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Withings | Bathroom scales accuracy

Scales accuracy in weight and body composition measurement


How can you know if bathroom scales are displaying your true weight, or if they’re weighing properly? If you’re curious about the accuracy of analog scales and smart scales, including impedance scales and other kinds of analytical scales, you’ve come to the right place. We’ll break down the science for you, so you can know if you’re getting a reliable weight measurement.

Learn about accuracy in weight measurement scales

Choosing a scale that help us keep track of our weight over time raises the question: how accurate are the measurements?


The question of accuracy is even more important when it comes to keeping track of other metrics, such as your body composition, or your body’s response to lifestyle changes when you have a chronic disease like diabetes or obesity.


“Weight assessment is a critical aid in patient care. It is particularly important in monitoring progression of pregnancies, heart failure status, and when adjusting medications."

Source: Precision in weighing: a comparison of scales found in physician offices, fitness centers, and weight loss centers, Public Health Reports, 2005

Scale accuracy is a major issue, especially in the clinical field, where population studies and medical decisions rely on measurement results.


Today’s smart scales are becoming more and more complex, and many have new features to ensure the best long-term accuracy.


Learn more about the criteria that determine accuracy in a reliable scale, and see how to optimize your weight measurements.

What is accuracy, and why is it important?

First, a couple of definitions: By “accuracy,” we mean how close the measurement comes to your actual weight. We also talk about “precision,” which means how reproducible the measurements are: when you weigh yourself several times, how much do the measurements differ?



A scale that is miscalibrated will still give the same results. Weight measurements are inaccurate, but reproducible.


Accuracy issues in bathroom scales may impact the estimation of people who are classified as overweight or obese. In a BMC Public Health study, “Impact of instrument error on the estimated prevalence of overweight and obesity in population-based surveys,” researchers found that “the estimated prevalence of overweight and obesity increased systematically with the size of instrument error.” In other words, accuracy errors may lead to an overestimation of the prevalence of overweight and obese individuals.


Some experts have said inaccurate measurements could potentially lead to improper medical treatment. In a February 2017 paper from the National Nurses Nutrition Group, “Obtaining an Accurate Body Weight Measurement in Adults and Children in Primary and Secondary Care Settings (not babies),” the authors state that an accurate body weight measurement is needed to assess and monitor nutritional status, calculate safe doses of medication, and establish and monitor fluid and nutritional status in a clinical setting.

What is the difference between analog, digital and smart scales?

Analog, or spring-based scales, work mechanically to measure the weight of an object.


analog scale functioning

A spring and plate move when you step on the scale, and four levers help to evenly distribute your weight. The up-and-down movements of a mechanical scale becomes a side-to-side motion, which then turns the dial to show your weight.


Over time, springs in analog scales may develop gradual wear and tear, which can lower both their accuracy and reproducibility.


You may also find it harder to read precise numbers, since these scales have a lower display granularity.

Unlike an analog scale, a digital scale converts force into weight via an electrical current. When you step on the scale, a component called a strain gauge measures stress, causing the resistance to change. A component called a load cell circuit measures this change, which is then converted into a signal communicated to your cell’s CPU. This number displays on your digital screen.


How digital scales work

Smart scales are generally digital scales that have additional features, so you can see more than just weight. Many smart scales use bioelectrical impedance analysis (BIA) to provide additional information about your body composition, which can give you a better picture of your overall health. Smart scales may sync to an app, so you can keep your measurements in one place and share them with your healthcare provider.

How do I know if my scale is accurate?

For a reliable weight measurement, you may want to test your scale for accuracy. Wikihow has some great advice on how to test your scale to see if it’s giving you an accurate weight measurement. Recommendations include:


  • Zero out your scale before you weigh in.

    If you have a digital scale, it should zero itself, but if you’re using an analog scale, manually turn the dial to zero before you step on.

  • Weigh an object you know well.

    Is your five-pound sack of flour actually five pounds?

  • Weigh the same object more than once.

    Again, this speaks to scales reproducibility and precision, which we mentioned above: When you weigh the same object, how much do the measurements vary?

  • Weigh two objects.

    First, weigh them separately. Then weigh them together and check the combined weight. Do the numbers add up?

  • Change the scale’s location.

    Do the measurements change? For the most accurate measurements, we suggest weighing yourself in the same place every day, on a flat, hard surface.


If you’re looking for a roundup of the best bathroom scales, Verywellfit has a great list of the 5 best bathroom scales of 2020—including the best smart scale and the best scale for accuracy.

Tom’s Guide also offers suggestions for affordable and stylish options.

position control

Weighing position: how to weigh yourself properly

For home scales accuracy, it’s important to ensure that you are weighing properly. We have several suggestions for getting the most accurate reading out of your scale.


Weight distribution

Make sure that when you step on the scale, your weight is distributed evenly. Try to keep the scale on a flat, hard surface. If the surface is not hard, use carpeted feet for example. Some premium scales are equipped with them by default. When you step on, space your feet evenly apart.


