Why Your Smartwatch Size Really Does Matter
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“Is my watch too big?” is a constant question on forums dedicated to smartwatches and fitness trackers of all kinds. (Some, but fewer, wonder if their watch is too small.) Personally, I don’t much care how a watch looks, but I’ve noticed that size can make a big difference in how accurately a watch functions. If your watch is too big, your numbers may be off.
How smartwatch size affects heart rate accuracy
Credit: Beth Skwarecki
Watches measure heart rate through an optical sensor on the back of the watch. (That’s what you see glowing green back there.) The heart rate sensor detects how that light reflects off the blood vessels in your skin. To get a proper reading, it needs to be snug against your skin, ideally in a way that blocks out ambient light.
If the sensor loses contact with your skin, or if too much light sneaks in, heart rate readings may be inaccurate. If a watch fits you well to start with, you can generally get an accurate heart rate by simply making sure the watch is snug during workouts.
But if a watch is too big, it can move around, letting light in. When you run, if a watch is bouncing around, the sensor may detect a flash of light with every footstep. This can lead to your watch reporting a heart rate that is the same as your cadence (footsteps per minute), which is sometimes called “cadence lock.” Some runners will cover their watch with a sleeve (especially in cold weather) to minimize this possibility.
No matter how you wear it, a big watch on a small wrist is likely to have a lot more difficulty than one that fits properly. Very small devices can have a similar problem: The device isn’t large enough or can’t be tightened enough to block light. This isn’t usually a problem with wristwatches, but it may explain why smart rings tend to be so poor at daytime heart rate tracking.
Now, is this a real-life concern, or just a theoretical quibble? I wasn’t sure, until I took the Forerunner 265 and 265S for a run at the same time. These are the same watch, with the same heart rate sensor, differing only in size. You can read about my results here, but the graph tells the story pretty well: The smaller 265S (42 mm) captured my heart rate more accurately than the 265 (46 mm).
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Why you may want a bigger (or smaller) watch
If heart rate accuracy were the only issue, I would tell you to get the smallest watch you can find. It wouldn’t even look weird; before smartwatches became popular, wristwatches were often much smaller than a person’s wrist. (Watch old movies and you’ll see some tiny watches, even on people who don’t have tiny wrists.)
But watches are big and seem to be getting bigger. A bigger screen means numbers, text, and icons are easier to read. This is great news if your vision isn’t the best anymore, or if you use the watch while exercising and want to be able to see what’s on your screen with a quick glance.
Bigger watches can provide bigger screens, but also bigger batteries and more room for other internals. This is probably at least partly why more “premium” watches come in bigger sizes. That said, it’s hard for me to distinguish when a watch is huge because it needs room to pack in its hardware features, versus when the company is making the watch bigger so it will seem fancier and thus justify a higher price.
Companies should make smaller watches
And this brings me to something I need to rant about. Smartwatch and fitness watch companies know that watch size matters: They often offer two sizes for at least some of their models. The Apple Watch Series 12 comes in 42 or 46 millimeters, while the Garmin Forerunner 570 comes in 42 or 47 millimeters, to give a couple of examples.
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But when it comes to premium models, those options often disappear. The Garmin Forerunner 970, far more feature-rich than the 570, only comes in a 47-millimeter size, matching the larger size of the 570 but without a smaller option. The Apple Watch Ultra 4 only comes in a 49-millimeter size, which is even larger than the larger size of the Series 12. I’m about to start testing a Hyrox feature that only comes on Amazfit’s Balance 3 or Balance Ultra watches; those watches are 51 and 52 millimeters, respectively. This is getting ridiculous.
As a woman reporting on tech, I can’t ignore the way gender plays into this. According to ANSUR II, a U.S. Army study, women have an average wrist size of 154 millimeters (about 6.1 inches), and men have an average wrist size of 176 mm (about 6.9 inches). My wrists are right around 6 inches, making me very average for a woman.
When companies make two sizes of watch, the smaller size usually fits the average woman’s wrist fairly well, and the larger size is a better fit for men’s wrists. This isn’t a perfect match, of course, since plenty of women may fit or prefer the larger size, and vice versa. I’m glad the companies market these as “small” and “large” or just by the numerical size.
But as a person with six-inch wrists, I can’t wear a large watch without dealing with bouncing, rubbing, and potentially inaccurate data. I’m no twig; I’m perfectly average for my half of the population. But those larger watches just aren’t going to be as accurate for me as they are for the average man. I noticed a similar phenomenon when I tested the heart rate tracking in Powerbeats Pro 2 earbuds: They barely worked for me, but did fine when I tried them on a man with larger ears.
There’s also an uncomfortable intersection between this gender dynamic and the way watch companies use size as a signifier of how fancy a watch is supposed to be. While Garmin only offers the 970 in a large size (47 mm), they only offer the cheaper 70 and 170 in smaller sizes (both 43 mm). Garmin’s Venu 4 comes in both 41 and 45 millimeter sizes, but the cheaper Vivoactive 6 is only available in 42. Having a small wrist can save you money, but it still shuts you off from some of the more premium options out there.
How to know if your watch is too big
For my work reviewing and testing watches for Lifehacker, I’ve worn smartwatches ranging from around 40 millimeters (an older Apple Watch) up to 48 millimeters (a tie between the Kospet Tank and the Amazfit T-Rex 3 Pro). I love a watch in the 40-42 millimeter range; I can tolerate one around 46 millimeters if it’s lightweight enough that it doesn’t bounce too much; and I really have a hard time with anything larger.
Those bigger watches are hard to snug up sufficiently for a workout; get them tight enough not to bounce, and they’ll feel like you’re nearly cutting off your circulation. After a long run with a big, heavy watch, I’ve sometimes gotten red, irritated spots on my wrist from the inevitable friction. You may not be able to judge how badly a watch will fit when it’s still in the box, but I do have a rule of thumb for when you try it on.
To judge visually, I find it helps to put a watch on your wrist, rest your arm on a table, and look directly down at it. If the lugs of the watch extend past your wrist, it’s too big. You want the watch to fit on your wrist, not overhang the sides.
And if you prefer a watch size that isn’t going to be the most accurate for heart rate, remember that you can always pair a chest strap to a watch that supports it, or to your phone if you don’t have a compatible watch. If you prefer a larger watch for more readable text or maps, or if you want a premium feature that doesn’t come in smaller sizes, a chest strap may be the way to get the best of both worlds.
