This May Be the Reason Your Fitness Watch Readings Aren’t Accurate

I used to live in Los Angeles, where I hiked up Griffith Park every morning with my dog, Ollie. Our climbs lasted, on average, about an hour and a half, and I was curious to know how much elevation I gained each week. So, I bought a fitness watch to track my activities and basic stats, like my distance. Because I didn’t want all the bells and whistles certain trackers offer, I went for a Fitbit Versa 2. I logged every climb and started paying attention to my heart rate and pace. I became obsessed—and started hiking faster and longer to compete with my weekly averages.

When we moved to Philadelphia in 2021, I traded in the mountains and dry heat for wooded trails and stifling humidity. I got into biking and running more frequently and noticed that I perspire way more on the East Coast, thanks to the thick, moist air out there. Even a quick two-mile jog leaves me drenched. When this happens, my watch slides up and down my wrist and occasionally loses its reading on my heart rate—and I can’t help but wonder if my Fitbit is less precise when I’m covered in a gooey soup of sweat and sunblock.

I scoured Reddit, searching for answers to see if other athletes were plagued by this phenomenon, and discovered that tons of people also believe their watch doesn’t work as well when sweat and sunscreen pool around their wrists. To get some much-needed answers, Outside spoke with a few experts to determine if sweat really impairs fitness watch accuracy.

Do Sweat and Sunscreen Impair Fitness Watch Accuracy?

Your standard tracker uses photoplethysmogram (PPG), a technology that utilizes infrared light to measure blood pressure and flow changes. The sensor that sits under the watch shoots light into your skin to get a detailed look at how your blood is passing by, says Stephen Carter, an assistant professor in the Department of Kinesiology at Indiana University. Then, the sensor sends those measurements through an algorithm, which estimates how fast your heart is beating, he says.

Basically, anything that messes with the sensor can cause inaccuracies, says Carter. Sweat, lotions, creams, and repellants, for example, can clog the sensors and impact how accurately they track your activity, says Michele Orini, an Associate Professor in Healthcare Engineering at King’s College London. Some evidence suggests that when a substance covers the sensor, it prevents the lights from drawing accurate heart rate, oxygen saturation, and respiratory rate readings from your skin, Orini says.

The big-name fitness watch brands provide guidance on caring for your watch. Garmin outright states that debris, like sweat and lotions, can make it harder for your watch to collect reliable heart rate data. Apple suggests keeping your watch clean and dry since water and sweat can result in an inaccurate recording.

Orini and his team of researchers conducted a study in 2022 where they dropped saline solution (to mimic sweat) onto the wrists of 17 people wearing fitness watches to investigate if and how sweat might impact heart rate readings. They found that the fake sweat caused heart rate errors, but only when they were moving around in a way someone would during an intense workout.

This happens because a high-intensity workout shifts the watch around and allows sunlight, if you’re outside, to shine between your skin and the sensor, which messes with the reading. The inaccuracies were even worse when the watches fit loosely instead of snugly around the wrist.

Orini’s major conclusion? “Sweat and lotions may reduce heart rate accuracy by functioning as a barrier between the sensor and the skin.” They also act as a lubricant and cause the watch to swim up and down your wrist, which makes it harder to stay in place and get an accurate reading, he adds.

Other studies have found more of the same. A 2023 study concluded that the proteins and lipids in your sweat can stick to the sensor and create buildup that, yep, blocks that sensor and prevents it from doing its job.

As for distance or steps, they’re unlikely to be affected by beads of sweat dripping down your arm because the watch’s embedded GPS and accelerometers track your distance, not the light sensors, says Orini.

There’s still a lot we don’t know, and more research is needed to uncover how much sunblock or sweat needs to get on the sensor to skew the readings and how drastically they can affect the numbers you see on your watch.

What Should You Do If Your Watch Is Dripping in Sweat?

Start with what you can easily control, like how tight the watch is strapped around your wrist. You want it to sit snugly because the accuracy can take a hit if it’s too loose, Orini says, but a too-tight strap can restrict blood flow and impair the estimations.

To ensure the band is secure but still comfortable, create a tiny bit of space, just enough so your skin can breathe. The watch should also sit a finger or two above your wrist bone. (Click here to see what the proper placement of your fitness watch should look like.)

You also want to get into a regular habit of cleaning the sensor. Garmin and Fitbit, for example, recommend regularly wiping it down with water and a cloth to remove buildup—especially if you’ve been sweating a ton or wearing sunscreen or insect repellent.

Obviously, it’s hard—if not impossible—to control how much you sweat, but the goal is to keep the sensor as clean and dry as possible, Orini says. Carter says that most trackers will do a good enough job tracking your workout, even if you’re perspiring a bit more than usual.

If you’re worried about not getting an accurate reading, it may be worth pausing your watch and wiping it—and your wrist—dry before carrying on. COROS recommends going this route if you’re soaked. But for many people—like Orini, who is a marathon runner—stopping in the middle of an activity isn’t really in the cards for many high-level athletes.

When pausing isn’t an option, it might be worth investing in a chest strap heart rate sensor such as the Polar H10, which uses electrocardiogram sensors to record your heart’s electrical signals. Though wearing a chest strap might not be the most comfortable device to wear, a 2019 study determined that they’re more accurate than watches.

Research appears to be inconclusive about whether or not sweat affects chest strap sensor accuracy. However, some folks on Reddit claim that sweat interfered with their readings; others said sweat had no impact. (FYI: If you’re a super sweater, the Polar H10 is waterproof.)

Fitness watches are incredible tools—they motivate people to exercise more and optimize their workouts—but they aren’t perfect, Carter says. Sure, there might be some errors, but at the end of the day, this shouldn’t dissuade you from using a tracker. The best thing you can do is taper your expectations.

So, if you’re in the middle of a grueling hike and your fitness watch says you have a heart rate of 11, don’t worry—you’re still alive; you might just have to give your watch a quick clean.