How to calculate your BMI
Body Mass Index is the most well-known indicator for assessing body size. It can be calculated in seconds and provides a useful initial indication. But it also has important limitations that you need to know to avoid jumping to conclusions. This guide gives you the formula, the interpretation, and the necessary perspective.
Steps
Weigh yourself under the right conditions
Weigh yourself in the morning on an empty stomach, after using the bathroom, in your underwear. Always use the same scale on a hard, flat surface. A single weigh-in can vary by 0.5 to 1.5 kg from one day to the next — that's normal.
Measure your height precisely
Stand up straight, barefoot, heels together against a wall. Use a rigid tape measure or ask someone to measure you. You lose 1-2 cm in height during the day due to disc compression — measure yourself in the morning for more consistency.
Apply the formula: weight (kg) / height (m)²
Divide your weight in kilograms by the square of your height in meters. Example: 75 kg / (1.75 m)² = 75 / 3.0625 = 24.5. It's a simple calculation that requires no special equipment or expertise — just a scale and a tape measure.
Interpret the result according to WHO categories
Under 18.5: underweight. 18.5-24.9: normal weight. 25.0-29.9: overweight. 30.0 and above: obesity. These thresholds are defined by the WHO for the general adult population. They don't take into account muscle mass, age, or ethnicity.
What BMI measures (and doesn't measure)
BMI is a statistical indicator designed to assess populations, not individuals. It correlates weight and height to estimate overall health risk. It doesn't distinguish between body fat and muscle mass: a rugby player with a BMI of 30 doesn't have the same risk profile as a sedentary person with the same BMI. It also doesn't account for fat distribution — yet visceral (abdominal) fat is more dangerous than subcutaneous fat.
BMI categories and their meaning
A BMI between 18.5 and 24.9 is associated with the lowest mortality risk in epidemiological studies. A BMI above 30 significantly increases the risks of type 2 diabetes, cardiovascular disease, and certain cancers. A BMI below 18.5 is associated with an increased risk of deficiencies, osteoporosis, and weakened immunity. These categories are screening tools, not diagnoses.
Limitations to keep in mind
BMI overestimates fat in muscular people and underestimates it in older adults who have lost muscle mass. WHO thresholds were defined based on Caucasian populations — the WHO recommends lower thresholds for Asian populations (overweight starting at 23). Waist circumference (> 94 cm for men, > 80 cm for women) is a better predictor of cardiovascular risk than BMI alone.
FoodCraft Tip
BMI calculator with interpretation
The FoodCraft BMI calculator displays your result with WHO interpretation, category, and clear visual markers. You get full context, not just a number.
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My BMI is "overweight" but I exercise a lot, should I worry?
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