BMI Calculator
Work out your Body Mass Index in metric or imperial units, see which weight category it falls into, and check the healthy weight range for your height.
Calculate Your BMI
Enter your height and weight to see your result instantly.
<18.5 Normal
18.5–24.9 Overweight
25–29.9 Obese
30+
BMI is a general screening measure, not a diagnosis. It doesn't account for muscle mass, bone density, age, sex, or distribution of body fat. For personal health guidance, speak with a doctor or registered dietitian.
What Is BMI?
Body Mass Index (BMI) is a simple screening number calculated from a person's height and weight. It's used by doctors, researchers, and public health bodies as a quick, population-level way to flag whether a person's weight is proportionate to their height.
BMI was originally devised in the 1830s by the Belgian mathematician Adolphe Quetelet as part of a broader study of "average man" statistics, and was known for over a century as the Quetelet Index. It was later renamed Body Mass Index in the 1970s and adopted by health researchers as a fast, inexpensive alternative to more detailed body composition measurements like skinfold calipers or hydrostatic weighing.
Today it's used by organizations such as the World Health Organization (WHO) and national health services worldwide as an initial screening tool — not a diagnostic test — to categorize adults into broad weight ranges: underweight, normal weight, overweight, and obese. It remains useful mainly because it requires nothing more than a height and a weight measurement, making it fast to apply across large populations.
The BMI Formula
BMI is calculated the same way everywhere — only the starting units change depending on whether you're working in metric or imperial measurements.
Metric Formula
BMI = weight (kg) ÷ height (m)²
Divide your weight in kilograms by your height in metres, squared. Height must be converted from centimetres to metres first (divide by 100).
Example: 65 kg ÷ (1.70 m)² = 65 ÷ 2.89 = 22.5
Imperial Formula
BMI = weight (lb) ÷ height (in)² × 703
Divide weight in pounds by height in inches squared, then multiply by 703 — a conversion constant that adjusts for the pound/inch units so the result matches the metric scale.
Example: 143 lb ÷ (67 in)² × 703 = 143 ÷ 4,489 × 703 ≈ 22.4
Because BMI only uses two inputs, it's quick to compute by hand, on a spreadsheet, or with the calculator above — the calculator handles the unit conversion automatically so you never need to convert centimetres to metres or worry about the 703 constant yourself.
BMI Categories Explained
The WHO's standard adult BMI classification splits results into four broad bands. These thresholds are for adults aged 20 and over — separate age- and sex-specific charts are used for children and teenagers.
| Category | BMI Range | General Description |
|---|---|---|
| Underweight | Below 18.5 | Weight may be low relative to height; can sometimes indicate inadequate nutrition or an underlying health issue |
| Normal weight | 18.5 – 24.9 | Associated with the lowest average health risk in most population studies |
| Overweight | 25.0 – 29.9 | Weight above the range associated with lowest average risk; often prompts a general health conversation |
| Obese | 30.0 and above | Further divided into Class I (30–34.9), Class II (35–39.9), and Class III (40+) in clinical settings |
Underweight
<18.5
Normal
18.5–24.9
Overweight
25–29.9
Obese
30+
Healthy Weight Range by Height
This table shows the weight range that falls within the "Normal" BMI band (18.5–24.9) for a selection of common heights, in both metric and imperial units.
| Height | Healthy Weight Range (metric) | Healthy Weight Range (imperial) |
|---|---|---|
| 150 cm / 4'11" | 41.6 – 56.1 kg | 91.7 – 123.7 lb |
| 155 cm / 5'1" | 44.4 – 59.8 kg | 97.9 – 131.8 lb |
| 160 cm / 5'3" | 47.4 – 63.7 kg | 104.4 – 140.4 lb |
| 165 cm / 5'5" | 50.4 – 67.8 kg | 111.1 – 149.5 lb |
| 170 cm / 5'7" | 53.5 – 72.0 kg | 117.9 – 158.7 lb |
| 175 cm / 5'9" | 56.7 – 76.3 kg | 125.0 – 168.2 lb |
| 180 cm / 5'11" | 59.9 – 80.6 kg | 132.1 – 177.7 lb |
| 185 cm / 6'1" | 63.3 – 85.2 kg | 139.6 – 187.8 lb |
| 190 cm / 6'3" | 66.8 – 89.9 kg | 147.2 – 198.2 lb |
These ranges come directly from the standard BMI formula and thresholds — they aren't personalized targets, since BMI alone doesn't account for muscle mass, frame size, age, or sex. The calculator above computes the exact range for any height, not just the ones listed here.
What BMI Doesn't Measure
BMI is popular because it's simple, but that simplicity is also its main limitation. Understanding what it leaves out helps put any single BMI result in proper context.
