How to use the bmi calculator
- Choose US or metric units, then enter your height and weight.
- Enter your age. This tool uses adult categories, so the age must be 20 or older.
- Read your BMI to one decimal, the category, and the weight range that corresponds to BMI 18.5 up to 25 at your height.
- Remember that BMI is a screening number. Use it with other information and talk with a health professional about what it means for you.
Formula
BMI = weight in kilograms / (height in meters)^2
BMI (US units) = 703 x weight in pounds / (height in inches)^2
Weight at a target BMI = BMI x (height in meters)^2
The 703 in the US form is a rounded conversion factor. This calculator converts to kilograms and meters first, using 1 lb = 0.45359237 kg and 1 in = 2.54 cm, so it does not depend on the rounded 703. The BMI is shown to one decimal, but the category is chosen from the unrounded value.
Worked example
A person who is 5 ft 10 in (70 in) and weighs 180 lb has a BMI of 703 x 180 / 70^2 = 25.8, which falls in the overweight category (25.0 to 29.9).
In metric units, a person who is 170 cm and weighs 65 kg has a BMI of 65 / 1.7^2 = 22.5, which is in the healthy weight category. For 170 cm, BMI 18.5 corresponds to 53.5 kg and BMI 25 corresponds to 72.3 kg.
A BMI of exactly 25.0, such as 81 kg at 180 cm, counts as overweight because the healthy category is defined as 18.5 up to but not including 25.
Adult BMI categories
The calculator uses the adult categories published by the US Centers for Disease Control and Prevention (CDC) for people age 20 and older.
- Underweight: below 18.5
- Healthy weight: 18.5 to 24.9
- Overweight: 25.0 to 29.9
- Obesity: 30.0 and above, divided into class 1 (30.0 to 34.9), class 2 (35.0 to 39.9) and class 3 (40.0 and above)
What BMI cannot tell you
BMI uses only height and weight. It does not separate muscle from fat, and it does not show where fat is carried or how healthy your blood pressure, blood sugar or fitness are. A muscular athlete can land in the overweight range, and an older adult who has lost muscle can land in the healthy range with a high share of body fat.
The CDC describes BMI as a screening measure, not a diagnostic tool. If you want a second look at body composition, the body fat calculator estimates fat percentage from tape measurements, and the ideal weight calculator compares formulas for your height.
Children and teens
Children and teens are not assessed with these cutoffs. Their BMI is compared with age and sex percentiles, which this tool deliberately does not offer. For anyone under 20, ask a pediatrician.
Assumptions and limits
- Adult categories only (age 20 and older). The tool refuses younger ages instead of giving a misleading label.
- The same cutoffs are applied to every adult regardless of sex, age or ethnicity, as in the CDC categories. Some health organizations use different cutoffs for some groups.
- The calculator does not account for pregnancy, large muscle mass, amputations, edema or medical conditions that change body weight.
- Height must be 120 to 250 cm and weight 20 to 450 kg (or the equivalent in feet, inches and pounds).
- Nothing you enter leaves your browser. The result is information, not medical advice or a diagnosis.
Frequently asked questions
What is a healthy BMI for adults?
The CDC lists 18.5 up to 25 as the healthy weight category for adults. Being inside it does not guarantee good health, and being outside it does not prove poor health. It is a starting point for a conversation with a clinician.
Is BMI accurate for athletes?
Often not. Muscle is dense, so a very muscular person can have a high BMI with low body fat. For people with lots of muscle, body fat estimates and a clinician visit give a better picture than BMI alone.
Why does the calculator say my BMI is 25.0 but still healthy?
The display rounds to one decimal, but the category uses the exact value. A value such as 24.96 displays as 25.0 and is still in the healthy range because it is below 25. A true 25.0 is classed as overweight.
Can I use this BMI calculator for my child?
No. Children and teens are compared with percentiles for their age and sex, and adult cutoffs would give wrong labels. This tool asks for an age of 20 or older and shows a message otherwise.
How is the weight range for my height calculated?
It multiplies BMI 18.5 and BMI 25 by your height in meters squared. The result is the span of weights that would fall in the healthy category at your height, shown in the units you chose.