Body mass index (BMI) is an estimate of body composition, based on weight and height. Doctors consider a moderate BMI for women to be 18.5–24.9. A BMI of 30 or above may indicate obesity.

BMI measurements can help someone understand whether they are underweight or overweight. However, BMI for people assigned female at birth has some limitations, as it does not measure body fat specifically.

In this article, we provide a BMI calculator, discuss the pros and cons of BMI measurements, and explain some other methods that people assigned female at birth may find useful for keeping track of their health.

Sex and gender exist on spectrums. This article will use the terms “male,” “female,” or both to refer to sex assigned at birth. Click here to learn more.

A group of women with a range of BMI for women huddle during a basketball game.Share on Pinterest
While BMI can act as a general indicator of health, it does not measure body fat.

The Centers for Disease Control and Prevention (CDC) defines BMI as “a person’s weight in kilograms divided by the square of their height in meters.”

A person can use this calculator to determine their BMI:

Once someone knows their BMI, they can find out where their measurement falls in the following standard weight categories from the National Heart, Lung, and Blood Institute:

BMIWeight standard
Below 18.5Underweight
18.5–24.9Normal weight
25.0–29.9Overweight
30.0 and higherObese

As a measurement, BMI is easy and inexpensive to measure.

BMI is a useful tool for understanding where someone’s weight falls on the scale from underweight to overweight. It can also help people maintain a moderate weight, which can reduce their risk for:

While it correlates with body fat levels, BMI does not distinguish between the weight that comes from fat, muscle, and bone.

As such, BMI alone may be a misleading measurement of someone’s overall health. This is particularly true for people who are very muscular, peri- or postmenopausal, or have abdominal fat but are otherwise a moderate weight.

BMI and muscle

Muscle tissue is denser than fat tissue. This means that people with high levels of muscle may have a high body weight that classifies them as being overweight or having obesity, even if they have a low body fat percentage.

People with high levels of muscle mass and a high BMI result typically have a lower risk of death than those with a high BMI score and lower muscle mass.

However, the limitation of BMI to account for this may lead to complications in treatment and affect a person’s insurance.

BMI and body fat

A high body fat percentage has an adverse effect on health. People with a moderate BMI but a high percentage of body fat may have a higher risk of developing metabolic syndrome.

Metabolic syndrome is a collection of risk factors that increase the risk of heart disease and type 2 diabetes. These risk factors include high blood pressure, abdominal obesity and high fasting glucose levels.

BMI and age

Additionally, the proportion of muscle, fat, and bone in the body typically changes as people age. The average female loses roughly 13 pounds of muscle and bone between the ages of 25 and 65, while belly fat increases to four times its previous amount.

As a result, BMI calculations may be inaccurate for postmenopausal women, who may have high body fat percentages but maintain moderate BMI. In severe cases, this may lead a person to miss out on preventive treatments for obesity-related conditions.

Learn more about the limitations of using BMI here.

In addition to BMI, people assigned female at birth can use other methods to understand their weight, body composition, and risk for certain diseases.

Waist circumference

This method measures belly fat, which is a key indicator of someone’s risk for weight-related disease. People can measure the circumference of their waist using a soft tape measure.

A Cameroonian population study found that waist circumference is the best predictor of screen-detected diabetes.

Waist-to-hip ratio (WHR)

This method also measures abdominal fat and can be a strong indicator of future health risks. However, some researchers claim that taking a person’s waist and hip circumference into consideration as separate figures provides a better assessment of health risk.

A person can calculate their WHR by dividing their waist measurement by the circumference of their hips. The World Health Organization (WHO) states that a WHR of 0.85 or below for women means that they have a low risk for weight-related disease.

Diagnostic methods

Diagnostic tests that a doctor may perform include:

  • Densitometry: This involves a doctor measuring someone’s body weight while they are in water. The test compares their weight measurements on land to their measurements in water to calculate body fat percentage. Densitometry generally only takes place in a research setting.
  • Dual energy X-ray absorptiometry: X-rays move through fat, muscle, and bone at different rates, so this approach passes two low-level X-rays through the body to calculate relative percentages.
  • Bioelectrical impedance (BIA): BIA estimates someone’s body fat percentage by passing a low-level electric current through the body.
  • Isotope dilution: In this test, a person drinks water that contains isotopes and then provides samples of bodily fluids. These samples give a doctor information on the individual’s body composition.
  • Skinfold measurements: During this test, a doctor will pinch folds of skin and measure their thickness. They will often repeat this at several sites on the body.

Learn more about calculating WHR and what the results mean.

BMI is one of the tools that individuals can use to determine a person’s risk of certain diseases, such as diabetes, heart disease, and stroke.

However, BMI is an outdated measurement of overall health and does not take into account multiple physical variables such as muscle mass, age, and menopausal status.

Body fat percentage, bone density, and muscle mass can all change over time. As BMI does not measure body fat specifically, it may not provide someone with all the information that they need about their risk for these diseases.