Siding acts like a skin for a house, offering both protection and beautification. With a seemingly endless list of siding types and specifications, it can seem difficult to estimate quantities. Using basic tools like a tape measure and calculator, the task of calculating siding needs becomes a simple one that can be completed by just about anyone.
Things You'll Need
 Tape measure
 Calculator
 Pencil
 Paper

Measure the height and length of a wall to be covered in siding using the tape measure. Multiply these numbers to find the total surface area (in square feet) of this wall. For gable walls that have a triangular shape, find the area of the lower rectangular section first, then find the area of the triangular portion by multiplying half the base length by its height, then add this amount to the rectangular area to find the total area of the wall.

Measure and multiply the height and length of any wall openings (doors, windows) to find their total area. Subtract these amounts from the total wall area to find the total amount of wall surface (in square feet) to be covered.

Repeat Steps 1 and 2 and on all remaining walls to be covered with siding. Add the surface area of all walls to find the total square footage of wall surface to be covered.

Divide the total wall square footage by the square foot coverage of the siding (per unit/package) you will be using to calculate the total number of siding units/packages you will need for the house. This coverage will be indicated on the product packaging.
Tips & Warnings
 Adding 10 percent to your final result will account for any wasted material during application.
 If the siding product does not indicate its coverage on the packaging, you can consult with a siding specialist at your local building supply store.
 It may be easier to measure walls and calculate their area in inches, then convert to square feet later. To convert square inches to square feet, divide the total square inch amount by 144 (the number of inches in a square foot).
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References
 "Estimating in Building Construction (6th Edition)"; Frank R. Dagostino and Leslie Feigenbaum; 2002