How Nurses Should Calculate BMI

Reviewed by Carole Jakucs, MSN, RN, PHN, CDCES

A Step-by-Step Guide

Body mass index (BMI) is a crucial metric in healthcare, providing a simple, yet effective way to assess whether a patient has a healthy body weight for a given height. For nurses, calculating BMI is a routine task that aids in diagnosing potential health issues like obesity, malnutrition, an exacerbation of congestive heart failure (CHF) and other weight-related conditions.

While many electronic medical record systems have a feature that calculates BMI automatically after you enter a patient’s height and weight, there may be times you need to calculate a BMI on your own using the BMI formula.

Below is a step-by-step guide on how nurses can accurately calculate BMI using the BMI formula, including practical tips for ensuring precision in your measurements.

What is the BMI formula, and why is it important?

The BMI formula provides a mathematical value derived from measuring a patient’s height and weight. It’s used to categorize a person’s overall size to classify them as either underweight, normal weight, overweight, or obese. The importance of BMI lies in its simplicity and effectiveness as an initial screening tool for various health risks.

Although BMI does not measure the amount of muscle mass or the percentage of body fat in a person, it’s widely used in clinical settings due to its relevance in assessing health risks.

Why? We know that the higher a person’s BMI is, the higher their risk for developing conditions such as hypertension, diabetes, hyperlipemia, liver disease such as nonalcoholic fatty liver disease, osteoarthritis, and several types of cancers linked to obesity.

The BMI categories for adults ages 20 years and older are:

Underweight: BMI less than 18.5
Normal or optimum weight: BMI from 18.5 to 24.9
Overweight: BMI 25 to 29.9
Obesity: BMI 30 or greater

Depending on the practice setting you’re working in, nurses may see obesity further defined and classified as follows:

Class I obesity: 30 to 34.9
Class II obesity: 35 to 39.9
Class III obesity: 40 or more

Understanding BMI categories is essential, as they inform clinical decisions regarding medical management, nursing diagnoses, nursing care and patient education needs.

The BMI formula explained

The BMI formula involves two key measurements: weight and height. Using weight and height in pounds and inches is known as using imperial measurements. Using measurements in kilograms and centimeters is known as using the metric formula, which is different from the imperial formula.

An online metric calculator for BMI can be found at this link to the National Institute of Health.

The focus here is on using the manual BMI formula using imperial measurements (pounds and inches) for adults ages 20 years and older and includes the following:

Multiply the patient’s weight in pounds by 703.
Divide that answer by the patient’s height in inches. (Remember 12 inches equals one foot.)
Divide that answer by the patient’s height in inches.

For example, if a patient weighs 160 pounds and is 5 feet, 8 inches tall (68 inches total) you would calculate their BMI as:

160 x 703 = 112,480
112,480 ÷ 68 = 1,654.12
1,654.12 ÷ 68 = 24.33

Their BMI would be 24.33.

Step-by-step guide for nurses on using the BMI formula

Below is a step-by-step guide to ensure you use the BMI formula correctly with every patient:

Step 1: Measure the patient's weight

Ensure accuracy. Use a calibrated scale and ensure that the patient is standing still, without support, wearing light clothing and preferably with their shoes off. Record their weight in pounds.

Step 2: Measure the patient's height using a stadiometer

A stadiometer is the standard tool used in many clinical settings to measure the height of patients who can stand. It’s comprised of a horizontal ruler and a sliding horizontally piece of metal which you need to lift and place just above the patient’s head to measure their height. When using a stadiometer, consider these aspects:

Make sure your patient is standing straight with their back against the stadiometer, with their shoes off, their heels together, and their eyes looking straight ahead. Measure the patient’s height in inches.
Record the patient’s height precisely as shown.

Step 3: Apply the BMI formula

Here is another example using pounds and inches:

If your patient weighs 128 pounds and their height is 63 inches, the formula would be:

128 (pounds) x 703 = 89,984
89,984 ÷ 63 (inches) = 1428.32
1428.32 ÷ 63 = 22.67
BMI = 22.67

Step 4: Interpret the results

Determine your patient’s BMI category. Once the BMI is calculated, compare it to the standard BMI categories. In our two examples above, both BMIs fall within the "normal weight" category.
Consider the patient's overall health. BMI is a general health indicator. It should be considered as one metric in the context of the patient's overall state of health, muscle mass, age, and sex, as these factors can influence the relevance and appropriateness of the BMI range.

Practical tips for nurses calculating BMI

Ensure consistent measurement techniques

Calibrate equipment regularly. Make sure your facilities’ scales and stadiometers are regularly calibrated to ensure correct measurements. Even slight inaccuracies in weight or height can lead to incorrect BMI calculations.
Standardize the process. Use the same type of equipment and measurement techniques across your clinical setting to maintain consistency. This helps when comparing BMI results over time in each individual patient.

Be aware of special considerations

Athletes and muscular individuals: BMI might overestimate body fat in individuals with a high muscle mass, as muscle weighs more than fat. In these patients, additional assessments like skinfold thickness and other techniques may be used when needed.
Older patients: In older adults, BMI may underestimate body fat. When a person ages, it can lead to an increase in body fat and a decrease in their muscle mass, even if their total weight remains the same.
Children and adolescents: For patients ages 19 years and younger, BMI interpretation requires age and sex-specific percentiles, as body composition changes with growth and development. Use evidence-based measurement methods and standard of care procedures when obtaining BMI for children and adolescents.

Use BMI as part of a comprehensive assessment

Combine with other measures. While BMI is a useful screening tool, it should be used in conjunction with other measurements like waist circumference, body fat percentage, and clinical assessments to provide a complete picture of a patient's health.
Educate patients. Help patients understand their BMI and what it means for their health. Discuss the importance of maintaining a healthy weight along with consuming a healthy diet, getting daily exercise, not smoking and having minimal to no alcohol intake.

Common pitfalls to avoid when calculating BMI

Misinterpreting the BMI categories

One common mistake is not considering the individual patient context when interpreting BMI. For example, a BMI of 25.5 categorizes a patient as overweight, but if that patient is an athlete, this BMI might not accurately reflect their health status due to their higher muscle mass than non-athletes.

Inaccurate measurements

Rushing through measurements can lead to errors. Always take the time to measure weight and height correctly. Double-check your calculations.

The role of BMI in nursing practice

Proper use of the BMI formula and BMI calculation is a vital skill for nurses to embrace. By following the steps outlined above and being mindful of the limitations of BMI, nurses can ensure they provide accurate assessments that contribute to better patient outcomes. The BMI formula is a powerful tool in your arsenal for promoting health and wellness with your patients and preventing disease.