BMI formula and categories, with worked examples

Body mass index is one of the few calculations the NHA CCMA exam can ask you to perform, and it lives in Patient Intake and Vitals, a subdomain worth 14 of 150 scored items. Height, weight and BMI come up as measurement technique, as arithmetic and as category recall, so a few minutes on the formula pays for itself. This post follows the height, weight and BMI chapter of unit 9 in the free NHA CCMA course, which takes its numbers from the CDC and NHLBI. It is a study aid, not clinical guidance; the provider decides what a BMI means for a given patient.

The metric formula

BMI = weight in kilograms divided by height in meters squared.

That is the CDC's definition and the only version you need if the question gives metric units. Square the height first, then divide.

Worked example: a patient weighs 70 kg and is 1.75 m tall.

1. 1.75 x 1.75 = 3.0625
2. 70 / 3.0625 = 22.9

A BMI of 22.9 falls in the healthy weight range. If you want to check your arithmetic on a few more cases, the BMI calculator accepts either unit system and shows the category, which makes it a fast way to generate practice problems for yourself.

The imperial formula, and the common wrong version

BMI = weight in pounds divided by height in inches squared, then multiplied by 703.

The 703 is a unit-conversion constant. It is what turns pounds and inches into the same number the metric formula gives. Some textbooks print a US formula with pounds and feet and no 703 factor; that version is wrong and gives nonsense results. If a question offers an imperial formula, look for inches and 703.

Worked example: a patient weighs 180 lb and is 70 inches (5 ft 10 in) tall.

1. 180 x 703 = 126,540
2. 70 x 70 = 4,900
3. 126,540 / 4,900 = 25.8

A BMI of 25.8 is in the overweight range.

Second example: 150 lb and 64 inches (5 ft 4 in).

1. 150 x 703 = 105,450
2. 64 x 64 = 4,096
3. 105,450 / 4,096 = 25.7

Also overweight, and a useful reminder that two people with different heights and weights can land on nearly the same BMI.

Height given in feet and inches has to be converted to inches before you square it: 5 ft 4 in is 64 inches, not 5.4 of anything. The exam has a built-in calculator, so the risk is not the multiplication but the setup. Write the formula, fill in the numbers, then compute. The same discipline applies to the other formula the clinical units lean on, covered in mean arterial pressure: formula and normal range.

CDC adult categories

For adults aged 20 and older, the CDC uses the same cutoffs regardless of age, sex or race.

Obesity is split into classes:

The boundaries matter on an exam. A BMI of exactly 25.0 is overweight, not healthy weight, and exactly 30.0 is obesity, not overweight. Our two worked examples (25.8 and 25.7) are both overweight; a result of 24.9 would be healthy weight. The CDC's own illustration: a 5 ft 9 in adult is healthy weight from 125 to 168 lb and overweight from 169 to 202 lb.

The CDC also recommends person-first language, for example "adults with obesity" rather than "obese adults", which is a reasonable habit for documentation and patient conversations.

Children use percentiles, not the adult table

BMI-for-age is not used before age 2. From age 2 through 19, a child's BMI is plotted on an age- and sex-specific growth chart and read as a percentile, never compared with the adult cutoffs.

The CDC defines severe obesity in children as a BMI at 120% of the 95th percentile or greater, or a BMI of 35 or greater. A percentile compares the child with peers of the same age and sex: a weight at the 75th percentile means heavier than 75 of 100 peers.

Two chart facts travel with this. CDC and AAP recommend the WHO growth charts from birth to age 2 and the CDC charts from age 2 on. Children under 24 months are measured lying down (recumbent length), and from 24 months standing height is used; recumbent length runs about 0.7 to 0.8 cm longer than standing height in the same child, so plotting a length on a stature chart is a charting error.

Measuring height and weight correctly

Measurement technique is the part of this chapter the test plan calls "special considerations by age, health status and disability", meaning choose the right equipment for the patient in front of you.

• Standing scale for patients who can stand unsupported. On a balance-beam scale, move the sliding weights until the bar is level, then add them.
• Sling scales for patients who cannot stand, and bariatric scales for patients over 350 lb.
• Bed scales are zeroed with the patient out of bed so only the patient's weight registers.
• Daily weights are common in cardiac, endocrine and kidney conditions and should be taken the same way each day so the numbers are comparable.
• Stadiometer for standing height: back, heels and head against the wall, headpiece resting on top of the head. Height is checked yearly in adults because height loss can signal bone loss in the vertebrae.

Weight is more than a BMI input. Chemotherapy and most pediatric medications are dosed by weight, so an intake error can become a dosing error downstream.

Waist circumference and the limits of BMI

BMI is a screening measure, not a diagnosis. Muscle is denser than fat, so a muscular patient can have a high BMI with little body fat, and fat distribution varies by race and ethnicity. The CDC says BMI should be considered alongside other factors, which is why the chapter pairs it with waist circumference.

The NHLBI method: the patient stands, the tape goes around the middle just above the hip bones (not at the widest part of the hips), and the reading is taken right after breathing out. A waist of more than 40 inches in men or more than 35 inches in women raises the risk of heart disease and type 2 diabetes. The distractor is tape placement; "around the hips" is wrong.

How this shows up on the exam

Expect three kinds of items: a calculation with numbers supplied, a category given a BMI, and a technique question about scales or the stadiometer. BMI sits beside the other intake measurements in vital signs normal ranges for the CCMA, and the vital signs normal ranges checker covers the rest of the intake sheet. Blood pressure has its own category table with the same kind of hard boundaries, explained in blood pressure categories explained.

For the cutoffs themselves, how to memorize a list, a table or a formula sheet is the method: quiz the boundaries one at a time rather than rereading the table. Then slot the whole subdomain into a plan using how to study for the NHA CCMA exam, and check how Patient Intake and Vitals is weighted against the rest in what's on the NHA CCMA exam. The height, weight and BMI chapter in the NHA CCMA flashcard course drills every number above.