iOS 8.1.3 reduces the free space you need to upgrade
Apple today made iOS 8.1.3, a minor update to its mobile operating system, available for public download. While the majority of the improvements offered by iOS 8.1.3 are bug fixes, the update reduces the amount of free storage required to perform an over-the-air update. When iOS 8 was first released to public, users were required to have up to 7GB of space to upgrade their devices, a problematic scenario for owners of 16B iPhones and iPads.
On top of the space required to download an update, an iOS device updating over-the-air also needs space to unpack and install updates. As such, users updating their iPhones and iPads through iTunes did not encounter similar problems.
Apple has also been testing iOS 8.2, a larger release designed to enable compatibility with the Apple Watch. This update is expected to arrive alongside Apple's wearable.