Want to Know If You’ve Gained Weight? All You Need is a Piece of String

Many of us have smartphones laden with fitness apps, many of which now count your calories and your steps, track your water intake, and tell you your BMI with the click of a button. We even have special synchronizing $200 scales that detail your heart rate and body fat percentage.

Though all that technology is great, there’s an even simpler way to find out if you’ve gained weight—just use a piece of string.

UK researchers conducted a study with 3,000 people which showed that one-third of these people would have been considered normal with a BMI test. However, the same group would have been deemed abnormal and in a high risk category with a string test.

Why the difference? A BMI test measures your body fat based on your height and weight. It then assigns a number and based on that number puts you into a category (18.5 to 25 is normal weight, 25-30 is overweight and anything over 30 is obese). The BMI test does not take into account that muscle weighs more than fat or the location of the fat, which means that top athletes could be placed in the same category as people who never exercise.

The new research shows that testing your body with a string is actually more effective than a BMI test. We do the string test with many patients in our office, and it’s actually pretty simple: measure your height with a piece of string, cut the string in half and put it around your waist. (Tip: Measure your waist after you take a deep breath and exhale—and please don’t cheat by sucking in your stomach).

If you have some space left after you wrap the string around your waist, that’s a good sign. You want your waist-to-height ratio to be half. For example, if you are 6 feet tall your waist should not be more than 36 inches. If you are 5 feet tall, it should be less than 30 inches.

The string test is so effective because white fat tends to cluster in the abdomen, and too much of it around your stomach has been linked to a higher risk for diabetes, heart disease and other metabolic disorders.

To be clear, I’m not saying that the BMI test isn’t effective. It’s a great starting point, but the string test enhances the information we get as a predictor of your overall health. If you do the test and the string doesn’t meet from one end to the other around your waist, you must find ways to improve your health. Talk to your doctor or nutritionist about how to increase your physical activity and improve your diet. And after you’ve follow doctor’s orders, just grab that same piece of string to see how much progress you’ve made.

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