To measure your BMI at home, you need two things: an accurate height and an accurate weight. Then you plug both into a simple formula. The whole process takes about five minutes, but small errors in how you measure can throw off your result, so technique matters more than equipment.
The BMI Formula
BMI divides your weight by the square of your height. If you’re working in pounds and inches, multiply the result by 703 to convert to the standard BMI scale.
- Metric: BMI = weight in kilograms ÷ (height in meters × height in meters)
- Imperial: BMI = weight in pounds ÷ (height in inches × height in inches) × 703
For example, if you weigh 170 pounds and stand 5’8″ (68 inches), the math looks like this: 170 ÷ (68 × 68) × 703 = 25.8. That’s your BMI. If you’d rather skip the arithmetic, the CDC has a free online calculator that does it instantly once you enter your numbers.
How to Measure Your Height Accurately
Find a flat section of wall on a hard, uncarpeted floor. Take off your shoes and remove anything from your head (hats, headbands, bulky hairstyles) that could add height. Stand with your feet flat on the floor and your heels touching the base of the wall. Your head, shoulders, and buttocks should all be in contact with the wall, and your eyes should look straight ahead so your chin is parallel to the floor.
Have someone place a flat, rigid object (a hardcover book or ruler) against the wall at a right angle, then lower it until it rests on top of your head. Mark the wall lightly with a pencil at the bottom edge of the object, then step away and measure from the floor to the mark with a tape measure.
If you’re doing this alone, a cereal box works better than a book because it’s easier to keep level on your head without a second pair of hands. Stand facing a mirror so you can confirm the box sits parallel to the floor before you mark the wall.
How to Weigh Yourself Correctly
The best time to step on the scale is first thing in the morning, after using the bathroom but before eating or drinking anything. Wear little to no clothing, or the same outfit each time. Weigh yourself barefoot.
Place your scale on a hard, flat surface. Carpet compresses unevenly under the scale’s feet and can skew your reading by several pounds. Stand tall and still with both feet equally spaced, distributing your weight evenly. Shifting your stance or leaning can change the number.
If you want to verify your scale’s accuracy, weigh yourself, then weigh yourself holding a dumbbell or other object with a known weight. The difference between the two readings should match the weight of that object. If it doesn’t, your scale may need replacing or repositioning.
What Your Number Means
The CDC defines the following BMI categories for adults 20 and older:
- Underweight: below 18.5
- Healthy weight: 18.5 to 24.9
- Overweight: 25 to 29.9
- Obesity (Class 1): 30 to 34.9
- Obesity (Class 2): 35 to 39.9
- Severe obesity (Class 3): 40 or higher
These ranges were designed as a quick screening tool, not a diagnosis. Two people with the same BMI can have very different body compositions and very different health profiles.
BMI for Children and Teens
The formula itself is identical for kids, but the result is interpreted differently. Because children are still growing, their BMI is plotted on age-and-sex-specific growth charts and expressed as a percentile rather than a flat number. A 10-year-old boy and a 15-year-old girl with the same BMI could fall into completely different categories. The CDC provides a separate Child and Teen BMI Calculator that does this comparison for you. For anyone 20 or older, the standard adult categories apply.
Where BMI Falls Short
BMI can’t distinguish between muscle and fat. Someone who lifts weights regularly may land in the “overweight” range despite having low body fat, while an older adult who has gradually lost muscle and gained fat may show a “healthy” BMI that masks real health risk. This is especially common after age 65, when muscle loss accelerates but body weight can stay relatively stable.
The standard cutoffs were also developed using data from adults of European ancestry. They don’t always reflect health risk accurately across other ethnicities. As one Johns Hopkins physician put it, BMI is “shorthand” that “doesn’t tell the whole story about race, age, fitness and body composition.”
Waist-to-Hip Ratio as a Complement
If you want a fuller picture, measure your waist-to-hip ratio. It takes 30 seconds and captures something BMI misses: how much fat you carry around your midsection. Visceral fat (the deep abdominal fat surrounding your organs) is more strongly linked to heart disease and metabolic problems than fat stored in your hips or thighs.
To measure it, wrap a flexible tape measure around the smallest part of your natural waistline, usually near your belly button, and note the number. Then measure around the widest part of your hips. Divide your waist measurement by your hip measurement. Research published by Harvard Health found that waist-to-hip ratio was a better predictor of future health issues than BMI alone, precisely because it reflects that dangerous abdominal fat more directly.
For general reference, a waist-to-hip ratio above 0.85 for women or above 0.90 for men is considered elevated risk. Combining this with your BMI gives you a more useful snapshot than either number on its own.