PyTorch CNN Shape Errors: Conv2d Output Sizes, Channels, Flatten Bugs and How to Debug Them

Sat, 08 Aug 2026 13:26:00 +0100

PyTorch: Zero to Hero — Step 06

CNN code is usually easy to write.

CNN shape bugs are usually easy to create.

A typical failure looks like this:

RuntimeError: Given groups=1, weight of size [32, 3, 3, 3],
expected input[64, 224, 224, 3] to have 3 channels,
but got 224 channels instead

Or this:

RuntimeError: mat1 and mat2 shapes cannot be multiplied

Or worse: the model runs, but the dimensions are wrong in a way that silently damages the architecture.