macOS Big Sur gives a message telling me I don't have enough free disk space

If you want to assure adequate free space for upgrading as well as optimal system and application performance try to maintain a minimum of 50 GB of free space on your boot drive.

The files that you have control over are located in the Documents, Downloads, Pictures, Music and Movies folders.  You can use either of these two free apps, GrandPerspective  or OmniDiscSweeper, to find the largest files on your drive so you can determine if they can be deleted or moved to an external HD for storage.  

Note: you can empty the Downloads folder after the apps and/or updates that were downloaded have been installed or applied.  Many users have found a couple of Gigabytes of files in their Downloads folder which are no longer needed. 

As already stated " Empty Space Requirements " .

There is no workaround except to remove unneeded Applications, Documents, Music, Videos, Podcasts Old iPhone Backups, and out dated TM Snap Shots lingering around.

In addition to all of the other excellent advice, you need to consider this:

Your storage space isn't just storage; it also contains the OS and all of its system files and they need an absolute minimum of 15 - 20 GB of empty space at all times to function properly. So, even if you were able to download and install it with your remaining 40 GB, you would experience serious problems after that. And, if you fill it up completely, it will simply stop functioning and very difficult to create some space unless you have access to and can boot into an external and access your drive from there.

If you are upgrading from High Sierra or earlier you need 44.5GBs.

There is no way "around" this issue. The documentation on the space required is somewhat inconsistent, probably because people doing the install/upgrade can have any of scores of different configurations, and the space required is somewhat configuration dependent. This article, which I bookmarked several months ago

https://www.iphonetricks.org/macos-big-sur-not-enough-free-space-to-upgrade/

explains some of the details and includes some suggestions.

Basically the update file requires slightly more than 12 GB for and an additional 32 GB to perform the installation. Approximately. May be slightly different on different Macs.

By the way, I strongly advise that you provide the required space plus some additional "margin," like another 10 GB or so. Because if you run out of disk space in the midst of the operating system install, the results could be a Mac that has no usable OS and won't start up. That is a hard problem to fix will take a lot longer to solve than the problem of moving files off your computer (temporarily), which is much easier to do. So I would take the easier route and not cut it close on disk space required.

Every little bit helps, be sure to empty the trash, and restarting the Mac in Safe Mode will clear some other temporary cache files. Look at your Downloads folder, often those files can be deleted. Search your Mac for all files whose name contains ".dmg" because those are installers and probably can be deleted. You can run OmniDiskSweeper (free) or WhatSize or DaisyDisk to identify the largest files on your system, some of which might be things you can delete. Another place that may have large files is your user Library, Application Support, MobileSync folder which contains iPhone backups. Those can be many GB, and can be temporarily moved to an external drive to free up some space (they can be moved back later, or just do a new backup in place).

Revision # 9 - June 2, 2021

Tips How to Upgrade to Big Sur 

There are several points to consider before attempting the upgrade to Big Sur. The most important is Due Diligence on the part of the User and is a Prerequisite for a smooth upgrade.  

The below points should all be consider Before attempting the upgrade.

Point # 1 - Are there pre-existing Unresolved Software Issues. They should be resolved prior to upgrading - they may carryover to Big Sur making the system unstable / unusable.

Point # 2 - Are there any AntiVirus, Disk Cleaner, Optimizers, Defrag, VPNs etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation. The Built-in Security of Big Sur is all that is required.

Point # 3 - What are the System Requirements ie: Empty Space Requirements specific to the Big Sur.

Point # 4 - Does the intended computer qualify for the upgrade.

Point # 5 - Are the installed Applications / Extensions compatible with Big Sur. Check with the Developer for compatible Apps / extension or do they offer updates. Same goes for Third Party Drivers for Printer, Scanners etc.

Point # 6 - If upgrading from Mojave and earlier versions of macOS - Remember that Big Sur does not support anything 32 bit. These include Applications, Extensions and Drivers for any devices. There is no workaround.

Point # 7 - Do I have a Rescue Plan In-Place. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup is very useful and can be used to Revert to Previous Working macOS. For addition purposes - two Tested Bootable Clones each to separate external drives. This is insurance incase the upgrades goes sideways. At least one Tested Clone and / or Time Machine Backup should be Off - Site

Point # 8  - To upgrade to Big Sur will require a quick, stable and reliable Internet Connection - preferred method is via Ethernet Connection. If not available and using Wifi - move computer as close to the Access Point / Router and disconnect all other devices connected to Wifi. Want the maximum bandwidth for the computer being upgraded to Big Sur.

Good Luck and come back to these Forums if there are questions, concerns or comments.