What is BMI?
Body Mass Index (BMI) is a simple numeric measure derived from your height and weight. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, it's used globally by doctors, insurers, and health organizations as a quick screening tool for weight categories.
The formula is straightforward:
- Metric — BMI = weight (kg) ÷ height (m)²
- Imperial — BMI = (weight (lbs) × 703) ÷ height (in)²
WHO BMI Categories
The World Health Organization defines these ranges for adults:
- Underweight — BMI below 18.5
- Normal weight — BMI 18.5 to 24.9
- Overweight — BMI 25.0 to 29.9
- Obese — BMI 30.0 and above
How the ToolKnit BMI Calculator Works
Enter your height and weight, toggle between metric (cm/kg) and imperial (ft-in/lbs), and see your BMI result instantly. The tool includes:
- Visual BMI scale — A color-coded bar showing exactly where you fall from underweight to obese
- Category label — Clear WHO classification with your exact BMI number
- Healthy weight range — Shows the weight range that would give you a normal BMI at your height
- Real-time calculation — Updates as you type, no submit button needed
Limitations of BMI
BMI is a useful screening tool but has known limitations:
- It doesn't distinguish between muscle and fat — athletes may score as "overweight"
- It doesn't account for body fat distribution (waist-to-hip ratio matters too)
- It may underestimate risk in older adults who have lost muscle mass
- Different ethnic groups may have different risk thresholds
BMI works best as a starting point, not a diagnosis. Always consult a healthcare professional for a complete health assessment.
Privacy
Your height and weight data are never sent to any server. The calculation runs entirely in your browser using JavaScript. Close the tab and everything is gone.
Try it now: Open BMI Calculator — free, instant, no sign-up required.