Body Mass Index, or BMI, is one of the most widely used screening tools for body weight. It's quick, free, and gives a rough sense of whether your weight is in a healthy range for your height. Here's what it means and how to work it out.
The BMI formula
BMI compares your weight to your height:
- Metric: BMI = weight (kg) ÷ height (m)²
- Imperial: BMI = 703 × weight (lb) ÷ height (in)²
Step by step (metric)
- Convert your height to metres: 175 cm = 1.75 m.
- Square it: 1.75 × 1.75 = 3.0625.
- Divide your weight by that number: 70 kg ÷ 3.0625 ≈ 22.9.
Step by step (imperial)
- Convert your height to inches: 5 ft 7 in = 67 in.
- Square it: 67 × 67 = 4,489.
- Multiply your weight by 703: 154 lb × 703 = 108,262.
- Divide: 108,262 ÷ 4,489 ≈ 24.1.
Working backwards: healthy weight for your height
Multiply your height squared by 18.5 and 24.9 to get the healthy weight range. At 1.75 m that's 3.0625 × 18.5 ≈ 56.7 kg to 3.0625 × 24.9 ≈ 76.3 kg.
Rather than doing the math by hand, you can use our BMI Calculator — just enter your height and weight (metric or imperial) and it shows your BMI, category and healthy weight range instantly.
Standard BMI categories
For most adults, the World Health Organization ranges are:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
What BMI does not tell you
BMI is a useful starting point, but it has real limitations:
- It doesn't distinguish muscle from fat. Athletes with high muscle mass can register as "overweight" despite being very fit.
- It ignores fat distribution. Where you carry weight matters for health.
- It varies by age, sex and ethnicity. The standard ranges don't fit everyone equally.
Think of BMI as a quick flag, not a diagnosis. For a full picture, combine it with other measures and professional advice.
Track changes over time
BMI is most useful as a trend. Checking it occasionally as your weight changes tells you more than a single reading. Bookmark the BMI Calculator and recalculate whenever you want a quick check.
Related tools
If you're working on broader health and finance goals, you might also find these handy:
- Percentage Calculator — for tracking progress percentages
- Age Calculator — exact age in years, months and days
Disclaimer: BMI is a general screening tool and not a substitute for medical advice. Always consult a healthcare professional for personal health decisions.