How many steps did you take today? Are you sure? How do you know?

If you’re one of the many people who wears a fitness band or smartwatch to count your steps, you may not be aware of one inescapable fact: they lie. Just because they tell you that you’ve reached your daily goal doesn’t mean that you actually took that many steps. The sad truth is that these devices can undercount or overcount the number of steps that you take in a day. In fact, your counts can differ widely depending on which brand you’re using.

Part of the reason is based on how they work. Today’s fitness bands use multiaxis inertial sensors called accelerometers to detect when the device is moving. Some also use gyroscopes to determine the direction and rotational movement. Because these sensors generate so much data that must be sifted through and interpreted by the device’s controller, results can often be misinterpreted and badly reported. In other words, what you see isn’t necessarily what you walked.

There can be several reasons for this.

Misunderstood motions

When your fitness band interprets the data from its motion sensors, it is supposed to ignore motions that are not associated with walking. However, it isn’t always successful. For example, banging on nails with a hammer can create vibrations that may be close enough to step motions that the data is misinterpreted as walking.

More subtle movements can also cause errors. Washing your hands, preparing food, petting your cat, or using a computer mouse can also result in steps being registered by your device. Using devices that vibrate — such as a random orbital sander that you’d use on a woodworking project — can cause your tracker to log hundreds of steps in just a few minutes.

It’s not just intentional motions that can be misinterpreted, however. Vibrations that affect all or part of your body can also result in inaccurate step counts. For example, riding in a car, bus, train, or subway can create motions that are interpreted as steps. You can add hundreds of steps while driving your car for an hour or less.

Because most trackers measure when you’re walking up stairs or an incline, they use a combination of motion sensors and sensors that detect air pressure. As you go higher, air pressure decreases. Unfortunately, changing air pressure levels can also mislead your tracker’s count of the number of flights you’ve climbed.

For example, riding in an elevator can register as walking up flights of stairs. Air pressure changes from riding in a car or other moving vehicle can throw off the count, as can entering and exiting high-rise buildings that have air pressure that is different from the outside. Even rapid changes in weather can register as climbing floors.

How to keep your numbers accurate

So trackers are not perfect at counting steps taken or flights climbed. But you don’t need perfect accuracy to know whether you have taken significantly more or fewer steps today than you did yesterday. There are several things you can do to be sure you’re getting the most accurate readings possible.

RTFM

The most important first step (so to speak) toward making sure your device is as accurate as possible is to read the manual. Follow the manufacturer’s instructions carefully when you set up the device and when you use it. Making sure you are using the tracker correctly is more likely to result in an accurate count.

Use your nondominant hand

Many trackers ask that you specify whether you’re wearing the device on your dominant or nondominant hand. Your dominant hand is likely to be more active — whether you’re wielding a tool or stirring a pot — so there will be more opportunities for mistakes. Even if your tracker’s instructions don’t cover this, consider wearing your device on your nondominant hand.

Be firm

Make sure that you wear the device so that it is firmly on your wrist. Some people don’t like a tight fit for their watch or bracelet, but if your fitness band flops around on your wrist, you’re likely to get false step counts. (A loose fit can also prevent heart rate sensors and other features from working correctly.)

Log your non-active times

If you’re a stickler for details, log your before and after step counts when you spend a long period of time sitting or doing some other activity that does not involve walking. You can then deduct these false steps from your day’s total score. This will also give you some measure of how many of these misinterpreted steps occur during a typical day. You can also take the tracker off before starting activities that you know will generate false results, such as using a sander or playing a musical instrument.