Why the Size of Your Smartwatch Really Matters
“Is my watch too big?” is a common question on forums dedicated to smartwatches and fitness trackers of all kinds. (A few, but still fewer, wonder if their watch is too small.) Personally, I don’t care much about the appearance of a watch, but I’ve noticed that size can significantly impact its accuracy. If your watch is too big, it may not be accurate.
How does smartwatch size affect heart rate measurement accuracy?
The watch measures your heart rate using an optical sensor on the back. (This is what you see as the green glow on the back.) The heart rate sensor detects how light reflects off the blood vessels in your skin. To get an accurate reading, the watch should be worn snugly against your skin, ideally in a way that blocks out ambient light.
If the sensor loses contact with your skin or is exposed to too much light, your heart rate readings may be inaccurate. If your watch fits well to begin with, you can usually get accurate heart rate readings simply by ensuring the watch is worn snugly during workouts.
But if the watch is too large, it can shift, letting in light. While running, if the watch is loose, the sensor may detect a flash of light with each step. This can cause the watch to display a heart rate that’s inconsistent with your cadence (steps per minute), sometimes called “cadence lock.” Some runners cover their watch with their sleeve (especially in cold weather) to minimize this possibility.
No matter how you wear a large watch on a thin wrist, it will likely fit much worse than a watch that fits properly. A similar issue can arise with very small devices: the device isn’t large enough or can’t be secured tightly enough to block out light. This isn’t typically a problem with wristwatches, but it could explain why smart rings perform so poorly in daytime heart rate tracking.
So is this a real issue or just a theoretical quibble? I wasn’t sure until I tested the Forerunner 265 and 265S side by side. They’re the same watch with the same heart rate sensor, differing only in size. You can read my results here , but the graph explains it pretty well: the smaller 265S (42mm) measured my heart rate more accurately than the 265 (46mm).
Why You Might Need a Larger (or Smaller) Watch
If heart rate accuracy were your only concern, I’d advise you to buy the smallest watch you can find. It won’t even look out of place; before smartwatches became popular, wristwatches were often much smaller than a human wrist. (Watch old movies, and you’ll see tiny watches, even on people with not-so-slim wrists.)
But the watch is big and seems to be getting bigger. The larger screen means the numbers, text, and icons are easier to read. This is great news if you have poor eyesight or if you use the watch while exercising and want to quickly see what’s on the screen.
Larger watches may have larger screens, but they also have larger batteries and more space for other internal components. This is likely, at least in part, why more “premium” watches come in larger sizes. However, I find it difficult to distinguish between watches that are oversized due to the need to accommodate all their hardware features, and cases where a company increases their size to make them look more refined and, therefore, justify a higher price.
Companies should produce smaller watches.
And that brings me to what I need to say. Smartwatch and fitness watch makers know that 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, and the Garmin Forerunner 570 comes in 42 or 47 millimeters, to name just a few.
But when it comes to premium models, these options often disappear. The Garmin Forerunner 970 , far more feature-rich than the 570 , only comes in a 47mm size, matching the larger 570 but lacking a smaller variant. The Apple Watch Ultra 4 only comes in a 49mm size, which is even larger than the larger Series 12. I’m about to start testing the Hyrox feature, which is only found on the Amazfit Balance 3 or Balance Ultra; those watches come in 51mm and 52mm sizes, respectively. This is ridiculous.
As a woman writing about technology, I can’t ignore the gender aspect. According to the ANSUR II study conducted by the US Army, the average wrist size for women is 154 millimeters (about 6.1 inches), while for men it’s 176 mm (about 6.9 inches). My wrist is about 6 inches, making me quite average.
When companies release watches in two sizes, the smaller size usually fits the average woman’s wrist, while the larger fits the average man’s. Of course, this isn’t a perfect match, as many women might find the larger size fits them better, and vice versa. I’m glad that companies market these watches as “small” and “large,” or simply by the numerical size designation.
But as someone with six-inch wrists, I can’t wear a large watch without experiencing bounce, friction, and potentially inaccurate data. I’m not skinny; I’m perfectly average for my half of the population. But these large watches simply won’t be as accurate for me as they are for the average man. I noticed a similar phenomenon when testing the heart rate tracking on the Powerbeats Pro 2 earbuds : they struggled on me, but performed perfectly when I tried them on a man with larger ears.
There’s also an awkward intersection between this gender dynamic and how watch companies use size as an indicator of how expensive a watch should be. While Garmin only offers the 970 in a large size (47mm), the cheaper 70 and 170 models are only available in smaller sizes (both 43mm). The Garmin Venu 4 comes in 41 and 45mm, while the cheaper Vivoactive 6 is only available in 42mm. A slim wrist can save you money, but it still limits access to some more expensive options.
How do you know if your watch is too big?
For my work reviewing and testing watches for Lifehacker, I’ve worn smartwatches ranging in size from 40mm (the old Apple Watch) to 48mm (equivalent to the Kospet Tank and Amazfit T-Rex 3 Pro ). I like watches in the 40-42mm range; I can live with a size around 46mm if they’re lightweight enough and not too loose; I actually find it difficult to wear watches larger than that.
These large watches are difficult to tighten tightly enough for exercise; if you tighten them so tightly that they don’t dangle, it feels like you’re almost cutting off circulation. After a long run with a large, heavy watch, I sometimes developed red, irritated spots on my wrist from the inevitable friction. You may not be able to judge how well a watch fits while it’s still in the box, but I have a rule of thumb for trying it on.
To visually assess the size, I find it helpful to put the watch on your wrist, rest your hand on a table, and look down at it. If the lugs protrude beyond your wrist, it’s too big. The watch should sit flush with your wrist, not protrude beyond its edges.
If you prefer a smaller watch that doesn’t offer the most accurate heart rate readings, remember that you can always connect a chest heart rate monitor to a watch that supports it, or to your phone if you don’t have a compatible watch. However, if you prefer a larger watch for easier reading of text or maps, or if you need a premium feature not available in smaller sizes, a chest heart rate monitor can be a solution that combines the benefits of both.