categories and the stated limitations of BMI.
View source - [3]
Centers for Disease Control and Prevention (CDC) (2022). About Adult BMI. CDC.gov.
BMI formula, categories, and guidance on its use as a screening tool.
View source
This calculator is a reference tool, not a substitute for professional advice. Verify important health, financial, legal, or safety decisions with a suitably qualified professional.
Last updated: October 8, 2026
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Frequently Asked Questions
How do I calculate BMI?
Divide your weight in kilograms by your height in metres squared. Someone 1.75 m tall weighing 70 kg has a BMI of 70 ÷ (1.75 × 1.75) = 22.9, which is in the healthy range.
What is a healthy BMI?
The WHO classes 18.5 to 24.9 as a healthy weight. Below 18.5 is underweight, 25 to 29.9 is overweight, and 30 or above is in the obesity range. These are general bands, not personal targets.
Is BMI accurate?
It's a useful screening tool but has real limitations. It doesn't distinguish muscle from fat, so athletes may be misclassified, and it can under- or over-estimate risk in older adults, children, and some ethnic groups. Pair it with other measures for a fuller picture.
Does BMI differ for men and women?
The BMI formula and the standard adult categories are the same for men and women. However, women naturally carry more body fat at a given BMI, so BMI alone doesn't capture body composition differences between the sexes.