How to Calculate Your BMI (With a Real Example)
Your Body Mass Index (BMI) is your weight in pounds divided by your height in inches squared, multiplied by 703. A 5’10”, 154 lb adult comes out to a BMI of 22.1, which falls in the “normal weight” range. The formula is the same for everyone; only the number changes.
What the BMI number means
BMI turns height and weight into a single number that public health bodies use to sort adults into four broad categories. It’s a screening tool, not a diagnosis, but it’s still the starting point most doctors and insurers use to flag weight-related risk.
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal weight |
| 25 to 29.9 | Overweight |
| 30 and above | Obese |
These ranges come from the World Health Organization and apply to adults. A BMI of 22.1, like the example above, sits comfortably in the middle of the normal range. A BMI of 27 would land in overweight, and 31 would be classed as obese, regardless of how that weight is distributed or where it comes from.
BMI stuck around as the default screening number for one practical reason: it needs nothing more than a scale and a tape measure. No lab, no imaging, no specialist equipment, just two measurements and a formula anyone can run by hand. That’s also its ceiling. A method this cheap and fast was never going to capture everything about a person’s health, and it doesn’t try to.
Worked example: 5’10”, 154 lbs
Here’s the full calculation for an adult who is 5 feet 10 inches tall and weighs 154 pounds.
| Step | Calculation | Result |
|---|---|---|
| Convert height to inches | (5 x 12) + 10 | 70 in |
| Square the height | 70 x 70 | 4,900 |
| Divide weight by height squared | 154 / 4,900 | 0.0314 |
| Multiply by 703 | 0.0314 x 703 | 22.1 |
That final 22.1 is the BMI, and it falls inside the 18.5 to 24.9 normal weight band. In metric terms, that’s about 177.8 cm (1.778 m) and 69.9 kg. The metric formula lands on the same answer: 69.9 divided by 1.778 squared is also 22.1. The 703 constant exists purely to convert pounds and inches into the units metric BMI uses natively: kilograms per square meter.
Small changes move the number more than people expect.
| Scenario | Height | Weight | BMI |
|---|---|---|---|
| Baseline | 5’10” (70 in) | 154 lbs | 22.1 |
| +10 lbs, same height | 5’10” (70 in) | 164 lbs | 23.5 |
| Same weight, shorter | 5’6” (66 in) | 154 lbs | 24.9 |
Ten extra pounds on the same 5’10” frame pushes BMI to about 23.5, still normal but closer to the overweight line. Keep the weight fixed and drop the height to 5’6” instead, and BMI climbs to roughly 24.9, right at the edge of the range. Height alone moves the result almost as much as weight does, which is easy to forget when the number gets treated as a pure weight measurement.
Calculate With Your Own Numbers
Enter your own height and weight, in either imperial or metric units, and get your BMI and category instantly. The calculation runs in your browser, no data is sent anywhere.
Common mistakes and limitations
Using BMI for children or teens. The 18.5 to 30-plus scale above only applies to adults. Kids and teenagers are still growing, so pediatricians use age- and sex-specific percentile charts instead of a fixed BMI range. Plugging a 12-year-old’s height and weight into the adult formula and reading the result against the adult categories will give a misleading answer.
Ignoring muscle mass. BMI can’t tell the difference between fat and muscle, it only sees total weight relative to height. A muscular athlete or a competitive weightlifter can post a BMI in the “overweight” or even “obese” range despite having very low body fat, simply because muscle weighs more than fat for the same volume. For anyone with significant muscle mass, BMI understates how lean they actually are.
Mixing up height units mid-calculation. The imperial formula needs height in inches, not feet, before you square it. Someone who squares “5.83” (5 feet 10 inches written as a decimal foot) instead of converting fully to 70 inches will get a wrong result that doesn’t match either the imperial or metric formula. Convert feet and inches into a single inches figure first, every time.
Treating BMI as a diagnosis. A BMI reading is a screening number, not a medical verdict. It doesn’t account for age, sex, bone density, or where fat is carried on the body, all factors that affect actual health risk. Doctors use BMI alongside other measurements, like waist circumference or blood work, rather than as a standalone conclusion. Pregnant women in particular shouldn’t apply standard adult BMI categories, since expected weight gain during pregnancy isn’t the same signal as weight gain outside of it.
Frequently Asked Questions
Is a BMI of 22.1 considered healthy? Yes, for most adults a BMI between 18.5 and 24.9 falls in the normal weight range, and 22.1 sits well inside that band. BMI is only one screening measure, so a full picture of health also includes factors like activity level, diet, and family history.
Why does the imperial formula multiply by 703? The 703 constant converts pounds and inches into the same scale as the metric formula, which uses kilograms and meters directly. Without it, dividing pounds by inches squared would produce a number on a completely different scale than kilograms per square meter, so 703 exists purely to make the two formulas land on the same result.
Does BMI account for muscle versus fat? No. BMI only uses total body weight and height, so it can’t distinguish lean muscle from fat tissue. This is why very muscular people, such as bodybuilders or strength athletes, sometimes get a BMI reading in the overweight or obese range even though their body fat percentage is low.
What should I do if my BMI falls outside the normal range? A BMI outside 18.5 to 24.9 is worth discussing with a doctor, especially alongside other measurements like waist circumference, blood pressure, and cholesterol. BMI is a useful starting point for a conversation, not a number to act on in isolation.
Can I use BMI to track a child’s growth? Not with the adult ranges shown here. Children and teens need age- and sex-specific percentile charts, since a healthy weight for an 8-year-old and a 16-year-old are very different even at the same height. A pediatrician is the right source for interpreting a child’s growth numbers.
Is BMI the same thing as body fat percentage? No, and this is one of the most common mix-ups. BMI only estimates a category from height and weight, it never actually measures fat. Body fat percentage comes from separate methods, like skinfold calipers, bioelectrical impedance scales, or a DEXA scan, that measure fat tissue directly rather than inferring it from two numbers on a chart. BMI is a quick proxy; those other methods are the real measurement.