Bathroom scales meant for home use display your weight with a dial or on a digital screen. These scales measure your weight in one of two ways: mechanically, with springs, or electronically, with circuits that bend under weight, altering the current coursing through them. In general, digital bathroom scales are more accurate than mechanical ones. But for the most accurate reading, any bathroom scale must be set up correctly and used consistently.

"First and foremost, you have to make sure you are weighing yourself on a hard, flat surface," said Tom Dorsey, e-commerce manager at Lifetime Brands, which sells both our top pick and runner-up bathroom scales. Placing the scale on carpeting or a slant will cause the pressure on the four sensors in the corner of the scale to be uneven, and the reading to be inaccurate. And ideally, you need to use the scale in the same place on that hard surface every time. Digital scales sold for at-home use can still be fairly accurate, but "when you move them, you can get funny readings," explained Kurt Labanowski, a regional sales director for Rice Lake Weighing Systems, who has been working in the medical scale industry for nearly two decades

For the most accurate reading, Meredith Yorkin, a dietician at University Hospital in Newark, New Jersey, advises that you "always calibrate before you hop on. Every single time." (The manufacturer suggests doing it after you move the scale, but more frequent calibration accounts for, say, someone bumping or shifting the scale between readings.) A few years ago, Yorkin and her colleagues studied whether bathroom scales were accurate enough for a weight loss program she was developing, and more broadly, for medical research in general. Could participants weigh themselves on whatever they had in their bathroom and get a suitable reading? Her team found that the digital bathroom scales they tested—a wide-ranging sample of scales that study participants brought in from their own homes—were sufficiently accurate.

When we've regularly calibrated our top and runner-up pick scales and used them on hard, flat surfaces at home, we've found that we also get acceptably accurate readings. Though these readings can bounce around a little from time to time—that is, they are not always precise—they still fall within acceptable spitting distance of the truth. Precision means that each measurement is close to the others; a measurement can be the same several times in a row but be far off from the truth. Accuracy is about getting close to the truth within an acceptable margin of error; a measurement can vary a bit, but still be good enough for all intents and purposes. Home digital bathroom scales have fewer—and lower-quality—electronic gauges compared with those in higher-quality digital scales in doctor's offices and hospitals, Labanowski told us. For a more consistently precise (and always accurate) reading, you'd need a scale with more sensors than found in affordable bathroom scales, said Labanowski, like a $1,400 one that he received as a parting gift from a former employer.

What's accurate enough when it comes to weight? It takes about 6 pounds to shift your body mass index by a full unit. In her study, Yorkin pointed out that this is well above the margin of error for even a crappy bathroom scale, which she found could be as much as a pound or so, a wider margin of error than that of our picks. But even so, such scales "provide sufficiently accurate and consistent results for use in public health research," the authors wrote.

Bottom line: Even a not-perfectly-accurate scale will let you know if you're losing or gaining weight over time. "The most important thing is looking at the trend," Yorkin said.

To minimize confusing fluctuations in the number you see on the dial, scale makers advise weighing yourself at a consistent time of day, like right when you wake up, which can help minimize shifts in your weight from hydrating and eating. Consider, for example, that a dietician who weighed herself 15 times over the course of a day found that she'd "gained" seven pounds.

Exactly how often you should weigh yourself depends on your goals. To simply understand your weight, Yorkin suggests that a weekly weigh-in will capture what's happening with your body. If weight loss or maintenance is your aim, research suggests that a daily weight check will keep you on track. Though if you have anxiety around your weight or disordered eating tendencies, regular weigh-ins may not benefit you at all; you and your doctors know best.

If you'd like to watch your weight, but want to alleviate some of the stress that comes with seeing numbers bounce around, try a smart scale that graphs readings over time. Although they're not for everyone, we generally enjoyed our experience with smart scales—and watching the overall trend of the line is more meaningful than assessing an individual weigh-in.

If you're truly fed up with numbers and have some money to spend, there's even a smart scale that displays no numbers at all.

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