While our free BMI calculator gives you an instant result, understanding how the calculation works helps you interpret your number with confidence. Here is a complete guide to calculating BMI manually in both metric and imperial units.
The Metric Formula
If you know your height in centimetres and weight in kilograms, use this formula:
Note that height must be converted from centimetres to metres before calculating. Divide your height in cm by 100 to get metres (e.g., 170 cm = 1.70 m).
Metric Example
Person: 68 kg, 165 cm tall
Step 1: Convert height to metres: 165 ÷ 100 = 1.65 m
Step 2: Square the height: 1.65 × 1.65 = 2.7225
Step 3: Divide weight by result: 68 ÷ 2.7225 = 24.98
Result: BMI of 25.0 — top of the Healthy Weight range
The Imperial Formula
If you prefer pounds and inches, use this formula:
The multiplier 703 is a conversion factor that adjusts the calculation to match the metric result.
Imperial Example
Person: 150 lb, 5 feet 5 inches tall (65 inches total)
Step 1: Convert height to total inches: (5 × 12) + 5 = 65 inches
Step 2: Square the height: 65 × 65 = 4,225
Step 3: Divide weight by result: 150 ÷ 4,225 = 0.03550
Step 4: Multiply by 703: 0.03550 × 703 = 24.96
Result: BMI of 25.0 — Healthy Weight
Converting Between Units
If you only know your measurements in one system, here are the conversion factors you need:
- Centimetres to inches: divide cm by 2.54 (e.g., 165 cm ÷ 2.54 = 65 inches)
- Kilograms to pounds: multiply kg by 2.205 (e.g., 68 kg × 2.205 = 150 lb)
- Feet to centimetres: multiply feet by 30.48 (e.g., 5.5 ft × 30.48 = 167.6 cm)
Common Calculation Mistakes
- Forgetting to convert cm to m: Using 170 instead of 1.70 gives a wildly incorrect result.
- Not squaring the height: The formula requires height multiplied by itself, not just height.
- Using feet without converting to inches: 5 feet 8 inches is 68 inches total, not 5.8.
Interpreting Your Result
Once you have your BMI number, use these categories from the World Health Organization:
- Below 18.5: Underweight
- 18.5 to 24.9: Healthy Weight
- 25.0 to 29.9: Overweight
- 30.0 and above: Obese
Skip the Maths
Get your BMI instantly with our free calculator — just enter your height and weight.
Calculate My BMIReading Your Result After 50
The formula does not change with age. The interpretation does.
At 55, a BMI of 26 and a BMI of 26 at 25 are not the same finding. After menopause, fat is redistributed towards the abdomen and lean muscle mass declines, so the same BMI generally represents a higher proportion of body fat and a larger waist. The category on the chart — "overweight", in that case — is technically correct and practically incomplete.
Two things follow. First, calculate it, then measure your waist and compare it with half your height; if the two disagree, the waist is the one to act on. Second, use BMI the way it works best after 50: as a trend of your own rather than a comparison against a chart. A BMI that has drifted up three points over five years says something worth acting on. A BMI of 26 that has been 26 for a decade, with a stable waist and good strength, says something far less alarming.
Why BMI Is Only One Piece of the Puzzle
Understanding how to calculate BMI is useful, but remember that BMI does not measure body fat directly. Athletes with high muscle mass, older adults, and people of certain ethnicities may get misleading results. Use BMI as a starting point, not a definitive health verdict.
Read next: what BMI cannot see, and why body fat percentage tells a different story. If the number surprised you, start with what BMI is and what it means after 50.