Loch Achall is a long narrow loch, at its widest point. It is dominated by the peaks of the Marilyn's Beinn Eilideach directly to the south, at and the peak of Meall Liath Choire to the north east at . The loch is fed by the Rivers Rhidorroch and Allt a'Ghiubhais, while the River Ullapool flows from its western end towards Loch Broom.