To check your BMI at home, you need two measurements (height and weight) and one simple formula. BMI equals your weight in kilograms divided by your height in meters squared. If you use pounds and inches, divide your weight by your height in inches squared, then multiply by 703. The whole process takes about five minutes, but getting accurate measurements matters more than most people realize.
The BMI Formula
There are two versions of the formula depending on which units you use.
Metric: BMI = weight (kg) ÷ height (m) ÷ height (m)
Imperial: BMI = weight (lb) ÷ height (in) ÷ height (in) × 703
Say you weigh 170 pounds and stand 5 feet 8 inches tall. First, convert your height entirely to inches: 5 × 12 + 8 = 68 inches. Then: 170 ÷ 68 ÷ 68 × 703 = 25.8. That’s your BMI. In metric, a person who weighs 77 kg and is 1.73 m tall would calculate 77 ÷ 1.73 ÷ 1.73 = 25.7. You can also punch your numbers into any free online BMI calculator, but knowing the math helps you spot errors.
How to Measure Your Height Accurately
Small errors in height have an outsized effect on your BMI because the formula squares that number. Being off by even an inch can shift your result by a full point or more. Here’s how to get it right at home without a clinical stadiometer.
Stand against a flat wall or door frame with no baseboard molding. Remove your shoes. Your heels, buttocks, shoulder blades, and the back of your head should all touch the wall. Keep your feet together with toes pointing slightly outward. Look straight ahead so the bottom of your eye socket and the opening of your ear canal form a line parallel to the floor. This head position, called the Frankfort plane, prevents you from tilting your chin up or down and adding or losing height.
Have someone place a flat object (a hardcover book or a ruler) on top of your head, pressing it firmly against the wall. Mark the wall lightly with a pencil at the bottom edge of the book, then step away and measure from the floor to the mark with a tape measure. Remove any hair buns, braids piled on top of your head, or thick headbands before measuring.
How to Weigh Yourself Consistently
Your body weight can fluctuate by several pounds over the course of a single day depending on meals, hydration, and activity. 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 as little clothing as possible, or at least the same items every time you weigh yourself.
Place your scale on a hard, flat surface. Carpet compresses unevenly under the scale’s feet and can throw off the reading by several pounds. If you only have carpeted floors, use a thin, rigid board under the scale. To check whether your scale is reasonably accurate, place a known weight on it, like a 10-pound dumbbell or a sealed 5 kg bag of flour (1 liter of water also weighs approximately 1 kg). If the reading is off by more than half a pound, try replacing the batteries or repositioning the scale before recalculating your BMI.
Consistency matters more than frequency. Whether you weigh yourself daily or once a week, do it on the same schedule and with the same routine. If you pick one day a week, stick to the same day each time rather than bouncing between weekdays and weekends.
What the Numbers Mean for Adults
The CDC defines BMI categories for adults 20 and older as follows:
- 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 thresholds are population-level guidelines. They tell you roughly where you fall on a statistical curve, not how healthy you are as an individual. More on that below.
BMI Works Differently for Kids and Teens
For anyone between ages 2 and 19, a raw BMI number doesn’t mean the same thing it does for an adult. Because children are still growing, their BMI is compared to other kids of the same age and sex using percentile charts. A child at the 75th percentile, for example, has a BMI equal to or higher than 75% of kids the same age and sex.
The categories for children and teens are:
- Underweight: below the 5th percentile
- Healthy weight: 5th to just under the 85th percentile
- Overweight: 85th to just under the 95th percentile
- Obesity: 95th percentile or above
You can calculate a child’s BMI with the same formula, but you’ll need the CDC’s age-and-sex-specific growth charts (or an online pediatric BMI calculator) to interpret the result. A BMI of 22 might be perfectly healthy for a 16-year-old boy but overweight for a 10-year-old girl.
Why BMI Is Useful but Incomplete
BMI is a screening tool, not a diagnosis. It estimates body fat based on height and weight alone, which means it can’t distinguish between muscle, fat, and bone. A muscular person and an overfat person of the same height can share an identical BMI. The American Medical Association adopted a policy recognizing that BMI is “an imperfect measure” that doesn’t account for differences across race, ethnicity, sex, or age. The formula was originally developed using data from predominantly white European populations, and its risk thresholds don’t apply equally to everyone.
None of this means BMI is useless. For most people, it provides a reasonable starting point. But it works best when combined with other measures that give a fuller picture of where your body stores fat and how that fat affects your health.
Waist Circumference Adds Context
Where you carry fat matters as much as how much you carry. Fat stored deep in the abdomen, around your organs, poses greater health risks than fat stored in your hips or thighs. Measuring your waist circumference at home takes 30 seconds and provides information BMI simply can’t.
To measure, stand up and wrap a flexible tape measure around your bare midsection at the level of your navel. Keep the tape snug but not compressing your skin, and take the reading at the end of a normal exhale. The World Health Organization sets elevated-risk thresholds at greater than 88 cm (about 34.6 inches) for women and greater than 102 cm (about 40.2 inches) for men.
If your BMI falls in the healthy range but your waist circumference exceeds those thresholds, the waist measurement is picking up abdominal fat that BMI misses. If your BMI is high but your waist is well below the threshold, some of that weight may be muscle rather than dangerous visceral fat. Using both numbers together gives you a more honest snapshot than either one alone.