I have a Macbook air 2015 with 128 GB storage and I just upgraded from Yosemite to Mojave. Surprisingly, after upgrade to Mojave, the amount of storage I had left went from 36 to 46 GB.

So my question is, if I update to Catalina or Big Sur, will this free up even more space or will it consume more?

2 Answers 2


Normally, one OS update replaces the file contents of the previous one. While there might be some small variation in size between OS versions, I would not expect an additional 10 GB to get used up.

However, some space will be needed to download the new OS installer before it is launched.

I would install the OS, and then check how much space you have. There are disk utilities like OmniDiskSweeper that sort your whole drive by size, showing you exactly what is taking up the space and letting you delete it, if that is advisable.

You may also want to check if you have any backups of iPhones or iPads saved to your Mac, as these can take up large amounts of space.


You should not have an issue upgrading to Catalina nor Big Sur. However, you should upgrade the storage to at Leary 256GB / 480Gb or 512GB. Depending on your budget. 128GB is not going to leave much room for Apps and data.

That model MacBook Air has an SSD you can replace. Otherworld Computing sells kits that include an SSD, screwdriver (pentalobe) and a USB3 external case for your old drive. Then you can clone the old drive to the new one and expand the volume to fill the disk. You could have a shop do this for you. You could buy the parts elsewhere. Fact is it’s doable. Plenty of options. OWC may not be the cheapest but they provide everything you need and tech support plus video guides. It’s not that hard. You remove the screws from the back. Make sure you remember where each screw goes as they are not all the same length. Then it’s just one screw to remove the SSD. Put the old SSD into an external case. SuperDuper or Carbon Disk Cloner can duplicate the old drive to the new drive and expand it to give maximum capacity.

  • 3
    128GB can be plenty of storage depending of what you do. This is only depending on your usage. Instead of changing the internal SSD it is cheaper and (IMO) mostly much easier to bu a fast external SSD. Further, it may seem rather easy for you top open a MacBook and change internals, but for less tech-savvy people this is something I would definitely not recommend!
    – X_841
    Jan 22, 2021 at 9:13
  • Concerning @X_841 comment above about not all being comfortable in doing a drive upgrade. While someone may not be comfortable to upgrade their drive it is possible and there are places that will do the upgrade for a reasonable fee. This service can include restoring the files and a secure refurbishment or recycling of the old drive, they keep the old drive for resale and put that as a credit towards the parts and service.
    – MacGuffin
    Jun 22, 2021 at 5:00

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