I don't know the true fabric weight, but the actual weight of my shirt, size XL, is about one pound. It is a substantial fabric, heavier than any of my other six or eight cotton flannel shirts of different brands.
The shirt is a mid-level flannel. Not light but not the heaviest I've had. I like it because it's warm enough on it's own but I can put a light sweater over it and not be too warm.