Body mass index (BMI) is determined by your body weight relative to your height. For most women, calculating their body composition using BMI ranges can help determine body fat levels. Knowing your BMI is important as it helps determine your risk for many diseases associated with having overweight.
The normal body mass index (BMI) for adult women (those over the age of 20 years) is between 18.5 and 24.9, according to the American Cancer Society.
What Is BMI?
BMI, which stands for body mass index, is calculated using a formula that takes into consideration your weight and height. It doesn't directly measure body fat, but the Centers for Disease Control and Prevention (CDC) states that it correlates with other methods of measuring body composition, including:
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- Measuring fat directly obtained from skinfold thickness (with calipers).
- Using a weak electrical current through your body to calculate resistance or bioelectrical impedance.
- Weighing oneself underwater.
- Measuring bone and lean muscle mass by means of a dual-energy X-ray.
When determining your BMI, it's important to know that several factors contribute to how much you weigh and the amount of body fat you have. Some of these include your environment, genetics, family history, metabolism and behaviors or habits, suggests the National Heart, Lung, and Blood Institute.
Your BMI values can help determine your risk level for various metabolic diseases. Although this isn't a diagnostic tool, it can tell whether you're underweight or have overweight. Based on this information, you can determine how many pounds to gain or lose to maintain optimal health.
Using BMI during pregnancy and lactation is not appropriate because of women's changes in body composition. However, a study published in BMC Pregnancy and Childbirth in December 2014 has found that women who had a BMI in the overweight and obesity range before pregnancy were at greater risk of complications during childbirth.
Researchers concluded that a normal pre-pregnancy BMI helps protect against adverse pregnancy and neonatal outcomes with fewer obstetrical interventions.
What Do the Numbers Mean?
BMI measurements broadly define different standard weight categories that apply equally to adult women and men over the age of 20. The division of ranges is as follows, according to the American Cancer Society:
- Underweight: BMI is less than 18.5.
- Normal weight: BMI is 18.5 to 24.9.
- Overweight: BMI is 25 to 29.9 —
having extra body weight from muscle, bone, fat and/or water.
- Obesity: BMI is 30 or more — a large amount of body fat in relation to height.
A BMI of 30 or higher is most prevalent in adult women aged 60 years and older, says the CDC. The higher the BMI, the greater the risk of heart disease, hypertension, Type 2 diabetes, osteoarthritis, gallstones, sleep apnea and certain cancers, warns the National Heart, Lung, and Blood Institute.
If you have a high BMI, other factors need to be taken into consideration to determine whether there is a health risk. These include evaluations of your diet, physical activity, family history and other health screenings, points out the CDC.
For most people, BMI is a quick and inexpensive assessment of body composition. However, using it for health assessment has some limitations, especially for people with a lot of muscle mass, such as bodybuilders and athletes. Since muscle weighs more than fat, individuals who have well-developed muscles may get an overestimated calculation of body fat and hence be rated as having obesity on the BMI chart, states the American Council on Exercise.
On the other side of the spectrum, elderly people who don't exercise and those who have lost muscle because of surgery or illness may get an underestimated body fat calculation.
Read more: Health Risks of a Low BMI
Measuring BMI for Women
There are free online tools that can help you quickly calculate your BMI. If you're looking for an easy-to-use online tool, check out the BMI calculator at the Mayo Clinic website.
You can also assess your BMI manually. To determine the ranges using the metric system, the formula is: your weight in kilograms (kg) divided by your height in meters squared (m2), according to the American Cancer Society.
If you prefer to use pounds and inches, multiply your weight in pounds by 703. Then divide that number by your height in inches squared (in2). In other words, BMI = (your weight in pounds x 703) / (your height in inches x your height in inches).
For example, if you weigh 120 pounds and you're 5 feet, 3 inches tall (or 63 inches), your BMI = (120 x 703) / (63 x 63) or 84,360 / 3,969 = 21.3. This amount is well within the healthy weight range of 18.5 to 24.9.
Read more: Diets to Lower the BMI
- Centers for Disease Control and Prevention: "About BMI for Adults"
- U.S. Department of Health & Human Services: National Heart, Lung, and Blood Institute: "Maintain a Healthy Weight"
- American Cancer Society: "Normal Weight Ranges: Body Mass Index (BMI)"
- U.S. Department of Health & Human Services: National Heart, Lung, and Blood Institute: "Body Mass Index"
- American Council on Exercise: "BMI Calculator"
- Mayo Clinic: "BMI Calculator"
- BMC Pregnancy and Childbirth: "Pre-Pregnancy Body Mass Index (BMI) and Delivery Outcomes in a Canadian Population"