Gross means "with no deductions," so the gross area of a ceiling is the total ceiling area including lights, extractor fans and all other fittings. It usually matches the gross floor area and is calculated in the same way. When the ceiling area matches the floor it is possible to determine the area by measuring the more accessible floor, however, the method is identical whether the measurements are taken at ceiling or floor level.
Things You'll Need
 Tape measure

Determine the length and width of the ceiling in a rectangular room by taking measurements from corner to corner. Multiply the length by the width to obtain the gross ceiling area. For example, a room 15 feet long and 10 feet wide has a gross ceiling area of 150 square feet  15 times 10 = 150.

Divide irregular shaped ceilings into more than one rectangle. If necessary divide the ceiling into other regular shapes too, such as triangles and circles. Calculating the area of a regular shape is much easier than attempting to calculate that of an irregular shape.
Determine the area of each section of the ceiling and then combine them to obtain the gross ceiling area. For example, a ceiling in a rectangular room containing a chimney breast, positioned centrally on one wall, may be divided into three rectangles; the main ceiling and two smaller rectangular ceiling areas on either side of the chimney breast. 
Find the area of triangle with the formula: area = (width by length) / 2 and of a circle with the formula: area = pi by radius squared. For example, a semicircular ceiling area above a window bay with a radius of 5 feet has an area of 39.27 square feet  3.1415 ( 5^2) = 78.54, and half of 78.54 is 39.27.
Tips & Warnings
 If the floor shape and size match those of the ceiling then it is both easier and safer to measure the floor.
 If you use a ladder to reach the ceiling, follow appropriate safety guidelines; don't lean too far and be aware of your center of balance at all times.
Related Searches
References
 "Webster's New World College Dictionary 4th Edition"; Michael Agnes, ed.; 2007
 MathIsFun: Area Calculation Tool
 Mathamnesia: Finding Complex Areas
 Clark University Department of Mathematics and Computer Science; Area of a Triangle; David E. Joyce; 2003
 Math Goodies: Area of a Circle
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