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Apple Watch 40mm vs 38mm (2021): Is It Time To Upgrade?

If you’re looking to buy an Apple Watch, the size of the model is a key consideration. That’s why the company has been coming out with two variants of their wearables. For those with smaller wrists, then the standard smaller models — the Series 5’s 40mm and the Series 3’s 38mm — should be perfect for you.

Although only 2mm separates the Apple Watch Series 5 40mm from the Apple Watch Series 3 38mm, there is a considerable difference in features between the two devices. With the 38mm coming in less than half the price of the 40mm, both in the GPS-only and GPS + Cellular models, there’s more to design and wrist size fit to take into account. To keep things simple, we’ve compared the 40mm and the 38mm to help those thinking of an upgrade or eyeing their first Apple Watch.

Apple Watch 40mm vs 38mm Comparison Chart

ModelApple Watch Series 5 40mmApple Watch Series 3 38mm
Amazon product Apple Watch Series 3 (38mm)
PriceAmazon product Check Price on Amazon
Dimensions40 x 38 x 10.7 mm38.6 x 33.3 x 11.4mm
Weight30.1 g (non-LTE)26.7g (Aluminum, GPS only)
DisplayAlways-On Retina LTPO OLED display with Force TouchRetina OLED display with Force Touch
Display Area759 sq mm563 sq mm
Display Size394 x 324 pixels272 x 340 pixels
Case Thickness10.7mm11.4mm
ColorsSilver, Space Gray, GoldSilver, Space Gray
Call & Text AlertsYesYes
Electronic Heart SensorYesNo
Optical Heart SensorYes (2nd Generation)Yes
Fall DetectionYesNo
Emergency SOSYesYes
4G/LTE ModelYesYes
Water ResistantUp to 50mUp to 50m
Battery LifeUp to 18 hrsUp to 18 hrs


The Apple Watch 40mm builds on the design and form of the 38mm, while offering a larger display and adding an ECG sensor on the digital crown.

In this front view, the Apple Watch 40mm’s (left) side button is less pronounced compared to the 38mm (right).

There is no mistaking that the 38mm and 40mm are from the same line of smartwatches. Both come in the same rectangular body and strap mechanism that has largely defined Apple Watches over the years, although the 40mm is 11% thinner. The two models also have the same rotating digital crown, and a side button that’s used for Apple Pay, sending out an SOS, or showing and hiding the Dock. However, the 40mm’s digital crown now houses the ECG sensor that was introduced back in the Series 4.

Put beside each other, the Apple Watch 38mm is surprisingly close in size to the 40mm. And that’s because the Series 3 model actually comes in at 38.6mm. Nonetheless, the display on the Series 5 40mm is 30% larger with more refined, rounded edges that give it a bolder appearance. Interestingly, the 40mm’s screen (759 sq mm) also comes in larger than the Series 3’s 42mm variant (740 sq mm). For those with big or thick fingers, the 40mm will definitely be easier to operate than the 38mm.

Both devices sport Retina-display, with the 40mm using LTPO OLED technology that claims to be 5 to 15% more efficient than regular OLED. They are also water resistant for up to 50 meters. With the 40mm, you get a ceramic and sapphire crystal back while the 30mm only offers this option for its GPS + Cellular variant. The switch to ceramic back should improve network reception. Lastly, you also get an additional gold aluminum color option with the 40mm.


The 40mm runs on a powerful new chip to support new sensors as well as enhanced features from the 38mm.

The new sensors provide metrics in an easy-to-read format on both the 40mm (left) and the 38mm (right). 

With a larger display and new health tracking features to support, the Apple Watch 40mm boasts of a next generation W3 wireless chip, as well as an S5 dual-core processor that’s twice as fast than the S3 on the 38mm. The new chip also allows support for Bluetooth 5.0. For a GPS model, the 40mm comes with 32GB capacity while the 38mm offers only 8GB. Both the 40mm and the 38mm, however, support WiFi and the optional LTE connectivity.

A key advantage of the 40mm is the inclusion of an electric heart sensor on the digital crown, a feature pioneered by Apple Watch with the release of its Series 4. Accessed through the ECG app, this added capability allows users to measure and record heartbeat and rhythm. With the 40mm, you also get a built-in compass as well as a second-generation optical heart sensor.

Although the Series 3’s 38mm shares some key features with the Series 5’s 40mm, the latter has enhanced hardware. For one, you get a speaker that’s 50% louder, perfect for users who frequently take calls with their Apple Watch. The accelerometer on the larger model has also been redesigned to measure up to the 32 g-forces as opposed to the 38m’s 15 g-forces. Combining the built-in accelerometer and gyroscope, the 40mm is also capable of detecting falls and automatically notifying emergency service—a critical feature that has been proven to save lives.


Both the 38mm and the 40mm Apple Watch support watchOS 6. 

The watchOS 6 brings new watch faces, complications, as well as health-specific features.

In terms of software, not much separates the Apple Watch 40mm and the 38mm largely because both support watchOS 6. This gives both smartwatches a dedicated App Store on their devices, as well as the option to install the apps even without the aid of an iPhone. Plus, you get new watch faces and complications that allow you to get more info with a glance.

With the watchOS6, there’s also a trend feature that tracks and records your fitness metrics to provide insights about your progress. For women, there’s a menstrual cycle tracking app that allows you to keep a log of your period, add a flow of information, and even record symptoms. All the information is displayed in an easy-to-read format that should help translate data into meaningful insights for the user.

Battery Life

The Apple Watch 40mm shares the same modest 18-hour battery life as the 38mm.

Active users who depend on GPS and the always-on display may find the battery life of the 40mm and 38mm lacking.

Given the plethora of fitness, health, and connectivity features in both the 40mm and the 38mm, it still is surprising you only get up to 18 hours of battery life. That’s not even a whole day. Despite a more efficient Retina-display, you’ll only get so much from the 40mm especially if you’ll be frequently using new features like the ECG, compass, and the power-draining Always-on display. For moderate users, you’ll probably be fine using the watch throughout the day but, just like with the 38mm, will still need to charge your watch before the day ends.


The 40mm comes with niche features suited for health and fitness buffs while the 38mm is ideal for those eyeing their first Apple Watch without the intimidating price tag.

Amazon product

The Apple Watch Series 5 40mm builds on the premium design of the 38mm while adding and enhancing hardware features. Both watches, however, enjoy the versatility and functionality of the watchOS 6. Appearance-wise, the 40mm isn’t a radical departure from the Series 3 38mm but its larger display will definitely be easier to operate.

If you’re looking to upgrade, the 40mm does come with great new features but these functionalities — like the ECG and compass — are not something the average person will be using every day. Health and fitness buffs will probably appreciate the niche features in the 40mm better. Keep in mind that if you’re coming from the 38mm, the 40mm is its size counterpart and will be the variant that fits you. You can go for the 44mm but it’s probably not going to be as comfortable wearing.

On the other hand, if you’re new to the smartwatch range and looking to get your first Apple Watch, the 38mm is the ideal entry point into the range. Not only is it less expensive than the 40mm, it’s just as capable, too. Plus, reports suggest that the Series 3 will be compatible with the upcoming watchOS 7, so you don’t have to worry whether your watch is future-proof or not.

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Mari Bassig

Senior Editor, writer and researcher passionate about gadgets, social media, and music.