Your body mass index, or BMI, is a measurement of the percentage of your body weight that is fat. One of the devices doctors or personal trainers routinely use is a handheld BMI machine. Knowing this measurement gives you an accurate picture of the progress of your fitness programs and indicates a degree of risk for diseases like cancer or diabetes.

A Pulse

Handheld BMI machines work by sending an electrical signal from one hand that travels through your body to the other hand. You won't feel a thing, but based on how quickly the pulse travels through your body, the machine will know how much of your body weight is fat. Because water is an electrical conductor and because unlike muscle, there's nearly no water in fat, the more fat you have, the slower the signal travels. The reading shows up as a percentage on the machine's digital screen.

Is it Accurate?

Handheld machines come up with a BMI reading instantly. With such quick results literally at your fingertips, you might wonder if they're accurate. While handheld BMI machines are capable of producing readings with an error margin of about 4 percent, the results can be skewed by skin temperature, hydration and food that might be present in your stomach. To ensure as accurate a reading as possible, recommends testing in the mornings after you've had a drink of water but before you eat breakfast.

What the Numbers Mean

Once you've gotten your BMI reading from a handheld device, you can apply it to a universal range that will indicate whether you're underweight, normal, or overweight. According to the National Institutes of Health, a BMI result of under 18.5 is underweight, 18.5 to 24.9 is normal, 25.0 to 29.9 is overweight, and a reading of 30 or over is obese.

Comparing Methods

Handheld BMI machines are just one way to determine your body fat percentage. The numerous methods and devices that have been developed to measure BMI include skin-fold measurements, underwater weighing and isotope dilution. Some require specialized equipment or expert training and can be costly. While you can purchase handheld BMI machines for home and personal use less expensively than buying a commercial model, the Centers for Disease Control recommends using a mathematical formula to calculate BMI. The formula -- weight in pounds divided by height in inches squared then multiplied by 703 -- doesn't require special equipment and is easy to use without training.