A method is used in sizing volatile memory (VM) cache based on flash-based cache usage. A user selection for a flash-based cache is received. Based on the selection, configuration and sizing factors are provided, by a flash based cache driver, to VM cache size determination logic. Based on the configuration and sizing factors and a sizing formula and rules, a requested VM cache size is produced by the VM cache size determination logic. Based on the requested VM cache size, the VM cache is caused, via VM cache resizing logic, to be resized to the requested VM cache size.