BMI Calculator.

Guide

How to use the BMI Calculator

Body mass index, or BMI, is a single number that compares your weight to your height. It is the screening measure most clinics still start with.

1. Choose your Units. Pick US (ft, in, lb) to enter feet, inches and pounds, or Metric (cm, kg) for centimeters and kilograms. The result is the same either way.
2. Enter your height, measured without shoes, standing straight: the Height (feet) and Height (inches) fields in US units, or the single Height field in centimeters.
3. Enter your Weight. For a consistent reading, weigh yourself in the morning, before breakfast, in light clothing.
4. Optionally fill in your Age and Sex. Neither changes the BMI number; the calculator uses them only for notes, such as flagging that the adult categories do not apply under 20.
5. Read the result: your BMI, its category, the weight range that counts as "normal" for your height, and how far you are from the nearest edge of that range. The visual scale shows where you sit.

How the math works

BMI is weight divided by height squared, with weight in kilograms and height in meters. Squaring the height is what makes the number roughly comparable across people of different heights: a taller person is expected to weigh more, and the formula scales for that, imperfectly.

If you enter US units, the calculator converts to centimeters at 2.54 per inch and kilograms at 0.45359237 per pound, then runs the metric formula. Multiplying pounds over inches squared by 703 lands on the same answer within rounding.

The healthy weight range is the same formula run backward: multiply the two edges of the normal category, 18.5 and 24.9, by your height in meters squared to get the lightest and heaviest weights that still count as normal at your height.

Worked example

Take someone who is 5 feet 11 inches tall and weighs 185 pounds. Convert the height: 71 inches × 2.54 = 180.34 cm, or 1.8034 meters, which squared is 3.2523. Convert the weight: 185 × 0.45359237 = 83.91 kg. Divide: 83.91 ÷ 3.2523 = 25.8, just inside the overweight category, which starts at 25.

For the healthy range, the low end is 18.5 × 3.2523 = 60.17 kg, or 132.6 lb, and the high end is 24.9 × 3.2523 = 80.98 kg, or 178.5 lb. This person is 185 − 178.5 = 6.5 pounds above the top of it.

Step	Calculation	Result
Height in meters	71 in × 2.54 ÷ 100	1.8034 m
Height squared	1.8034²	3.2523 m²
Weight in kilograms	185 lb × 0.45359237	83.91 kg
BMI	83.91 ÷ 3.2523	25.8 (overweight)
Healthy range	18.5 × 3.2523 to 24.9 × 3.2523	132.6 to 178.5 lb
Distance to normal	185 − 178.5	6.5 lb above

The US shortcut gives the same answer: 703 × 185 ÷ 5,041 = 25.8. At this height one BMI point is about 7.2 pounds, so the cutoffs translate to roughly 179.2 lb for 25 and 215.1 lb for 30, which is easier to act on than an abstract index.

What BMI gets wrong

BMI only knows two things about you: how tall you are and how much you weigh. It has no idea what that weight is made of, where it sits, or how old you are. That leads to predictable blind spots.

• Muscle. Muscle is denser than fat. A 5-foot-11 lifter at 216 pounds with a lean waist scores a BMI of 30.1 and gets labeled obese. People who train hard routinely get overweight readings that have nothing to do with fat.
• Age. People tend to lose muscle and gain fat as they age without the scale moving much, so an older adult with a "normal" BMI can carry more fat than a younger adult at the same number.
• Sex. At the same BMI, women generally carry more body fat than men, yet the thresholds are the same for both.
• Ethnicity. Several health bodies have noted that people of South Asian, East Asian and some other backgrounds face higher metabolic risk at lower BMI values than the standard cutoffs suggest.
• Fat location. Fat around the organs in your midsection is more strongly tied to heart disease and diabetes than fat on your hips and thighs, and BMI cannot tell the difference. A tape around your waist adds real information in seconds.

None of this makes BMI useless. For most adults who are not unusually muscular, it tracks body fat reasonably well, and it is cheap and repeatable. It is a screening number: it tells you whether a closer look is worth taking, not what you will find.

How to read the result

Treat the category as a prompt, not a verdict. A reading of 25.8 that is 6.5 pounds above the normal range is a very different situation from a reading of 36. Near a boundary, notice how little it takes to cross: in the example, losing 5.8 pounds drops the BMI under 25. The boundaries are round numbers, and your body does not change on the day you cross one.

If your BMI is high and you want to know whether it is fat or muscle, the Body Fat Calculator estimates body fat from a tape measurement. If you plan to change your weight, the Calorie Calculator turns that goal into a daily calorie target.

These results are estimates for healthy adults aged 20 and over, and they are not a diagnosis. If you have a medical condition, are pregnant, are an athlete, or are under 18, talk to a clinician about what your weight means for your health rather than relying on these categories.

What this calculator doesn't do

It does not measure body fat, muscle or bone, and it does not adjust the number for age, sex or ethnicity. BMI for children and teenagers is read against growth charts as a percentile for their age and sex, which this calculator does not do, so it is not suitable for anyone under 20.

It also cannot tell you what your weight is doing to your health. Blood pressure, blood sugar, cholesterol, fitness and waist size all matter, and a clinician who can see those alongside your BMI can interpret it far better than a formula can.

Frequently asked questions

Estimates for healthy adults, not a diagnosis or medical advice. Formulas like these are population averages and can be off for athletes, older adults, pregnant people and anyone with a medical condition. Talk to a clinician before changing your diet or training. Nothing you type here leaves your browser. See the terms of use.