Why can you only generate at resolutions that are multiples of 8?

If you look at ComfyUI's Empty Latent Image node, width and height can only be set in steps of 8. Even if you force in something like 513px, the output is always rounded to a multiple of 8.

This comes from a structural constraint of latent diffusion models and the VAE.

Latent diffusion models and the VAE's compression ratio

Latent diffusion models such as Stable Diffusion first compress the image into a low-resolution latent space and then do their calculations there. The VAE is what handles this "compression" and "restoration".

Commonly used VAEs (the SD1.5 family and many derived models) are built to:

  • Compress height and width to 1/8 (1/64 in area)

Since the latent resolution has to be a whole number, any remainder that is not divisible by 8 is dropped.

For example, if you input a 513px image, the extra 1px is ignored and it is effectively processed as 512px.

VAEs with other compression ratios

Not every VAE compresses by 8x. Depending on the model, it can be 4 / 8 / 16 / 32, and so on.

When you use a new model, it is a good idea to check which VAE (and which compression ratio) it uses.

Video model VAEs and frame counts

Some video VAEs also compress along the time axis (the number of frames).

  • Wan 2.1 VAE: a VAE that packs 4 frames into one latent

In this case, the number of frames you can generate takes a form like "1 + 4n", and fine-grained choices such as outputting just 2 frames may simply not be possible.

Just as the spatial dimensions (width and height) are tied to multiples of 8, the time axis can also come with a "only in multiples of this number" constraint. Just keep that in the back of your mind.

Caveats for image2image and image editing models

You can use a resolution that is not a multiple of 8, and generation will still work without errors. In that case, though, watch out for misalignment.

  • Input image: the original resolution (e.g. 513px × 769px)
  • Output image: the resolution rounded to a multiple of 8 (e.g. 512px × 768px)

Because input and output pixels no longer line up 1-to-1, if you need pixel-perfect edits, crop or pad the image to a multiple of 8 first.

If something feels off, like "the edge seems to be cut off" or "the size is slightly different", check whether the resolution is a multiple of the VAE's compression ratio.