The apparatus used to measure blood pressure is a sphygmomanometer. The portion which wraps around the upper arm is called the cuff. The bladder is the inflatable bag inside of double layers of material of the cuff.
Accuracy Depends on Size
To obtain an accurate blood pressure reading, the cuff bladder should be at least as long as 80 percent of the middle of the upper arm and at least 40 percent of the width.
Effects of Size
A cuff bladder that is too small can result in a blood pressure reading that is artificially high. Conversely, if the cuff bladder is too big, the reading can be artificially low. Err on using the larger size cuff if the size is borderline.
How to Determine Size
To determine the appropriate size cuff bladder, measure the upper arm circumference (distance around) midway between the shoulder and the elbow.
Basic Sized Cuff Bladders
There are three basic sizes for blood pressure cuff bladders. The child/small adult cuff fits arms 7 to 9 inches in circumference. The standard adult cuff fits arms with a circumference of 9 to 13 inches. The large adult cuff bladder fits arms between 13 and 17 inches around.
Cuff Bladder for Obese Arms
For obese arms (17 to 20 inches), a thigh cuff may be needed for accurate blood pressure readings.