A reliable estimate begins with a small list of measured rectangles, not one unexplained room total. Record each distinct wall and the ceiling separately, and use quantity only for surfaces that truly share the same dimensions.
Create one row per distinct surface
Measure the face width and height of each wall in one unit system. Two opposite walls may share a row when their measurements match; otherwise keep them separate. A flat ceiling is another rectangle whose length and width can use the same two fields. This keeps every multiplication visible: width × height × quantity. Avoid combining measurements mentally, because a misplaced unit or forgotten wall is harder to find inside one total.
Add gross areas before waste
For each row, multiply the two dimensions and quantity, then add the row areas. That sum is gross surface area. DrywallCalc does not subtract ordinary doors and windows: sheets still need cuts around their edges, and simple area arithmetic cannot know whether each offcut fits elsewhere. Keeping openings inside the total is a conservative boundary, not a claim that material can never be reused. A real layout plan may find savings that this calculator deliberately does not promise.
Keep units explicit
Metric project surfaces use metres, while the metric custom sheet fields use millimetres because that is how many products are labelled. Imperial surfaces and custom sheets use feet. Presets remain exact: 1200 × 2400 mm and 4 × 8 ft are not treated as interchangeable rounded sizes. Switching systems loads fresh starter values instead of silently converting work in progress. Recheck every row after measuring and before adding your chosen whole waste percentage.