Comment 34 for bug 132311

@Kevin: The way UM does its calculation is to count up all the kernels that are marked to-be-installed, and then simply allocates N KB per new kernel, which includes the initrd, vmlinux, etc files. Through observation I noticed that it's assigning 18*1024*1024 bytes for this, which on one of my systems is undercounting by 260KB (although on another system, it's just about right, so hmm...)

IIUC, the code uses statvfs on the underlying /boot file system to determine how much space is already consumed, so it doesn't have to count kernels that are already installed specifically.

Do you know how much space you had available in the /boot filesystem when it failed?

You generally only need one active kernel at a time. You can probably use Computer Janitor to clean up those old kernels and free up /boot space. Some people feel it's good to keep at least one old (i.e. not active) known-good kernel around just in case you find a problem with the one that is active, i.e. the last installed.