How to use the bmi calculator
- Select Metric or Imperial from the unit system dropdown.
- Enter your weight in kilograms or pounds.
- Enter your height in centimeters or inches.
- Read your calculated body mass index and the corresponding WHO category below.
How the body mass index formula works
The formula for body mass index is straightforward. In metric units, divide your weight in kilograms by your height in meters squared:
BMI = weight (kg) / height (m)2
For example, a person weighing 70 kg who is 175 cm tall (1.75 m) has a BMI of 70 / (1.75 x 1.75) = 70 / 3.0625 = 22.86.
When using imperial units, the same ratio requires a conversion factor because pounds and inches do not cancel out cleanly. The adjusted formula is:
BMI = (weight (lb) / height (in)2) x 703
The constant 703 comes from converting pounds to kilograms (1 lb = 0.45359237 kg) and inches to meters (1 in = 0.0254 m), then simplifying the fraction.
WHO BMI categories
The World Health Organization defines standard ranges for adults aged 20 and older. This bmi calculator uses those exact thresholds to label your result.
| Category | BMI range (kg/m2) |
|---|---|
| Underweight | Below 18.5 |
| Normal weight | 18.5 to 24.9 |
| Overweight | 25.0 to 29.9 |
| Obese | 30.0 and above |
These boundaries were established through large population studies linking body mass index ranges to health risks such as cardiovascular disease and type 2 diabetes. They serve as a screening tool rather than a diagnostic measure.
Reference table of common values
The table below shows what different weights look like at a fixed height of 170 cm (about 5 feet 7 inches). It helps illustrate how quickly the body mass index changes with each additional kilogram.
| Weight (kg) | Weight (lb) | BMI | Category |
|---|---|---|---|
| 50 | 110.2 | 17.30 | Underweight |
| 60 | 132.3 | 20.76 | Normal weight |
| 70 | 154.3 | 24.22 | Normal weight |
| 80 | 176.4 | 27.68 | Overweight |
| 90 | 198.4 | 31.14 | Obese |
At 170 cm, every extra 5 kg adds roughly 1.73 points to your BMI. You can use this bmi calculator with your own height to build a similar personal reference.
Limitations you should know
Body mass index does not distinguish between muscle, fat, bone, and water. A competitive weightlifter may register as obese on this scale despite having very low body fat. For a more detailed picture of body composition, consider pairing this result with a body fat calculator.
The standard WHO categories apply to adults over 20. Children and teenagers are evaluated against age- and sex-specific percentiles, not these fixed cutoffs. Pregnant individuals should also use clinical guidance rather than a general bmi calculator.
Ethnic differences matter too. Research suggests that some Asian populations face elevated metabolic risk at lower thresholds, prompting regional health authorities to define overweight starting at 23 rather than 25. Always interpret your number alongside other markers like waist circumference, blood pressure, and lipid panels.
Using BMI alongside other health metrics
Your body mass index is one data point among many. If your goal is to manage energy balance, you might next estimate your daily calorie needs with a tdee calculator, which factors in activity level and basal metabolic rate.
For fitness planning, knowing your healthy weight range lets you set realistic targets. Someone who is 180 cm tall and wants to stay within the normal BMI band (18.5 to 24.9) should aim for a weight between about 60 kg and 81 kg. Pairing that target with a water intake calculator and a protein intake calculator gives a practical daily framework.
Athletes tracking strength progress often monitor their one rep max alongside weight changes to ensure they are gaining muscle rather than just adding mass.
An estimate, not medical advice
The bmi calculator uses a standard published formula, and formulas describe averages, not you. Age, body composition, medical conditions, medication, pregnancy and training history can all move the real number. Use the result as a starting point for planning, and speak to a doctor, dietitian or qualified coach before making health, diet or training decisions, especially if you have a medical condition.
Worked examples
Each result below was produced by the same code the tool runs, so you can check your own numbers against them.
| Example | Input | Result |
|---|---|---|
| Average adult (metric) | Unit system: metric; Weight (kg): 70; Height (cm): 175 | BMI: 22.86; Category: Normal weight |
| Average adult (imperial) | Unit system: imperial; Weight (lb): 154; Height (inches): 69 | BMI: 22.74; Category: Normal weight |
| Tall and lean | Unit system: metric; Weight (kg): 65; Height (cm): 190 | BMI: 18.01; Category: Underweight |
| Short and heavy | Unit system: imperial; Weight (lb): 200; Height (inches): 63 | BMI: 35.43; Category: Obese |
FAQ
What is a good BMI?
For most adults, a body mass index between 18.5 and 24.9 is classified as normal weight by the World Health Organization. Values outside this range may warrant a conversation with a healthcare provider.
Is BMI accurate for muscular people?
No. Because the formula only uses total weight and height, it cannot tell muscle apart from fat. Athletes with high muscle mass often score in the overweight or obese range despite being lean.
Does this bmi calculator work for children?
This tool applies the adult WHO formula. Children and teens require age- and sex-specific percentile charts, so consult a pediatrician for anyone under 20.
Why does the imperial formula multiply by 703?
The factor 703 converts pounds per square inch into kilograms per square meter. It is derived from the exact definitions: 1 pound equals 0.45359237 kg and 1 inch equals 0.0254 m.
Can I use this if I am pregnant?
Standard BMI categories are not designed for pregnancy. Weight gain recommendations during pregnancy depend on your pre-pregnancy BMI and should come from your doctor.
Is my data stored or sent anywhere?
No. The calculation happens entirely inside your browser. Nothing you type is transmitted to any server.