Trim the composition you want to loop so that it's one frame longer than the time it takes to loop. You can do this by going to the Composition Settings (command/Ctrl K) and adjusting the duration.
The reason for this step is that nothing will show up on the last frame of the composition (when we add the Time Remap property in step 4). So if the composition is the exact duration that it takes to complete a loop, the last frame won't be there, so it won't be a perfect loop.
Put this composition into another composition. An easy way to do this is to drag the composition in the Project panel onto the New Composition button.
With the new composition (the one that has another composition nested inside it) open, hit command/ctrl K to get to the Composition Settings. Adjust the duration to be however long you want it to be.
If you want this entire composition to loop too, make the duration a multiple of the duration of the nested composition. For example, my nested composition is 1:02 and I multiplied that by 10 and made this composition 10:20.
Right click on the nested composition and choose Time -> Enable Time Remapping.
Drag the end of the nested composition to fill the timeline.
Applying Time Remapping will automatically set two keyframes. Move your playhead to the frame before the second keyframe. Set a new Time Remap keyframe. Delete the last keyframe.
The reason for this step is that nothing is visible on the last frame of the composition (in my example, this is 1:03). So if you skip this step, there will be a blank frame every time it loops.
Option/alt click the stopwatch for the Time Remap property. Type in the expression: loopOut()
Now your this nested composition should loop!
Learn how to create seamlessly looping animations for websites, social media, or as part of longer, narrated animations.
