How do I measure my door opening correctly before ordering?

Take three width readings (top, middle, bottom) and three height readings (left, centre, right) and use the smallest of each. Measure wall thickness for the jamb depth, and check the floor is level. If you are replacing a slab only, measure the existing leaf itself — width, height and thickness — not the opening. Photograph the hinge side and swing direction; this prevents most ordering errors.