Good measurement is less about complicated maths and more about being systematic. Measure each room separately, write the dimensions down and do not rely on a single overall guess.
Room-by-room method
For a rectangular room, multiply length by width. Record the unit clearly and keep a consistent measurement convention throughout the project. For irregular rooms, split the plan into rectangles, calculate each part and add them together.
Openings and unusable areas
Door thresholds, fixed shafts, large built-in furniture or other areas that will not receive the selected finish can affect the net area. Mark them separately rather than silently subtracting them.
Waste is not a universal number
Diagonal layouts, staggered patterns, large-format pieces and complex cuts can require more allowance than a straightforward grid. The supplier or installer can confirm the practical yield once the actual layout is known.
Why site verification still matters
Walls are not always square, levels can vary, and site conditions can change the final cut plan. Treat an online calculation as a planning estimate — not a final measurement certificate.