BMI Calculator
Calculate your Body Mass Index and discover your weight category using metric or imperial units.
What is BMI Calculator?
A BMI calculator is a fast screening tool that tells you whether your weight is in a healthy range for your height. Body Mass Index (BMI) is a single number — weight divided by height squared — used by doctors and the World Health Organization (WHO) to classify adults as underweight, normal weight, overweight, or obese. Although BMI does not measure body fat directly, research consistently links BMIs outside the normal range to higher risks of heart disease, type 2 diabetes, and high blood pressure.
This free BMI calculator works with both metric and imperial units and reports five results at once: your BMI, your WHO weight category, the healthy weight range for your height, BMI Prime (your BMI relative to the upper healthy limit of 25), and the Ponderal Index, an alternative ratio that is more consistent for very tall or very short people.
How to Use
- Select your preferred unit system: Metric (centimeters and kilograms) or Imperial (feet, inches, and pounds).
- Enter your height and weight into the corresponding fields.
- Click Calculate BMI to see your BMI, category, healthy weight range, BMI Prime, and Ponderal Index.
- Use the color gauge bar to visualize where your BMI falls across the standard categories.
The healthy weight range shows the lightest and heaviest weights that still count as a normal BMI for your height.
BMI Formula
BMI uses the same calculation for men and women; only the units differ.
Metric formula
BMI = weight (kg) ÷ height² (m²)
Imperial formula
BMI = 703 × weight (lb) ÷ height² (in²)
- weight — your body mass in kilograms (metric) or pounds (imperial).
- height — your height in meters (metric) or inches (imperial). Because height is squared, always convert feet to inches first: 5'9" equals 69 inches.
- 703 — the conversion factor that makes the imperial formula produce the same result as the metric one (the exact value is about 703.07).
Worked Examples
Example 1: metric — 70 kg and 1.75 m
Step 1: square the height: 1.75 × 1.75 = 3.0625.
Step 2: divide the weight by that result: 70 ÷ 3.0625 ≈ 22.9.
A BMI of 22.9 falls in the normal range. For this height, the healthy weight window is 56.7–76.3 kg, BMI Prime is 22.86 ÷ 25 ≈ 0.91, and the Ponderal Index is 70 ÷ 1.75³ ≈ 13.1 kg/m³.
Example 2: imperial — 154 lb and 5'9"
Step 1: convert the height to inches: 5 × 12 + 9 = 69 in.
Step 2: apply the formula: 703 × 154 ÷ (69 × 69) = 108,262 ÷ 4,761 ≈ 22.7.
A BMI of 22.7 is also comfortably within the normal range; both formulas give equivalent results for the same measurements.
WHO BMI Categories for Adults
The World Health Organization classification below is the standard used worldwide for adults aged 20 and over.
| Category | BMI (kg/m²) |
|---|---|
| Underweight | Below 18.5 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese Class I | 30.0 – 34.9 |
| Obese Class II | 35.0 – 39.9 |
| Obese Class III | 40.0 and above |
Some clinical guidelines split underweight further into severe (below 16), moderate (16–16.99), and mild (17–18.49) thinness.
BMI Chart for Common Heights and Weights
Find your height in the left column, then read across to the weight closest to yours for an approximate BMI.
| Height | 100 lb | 120 lb | 140 lb | 160 lb | 180 lb | 200 lb | 220 lb |
|---|---|---|---|---|---|---|---|
| 5'0" (152 cm) | 19.5 | 23.4 | 27.3 | 31.2 | 35.1 | 39.1 | 43.0 |
| 5'3" (160 cm) | 17.7 | 21.3 | 24.8 | 28.3 | 31.9 | 35.4 | 39.0 |
| 5'6" (168 cm) | 16.1 | 19.4 | 22.6 | 25.8 | 29.0 | 32.3 | 35.5 |
| 5'9" (175 cm) | 14.8 | 17.7 | 20.7 | 23.6 | 26.6 | 29.5 | 32.5 |
| 6'0" (183 cm) | 13.6 | 16.3 | 19.0 | 21.7 | 24.4 | 27.1 | 29.8 |
| 6'3" (190 cm) | 12.5 | 15.0 | 17.5 | 20.0 | 22.5 | 25.0 | 27.5 |
As a rule of thumb, a healthy weight (BMI 18.5–24.9) spans roughly 95–128 lb at 5'0", 115–154 lb at 5'6", and 136–184 lb at 6'0".
Limitations of BMI
BMI is a population-level screening tool, not a diagnosis, and it can misclassify individuals in several common situations:
- Athletes and muscular builds: muscle is denser than fat, so a muscular person can register as overweight or even obese while carrying very little body fat.
- Older adults: muscle mass declines with age, so an older adult can sit in the normal range while carrying more fat than the number suggests.
- Fat distribution: BMI says nothing about where fat is stored. Abdominal (visceral) fat carries more health risk, which is why waist circumference is often measured alongside BMI.
- Children and teens: for anyone under 20, BMI must be interpreted using age- and sex-specific percentile charts, not the fixed adult cutoffs above.
- Pregnancy: BMI is not meaningful during pregnancy; pre-pregnancy BMI is used instead for weight-gain guidance.
If your result surprises you, pair it with a body fat percentage or waist measurement, and talk to a healthcare professional before changing your diet or exercise routine.
Frequently Asked Questions
What is a healthy BMI range?
A healthy BMI for most adults is between 18.5 and 24.9. Below 18.5 is considered underweight, 25 to 29.9 is overweight, and 30 or above is obese, split into Class I (30–34.9), Class II (35–39.9), and Class III (40 and above). Ideal weight also depends on age and muscle mass, so treat BMI as a starting point, not a diagnosis.
How do I calculate BMI by hand?
Divide your weight in kilograms by the square of your height in meters: BMI = kg ÷ m². With pounds and inches, multiply your weight by 703 first: BMI = 703 × lb ÷ in². For example, 154 lb at 5'9" (69 in) gives 703 × 154 ÷ 4,761 ≈ 22.7.
Is BMI accurate for everyone?
BMI is a useful screening tool but does not account for muscle mass, bone density, age, or fat distribution. Athletes with high muscle mass may have a higher BMI without excess body fat, while older adults can have a normal BMI despite having lost muscle. Waist circumference and body fat percentage are helpful complements.
What does BMI Prime mean?
BMI Prime is your BMI divided by 25 (the upper limit of the healthy range). A value of 1.0 means you are at the top of the healthy range; values above 1.0 indicate overweight status. For example, a BMI of 22.5 gives a BMI Prime of 0.90.
Is BMI calculated differently for men and women?
No. The BMI formula is identical for men and women. However, at the same BMI, women typically carry more body fat than men, and health risks can differ, so interpretation should also consider sex, age, and body composition.
Can I use this calculator for children?
This calculator is designed for adults aged 20 and over. For children and teens, BMI is calculated the same way but interpreted using age- and sex-specific percentile charts, such as the CDC growth charts, rather than the fixed adult categories.
What is the difference between BMI and body fat percentage?
BMI estimates weight status from height and weight alone, while body fat percentage measures the actual share of fat in your body. Two people can share the same BMI yet have very different body fat levels, which is why athletes often pair BMI with a direct body fat measurement.