When you do the online update, not using iTunes, it also needs about 3 GB to build the new system image. Once the update is completed, the effect on storage will be minimal or none.

How much data is needed to update iOS?

An iOS update typically weighs anywhere between 1.5 GB and 2 GB. Plus, you need about the same amount of temporary space to complete the installation. That adds up to 4 GB of available storage, which can be a problem if you have a 16 GB device. To free up several gigabytes on your iPhone, try doing the following.

How many GB does it take to update to iOS 13?

iOS 13 update will require at least 2GB of free space, so if you’re running low on free space on your iPhone or iPad, it might be a good idea to free up some space by deleting unwanted stuff from your device. You should have at least 2.5GB or more free space to be on the safe side.