converting pdfs to epubs using calibre is very slow. It has takes 2 hours+ for 40mb pdfs to convert to epub

I’m using calibre to convert from PDFs to ePub format for books but it is exceedingly slow.


Is there a faster app I can use that you have tested vs calibre and found the performance much better?


I looked online for alternatives but they all seem like ads pushing their product, so I thought it best to ask the community who won’t have a financial incentive and will have experience.

System: MacBook Air 2018 model

Processor: 1.6GHz dual-core Intel Core i5

Storage1: 128GB PCIe-based SSD

OS: Big Sur latest update (yesterday)

  • 1
    Converting from PDF to ePub is a lot of work. You've got to generate HTML-style tags to match the layout of the PDF page. Your CPU is 'not the fastest'. A 40Mb PDF of text and graphics is quite large.
    – benwiggy
    Commented May 28, 2021 at 12:06
  • Ok @benwiggy, I’m just asking is the performance among all apps therefore going to be similar?
    – Nickotine
    Commented May 28, 2021 at 12:07
  • @Nickotine You might see fractional improvements, depending on implementation, but it still going to take hours.
    – benwiggy
    Commented May 28, 2021 at 12:13
  • 1
    Some of the alternatives seem to be free, maybe just try one? It took a few minutes to convert a 40MB PDF on an M1, with one CPU core at 100%. This easily explains why it takes ages on the CPU in your Mac. Unless there are apps out there which can use several CPU cores in parallel for this you most likely will not see significant improvements.
    – nohillside
    Commented May 28, 2021 at 12:18
  • 1
    I stared at it for about 40 minutes, after which it still said 1%, at which point I decided life was too short ;)
    – Tetsujin
    Commented Jun 1, 2021 at 6:25

1 Answer 1


Using the identical 30mb pdf book:

Calibre GUI: 1hr47 mins to convert

Calibre cmdline: 37 mins to convert

The GUI converted ePub is in the calibre library dir

The cmdline converted ePub is in the calibre cmdline dir

Both working as expected


cd ~/Applications/calibre.app/Contents/MacOS

Test for yourself with:

time ./ebook-convert <pathToPdf>.pdf <pathWhereEver>.epub

Compare with GUI convert and be amazed

Add the following to ~/.bash_profile so you can just do ebook-convert:

export PATH=“/Users/$HOME/Applications/calibre.app/Contents/MacOS:$PATH”

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