Estimating/Predicting CRF bitrate or file size

[Makaveli84]

As the title says, is it possible to estimate beforehand the expected bitrate or file size of a given encode at a given CRF factor??
The reason I ask is because I would like to estimate the output size of an encode at CRF 20 for example, then actually encode the video in a size based 2-pass mode. I am not trying to keep the encodes below a certain file size, but rather, trying to maximize quality at the best file size possible. So encoding in 2-pass without a quality reference would be flying blindly. That's why I rely on a constant quality CRF pass to get the needed bitrate for a 2-pass encode to achieve a certain level of quality.
But as you can imagine, this is a very slow and time/resources consuming approach. This is why I'm looking for some kind of estimation method or quick virtual CRF pass that estimates the bitrate quickly without actually encoding the video.

Thanks.




P.S: Please, let the CRF vs 2-pass debate for another thread. To me, based on the testing that I've done, 2-pass produces slightly better (but pretty noticeable) quality than CRF at a given bitrate.