Assuming the smooth bottom version, there is a notable radius at the corners and a lesser one where the sides meet the bottom, plus there is some draft to the sidewalls. As such it's difficult to be too precise. They are roughly 9.5 x 11.5 inches inside. There is a significant rolled edge around the outsides for stiffening and they are about 11 x 13 overall on the outside with an additional quarter inch "jog" in length and width at one end where there is a pouring spout at one corner. I use mine for tray developing 8x10 X-ray film for what it's worth.
Hope that helps,