If you’re using a smart scale, it may show your on-screen prompts to guide your feet to the correct positions so you can center your weight easily.


Time of day

Keep the scale in the same place, and follow the same consistent weigh-in routine at least twice a week: at the same time and under the same conditions (naked, fasting...). Step on the scale at the same time each day, as weight fluctuations and changes in body composition measurements may occur throughout the day. If you’re frustrated by these fluctuations, consider checking your trends—say, over the course of a week—vs. individual numbers, to give you a more accurate idea of your progress.


Gravity matters

Did you know that gravity can affect your weight? Gravity can vary according to where you are on the globe, up to 1.3kg*. Some smart scales have a gravity compensation feature to ensure that you get the most accurate reading. They may also provide on-screen positioning guidance to ensure that your feet are placed correctly.

* Example: Weight = Mass*Gravitational acceleration. Mount Nevado Huascarán in Peru has the lowest gravitational acceleration, at 9.7639 m/s2, while the highest is at the surface of the Arctic Ocean, at 9.8337 m/s2. For random Mass=18, we would have: Weight=175.7kg (Mount Nevado Huascarán) or =177.0kg (in the Arctic Ocean). Difference of 1.3kg.

weight loss setbacks

How accurate are body composition scales?

First, a word on what body composition is and why it’s important: Body composition describes the percentages of fat, bone and muscle mass in your body.


Body composition scales typically use technology called bioelectrical impedance analysis (BIA) to measure body fat. The scale sends a tiny, safe electrical current through your body to measure your total body water, and then estimates your fat mass based on that measurement. Body composition can provide a useful metric for assessing your overall health.


What research says

As for the question of accuracy: while digital scales are generally more accurate than analog scales, scale accuracy can vary widely. Withings smart body composition scales, such as Body+, have been shown to give highly precise measurements of within .2 lbs. In a study in the Journal of Personalized Medicine entitled ”Withings Body Cardio Versus Gold Standards of Pulse Wave Velocity and Body Composition,” researchers found that the Body Cardio scale accurately measured body mass and pulse wave velocity, a measure of heart health.


It is important to understand that body composition scales indirectly measure the amount of fat, water and skeletal muscle mass through the interpretation of an electrical current. If the measurement of the current is reliable and reproducible, the interpretation algorithms can vary from model to model, with more or less significant biases.


These algorithms are crucial when assessing accuracy in body composition scales.

What technologies contribute to accuracy in smart scales?

Overall, the technology in digital scales has been shown to be more accurate than that in analog scales. In a BMC Public Health study entitled “Accuracy and consistency of weights provided by home bathroom scales,” researchers evaluated the accuracy of dial-type bathroom scales vs. digital scales. They found that “digital scales accurately weighed test loads, whereas dial-type scale weights differed significantly.”



Auto-calibration


Auto-calibration consists of checking a weight measurement by comparison with a standard, in order to verify that the scale keeps the same references over time, and in temperature variations or even humidity.

Gravity compensation


Certain cellular scales or Wi-Fi connected scales sometimes have scale location information. This allows for more accurate calibration, which takes into account the acceleration of gravity at the location where the scale is used.

Position control


While detecting your weight distribution among the different load cells, small arrows in the corners of your scale's screen show you where to stand for the best accuracy.

Step-on & auto-zeroing


If these features are user-friendly, the scale may “have fewer moving parts to get out of alignment or become damaged and have mechanisms to automatically set the starting weight to zero,” according to BMC Public Health.

High-sensitivity load cells


When it comes to digital scales accuracy, highly sensitive load cells enable the smallest voltage variation to be detected with the highest reproducibility.

Are smart scales more accurate than any other digital scale?

Scale accuracy can vary widely, depending on the technology they use, so it’s hard to say whether smart scales are more accurate than other kinds of digital scales..


Smart scales do generally offer more features than digital scales. Many smart scales have features that show you more just weight, including body composition, heart rate, body mass index (BMI), and you may even be able to sync your data to an app to keep everything stored in one place, so you can share it with your doctor.


The accuracy of smart scales also varies greatly depending on the quality of its manufacture and, therefore, the quality of the materials & sensors used. It is important not to neglect the experience and knowledge a manufacturer has accumulated in the field. This allows them to offer new features for a complete view of health, such as pulse wave velocity.


So if you’re interested in going beyond just weight, a smart scale may be for you.

Can bathroom scales lose accuracy?

Yes. Age, wear and tear can take their toll on your bathroom scale.


Analog scales may be more prone to damage—for example, if the levers bend out of shape or if dirt gets into the springs.


Digital scales may be less likely to become contaminated. If you have a smart scale that’s connected to an app, make sure you install any suggested updates to keep your scale running smoothly.

The bottom line

If you want a bathroom scale that measures your true weight, you may want to look for an accurate smart scale with measurement precision. Weighing yourself properly and looking at the reproducibility of results may help. However you look for accuracy, make sure you keep your weighing routine consistent, and try to weigh yourself every day. Consistency in your weight measurement routine can make a big difference!

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