Loading tool…
How to Calculate BMI Manually
Body Mass Index is a simple ratio of weight to height squared. You can calculate it with a calculator or even pen and paper.
- Measure your weight in kilograms. If you have pounds, divide by 2.205 to convert.
- Measure your height in metres. If you have centimetres, divide by 100. If you have feet and inches, multiply feet by 0.3048 and inches by 0.0254, then add.
- Square your height: height × height.
- Divide your weight by the squared height: BMI = weight ÷ height².
- Compare the result to standard categories: under 18.5 is underweight, 18.5–24.9 is normal, 25–29.9 is overweight, 30+ is obese.
BMI Formula
Metric: BMI = weight (kg) ÷ height² (m²). Imperial: BMI = (weight (lbs) × 703) ÷ height² (in²). A BMI of 22 means you weigh 22 kg for every square metre of height. The World Health Organization defines Normal as 18.5–24.9. Note that BMI does not distinguish between muscle and fat — athletes with high muscle mass can have an 'overweight' BMI while being perfectly healthy.
Why Use Our BMI Calculator?
- Your weight stays private — no server call, no health data stored, no tracking cookies.
- Visual gauge — instantly see where you fall on the spectrum with a color-coded arc.
- Metric and Imperial — switch freely between kg/cm and lbs/ft-in; the tool converts for you.
- Ideal weight range — see the weight range that corresponds to a 'normal' BMI for your height.
- Educational only — we include a clear medical disclaimer because BMI has known limitations.
Frequently Asked Questions
What is a healthy BMI?
According to the WHO, a BMI between 18.5 and 24.9 is classified as 'normal weight' for adults. However, optimal BMI can vary by age, sex, ethnicity, and muscle mass. It is one screening tool, not a diagnosis.
Is BMI accurate for athletes or bodybuilders?
Not always. BMI does not differentiate between lean muscle and body fat. A muscular person may register as 'overweight' despite having low body fat. For athletes, body-fat percentage or a DEXA scan is a better measure.
Should children use this BMI calculator?
Children and teens should use age-and-sex-specific BMI percentile charts (CDC growth charts). This calculator uses the standard adult formula and is not designed for anyone under 18.
How is ideal weight range calculated?
We solve the BMI formula in reverse: Ideal weight = BMI × height². Using BMI boundaries of 18.5 and 24.9 gives the lower and upper bounds of the 'normal' range for your height.
What are the limitations of BMI?
BMI ignores body composition, bone density, age, sex, and ethnicity. It can overestimate fat in muscular people and underestimate it in older adults who have lost muscle. It should be used alongside other health metrics, not as a standalone diagnosis.
Is my weight data stored?
No. The calculation happens entirely in your browser. ZeroKit never sees, stores, or transmits your weight, height, or BMI result.