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How to calculate ceiling square footage?

For a flat ceiling, calculate square footage the same way as floor area: multiply the room's length by its width. A 14-foot by 12-foot room has a 168-square-foot flat ceiling. The ceiling matches the floor directly beneath it as long as the walls are vertical and the ceiling is level.

A vaulted, sloped, cathedral, tray, or coffered ceiling has more surface than the floor below it. Measure the length of the room and the sloped distance from ridge to wall, then multiply those two figures for each sloped plane. On a symmetrical gable ceiling, calculate one sloped side and double it.

When estimating paint, drywall, insulation, or ceiling tile, measure each distinct surface on its own. Subtract large skylights or openings only when their size actually affects the order.

For paint, the ceiling area is just one input. You also need the product's coverage rate, the number of coats, surface texture, primer requirements, and a reasonable allowance for waste.