Body Mass Index (BMI) is one of the most widely used health screening tools. Whether you want to understand your results or simply learn the math behind it, calculating BMI manually takes less than a minute.
What Is BMI?
BMI is a numerical value derived from your height and weight. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and is used today by doctors, researchers, and fitness professionals as a quick screening tool for weight-related health risks.
BMI does not directly measure body fat β it estimates it from height and weight. That said, it correlates well with body fat for most adults and is accepted by the WHO, CDC, and NHS as a standard health metric.
The BMI Formula
There are two versions depending on the unit system you use:
Metric Formula (kg and cm)
Note: Convert height to metres first (e.g. 175 cm = 1.75 m)
Imperial Formula (lbs and inches)
Step-by-Step Example (Metric)
Let’s calculate BMI for someone who is 70 kg and 175 cm tall:
- Convert height to metres: 175 cm Γ· 100 = 1.75 m
- Square the height: 1.75 Γ 1.75 = 3.0625
- Divide weight by height squared: 70 Γ· 3.0625 = 22.9
- Result: BMI = 22.9 (Normal weight β )
Step-by-Step Example (Imperial)
For someone weighing 154 lbs and 5’9β³ (69 inches) tall:
- Square the height: 69 Γ 69 = 4,761
- Divide weight by result: 154 Γ· 4,761 = 0.03234
- Multiply by 703: 0.03234 Γ 703 = 22.7
- Result: BMI = 22.7 (Normal weight β )
BMI Categories (WHO Standard)
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Increased risk |
| 18.5 β 24.9 | Normal Weight | Lowest risk |
| 25.0 β 29.9 | Overweight | Moderate risk |
| 30.0 β 34.9 | Obese Class I | High risk |
| 35.0 β 39.9 | Obese Class II | Very high risk |
| 40.0 and above | Obese Class III | Extremely high risk |
Limitations of BMI
BMI is a useful starting point, but it has known limitations:
- Athletes β Muscle weighs more than fat. A bodybuilder may have a “overweight” BMI despite having very low body fat.
- Elderly β Older adults often have more fat but less muscle, making BMI less accurate.
- Ethnic variation β Some health organisations recommend lower thresholds for Asian populations (β₯23 as overweight).
- Pregnancy β Standard BMI does not apply during pregnancy.
For a more complete picture, consider measuring body fat percentage or waist circumference alongside BMI.
Use the Free BMI Calculator
Skip the manual calculation β use our free BMI calculator below for instant results in both metric and imperial units: