The global shipments of wearable devices are expected to grow from 113.2 million units in 2017 to 222.3 million in 2021 with a compound annual growth rate (CAGR) of 18.4%, according to IDC.

The most popular wearables to date have been basic wristbands like the Xiaomi Mi Band or Fitbit Charge. However, such wearables are quickly becoming commodities and IDC anticipates low single-digit growth in this category throughout the forecast period. Meanwhile, watches (both smart and basic) are on track to take the lead and are expected to grow from 61.5 million in 2017 to 149.5 million in 2021 as more vendors and cellular connectivity built into smartwatches help to drive growth.

By categories, the simplicity and low cost of basic wristbands (fitness bands) make them highly accessible, particularly in emerging markets around the world. Although they will account for the majority of device shipments in 2017 (39.8%), this category is expected to decline in share to 21.5% by 2021.

In many ways, basic watches (hybrid watches) are the evolution of fitness bands but they include significantly better design and fashion appeal. The entry of additional fashion brands along with their unique customer base and distribution network will help to drive growth.

Shipments of smartwatch, led primarily by the Apple Watch, are expected to total 71.5 million units by 2021, up from 31.6 million in 2017. Contributing factors include the adoption of cellular connectivity, additional SKUs from fashion brands, and the transition of kids watches (a phenomenon largely relegated to China) from basic location tracking watches to more sophisticated watches that allow kids to play games, run apps, and communicate with friends/family.

The emerging category of sensor-laden clothing, led mainly by step-counting shoes, will be a distant fourth as many of the features will be copies of wristbands and watches.

Shipments of earware devices are expected to reach 10.6 million units by the end of 2021 with a 58.5% CAGR from 2017-2021. It's important to note that this category excludes traditional Bluetooth headsets. Instead, it is comprised of wireless headphones that offer additional features such as fitness tracking or audio augmentation.