Doubling Framerate Without Shortening Clip

About a year ago i was able to take a 1080p 30fps video and convert it to 60fps. i was able to do this without the clip playback speed increasing and without the clip shortening itself. Infact since I doubled the frame rate the length of the clip also almost doubled because of the added frames (which is exactly what i wanted) Unfortunately I can't remember how i did it, and so far every setting I've tried has sped up the video clip and shortened the video clip. So can someone tell me how I can double the frame rate of a clip (the clip i want to double the frame rate of currently has a frame rate of 20fps which i want to increase to 40fps) Again i want to double the frame rate without the video speeding up and without the video shortening. I am currently using Davinci Resolve version 17.1 (the free version) on a Windows 11 PC.

1，Create a timeline with framerate 60.
2, Drop your 30 framerate vedio into the 60 framerate timeline.
3, Change the Retime Process to Optical Flow.

Optical Flow, for the win.
If your system can handle it, use "Enhanced Better" or the bottom option.
Once you do that, if it slows down your editing too much just "Render In Place" your clip.
The results for most types of content are incredibly convincing.

"Speed Warp", which is only available for single clips, is even better.
But dead slow if you don't have a powerful GPU.

"Speed Warp", which is only available for single clips, is even better.
But dead slow if you don't have a powerful GPU.

where is that setting located? I have a RTX 2070 Super so I don't think it will be a problem.

Optical Flow, for the win.
If your system can handle it, use "Enhanced Better" or the bottom option.
Once you do that, if it slows down your editing too much just "Render In Place" your clip.
The results for most types of content are incredibly convincing.

Okay I'll give it a try tonight. Also someone mentioned Speed Warp, do you have any experiance with that setting?

"Speed Warp", which is only available for single clips, is even better.
But dead slow if you don't have a powerful GPU.

where is that setting located? I have a RTX 2070 Super so I don't think it will be a problem.

Speed warp is in paid version only, can't tell if you are using free version or not

And it shows only in the Inspector settings for retiming, not in the project.

Okay I'll give it a try tonight. Also someone mentioned Speed Warp, do you have any experiance with that setting?

As pointed out, Speed Warp is in the Studio version.
Nothing of this sort is ever going to be "perfect", but Speed Warp is pretty close.
There are often issues where stationary objects get distorted by a moving object, and Speed Warp seems to do the best job of managing that... but sometimes one of the other options actually does a better job, so you have to check on a per-clip basis.
I had some footage shot on a webcam in 2007, at 15 fps at 640x480 that I managed to zoom up to 1080 using Topaz Video Enhance and convert to 60fps using Resolve. It's not perfect, but it is still really good compared to the original.

1，Create a timeline with framerate 60.
2, Drop your 30 framerate vedio into the 60 framerate timeline.
3, Change the Retime Process to Optical Flow.

Okay so i just tried this and while it did add frames it also shortened the video because it sped it up and sped up the audio so now it sounds like a Munchkin talking. is there a way to get it to add frames but not sped up the video? I did it with that 30fps to 60fps video a while back I changed it to 60 fps and it extended the length of the video and didn't sped up the video or audio. but again i can't remember how i did it.

I have a RTX 2070 Super so I don't think it will be a problem.

So do I and it rendered a clip at 9fps max on a system with 64GB RAM and an AMD Ryzen 2700X.

So instead of creating new frames which always will have side effects (this is good for single shots) use the same technique, but set mode to Nearest, so you will get repeated frames.
Why do you want 60fps video out of 30 anyway ?
You have many other 60p sources and just one 30p and you want to include it in the edit?

Well, Andrew, it seems he wants the smoothness of 60 fps and is inserting a few clips with 30 fps, maybe even with short exposure times or panning too fast. In such cases, Nearest will show stutter.
Speed Warp can do miracles for this, if there's not too much movement of objects crossing over each other.