Count faces and layers deliberately
A wall face and its opposite face are separate installation surfaces. A second layer adds another sheet area. In the drywall calculator, quantity repeats the complete surface, including its openings; layers repeat the net sheet coverage.
Decide how to handle openings
A large opening reduces installed area, but a sheet spanning the opening may still be needed before cutting. Deduct only the openings that suit your installation plan. The tool rejects an opening wider or taller than its face and a combined opening area that consumes the complete face.
Compare the two rounding methods
Separate surfaces keeps each face and layer independent. Combined area allows all net area to share a pool of sheets. Two 1 m² faces and a 2.88 m² sheet illustrate the difference: the methods produce two sheets and one sheet respectively before any allowance.
Neither method draws the cut pattern. Check face dimensions against sheet dimensions and the intended joint locations before relying on the lower number.
Confirm the product specification
Use the actual sheet size. A metric sheet and a similarly described US sheet need not have the same area. Board type, required layers, fixing details and suitability come from the installation specification; this tool estimates quantities after you supply those choices.