How it works
Body mass index compares your weight with your height: BMI = weight (kg) ÷ height (m)². In pounds and inches the same thing is written BMI = 703 × lb ÷ in².
- Convert your height to metres — 5 ft 9 in is 1.75 m.
- Square it: 1.75 × 1.75 = 3.0625.
- Divide your weight in kilograms by that number.
| BMI | Category |
|---|---|
| Under 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
BMI says nothing about where fat sits or how much of your weight is muscle, which is why a lean athlete can score "overweight". Body fat percentage and waist measurement describe health risk better, and the ideal weight calculator turns the healthy BMI band back into a weight range.
Examples
Two worked examples:
- 70 kg at 1.75 m: 70 ÷ 3.0625 = 22.9 — healthy weight.
- 85 kg at 1.68 m: 85 ÷ 2.8224 = 30.1 — overweight.
For the same height, every 3 kg moves BMI by about 1 point, so small changes rarely shift the category.
These formulas are population averages published in peer-reviewed research. They cannot see your muscle mass, frame or medical history, so treat the result as a starting point and talk to a doctor or dietitian before making big changes.
BMI Calculator: frequently asked questions
- How is BMI calculated?
- Divide weight in kilograms by height in metres squared. In pounds and inches: BMI = 703 × lb ÷ in².
- What is a healthy BMI?
- 18.5 to 24.9 for adults. Some Asian health bodies use lower cut-offs (overweight from 23).
- What are the limits of BMI?
- BMI doesn't distinguish muscle from fat or show where fat is stored. Athletes can be “overweight” by BMI; waist size and body fat % add context.