- Muscle vs. fat. BMI can't distinguish muscle from fat. A muscular athlete can register as "overweight" on the BMI scale despite having low body fat, because muscle is denser than fat.
- Where fat is stored. Two people with the same BMI can have very different amounts of visceral (abdominal) fat, which research links more directly to cardiovascular risk than total weight alone.
- Age and sex differences. Body composition shifts with age, and men and women typically carry different proportions of muscle and fat at the same BMI — factors the formula doesn't adjust for in adults.
- Frame size and ethnicity. Some population studies suggest certain groups face elevated health risks at lower BMI thresholds than the standard cutoffs reflect, which is why some national health bodies use adjusted thresholds for specific populations.
- It's a screening tool, not a diagnosis. A single BMI number is a starting point for a conversation with a healthcare professional, not a standalone verdict on someone's health.
Because of these gaps, many clinicians pair BMI with other measurements — waist circumference, waist-to-hip ratio, or direct body composition scans — for a fuller picture, rather than relying on BMI in isolation.
Worked Examples: Step by Step
Example 1: Metric — 160 cm, 55 kg
- Convert height to metres: 160 cm ÷ 100 = 1.60 m
- Square the height: 1.60² = 2.56
- Divide weight by that figure: 55 ÷ 2.56 = 21.5
- Answer: BMI = 21.5 → Normal weight
Example 2: Imperial — 5'4" (64 in), 150 lb
- Square the height in inches: 64² = 4,096
- Weight ÷ height² × 703: 150 ÷ 4,096 × 703 ≈ 25.7
- Answer: BMI ≈ 25.7 → Overweight
Example 3: Finding a target weight for a given BMI
- Rearranged formula: weight (kg) = BMI × height (m)²
- For a BMI of 22 at 1.75 m: 22 × (1.75)² = 22 × 3.0625 = 67.4 kg
- Answer: A BMI of exactly 22 at 175 cm corresponds to about 67.4 kg
Where BMI Is Actually Used
Routine Health Checkups
Doctors and nurses often record BMI as part of a standard checkup, alongside blood pressure and other vitals, as one input among several for a broader health conversation.
Population Health Research
Public health agencies use average BMI trends across large populations to track obesity trends over time — a use case BMI was originally built for, since it works well as an aggregate statistic even where it's a blunt instrument for any single individual.
Insurance and Occupational Health
Some insurers and employers (for roles with physical fitness requirements, such as certain military or aviation positions) use BMI as one preliminary screening criterion, typically alongside other assessments.
Sports and Fitness Screening
Coaches and fitness professionals sometimes use BMI as a rough starting point, though most pair it with body composition testing for athletes, since BMI alone can misclassify muscular individuals.
Cheat Sheet: BMI at a Glance
| BMI | Category |
|---|---|
| Below 16.0 | Severely underweight |
| 16.0 – 18.4 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese Class I |
| 35.0 – 39.9 | Obese Class II |
| 40.0 and above | Obese Class III |
Use the calculator at the top of the page for your exact figure — this table is a quick lookup reference only.
Frequently Asked Questions
What is a healthy BMI?
A BMI between 18.5 and 24.9 falls within the range associated with the lowest average health risk in most adult population studies, according to WHO classification.
Is BMI accurate for athletes and bodybuilders?
Not reliably. Because BMI can't distinguish muscle from fat, muscular individuals with low body fat can register as "overweight" or "obese" despite having a healthy body composition.
Does BMI differ for men and women?
The standard adult BMI formula and categories are the same for men and women, though body composition at a given BMI often differs by sex — which is one reason BMI is treated as a screening tool rather than a precise individual measure.
Is BMI calculated differently for children?
Yes. Children and teenagers are assessed using age- and sex-specific BMI percentile charts rather than the fixed adult thresholds, since healthy body composition changes significantly during growth.
What is the 703 number in the imperial BMI formula?
703 is a conversion constant that adjusts the pound/inch based calculation so it produces the same numeric result as the metric kilogram/metre formula.
Can BMI be too low even without being underweight in appearance?
Yes — BMI is a numeric ratio and doesn't reflect appearance directly. Frame size, muscle mass, and individual body composition all affect how a given BMI number actually looks on a person.
Should I make health decisions based on BMI alone?
BMI is best treated as one general screening data point. For any personal health decision, it's best discussed alongside other measures and a healthcare professional's assessment rather than used on its own.
How often should BMI be checked?
There's no fixed rule — many people see it recorded during routine doctor visits. It's not something that needs frequent independent tracking, since it shifts slowly and reflects long-term patterns rather than day-to-day changes.