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Siding Calculator
Use this free siding calculator to find out how much siding your house needs before you order. Enter up to four walls, subtract the windows and doors, pick a siding type, and add a waste allowance for cutting. The calculator gives you the net wall area, the number of squares, and the boxes and pieces to order, rounded up so you do not run short.
Set a wall to 0 × 0 to leave it out.
How it works
Siding is sold by the square, and one square covers 100 square feet of wall. The calculator starts by finding the total wall area: the length of each wall times its height, added up across all four walls. Then it subtracts the area of every window and door, because those openings do not get sided. What is left is the net wall area that actually needs covering, and that is the number everything else is based on.
Waste is the allowance for cutting boards to fit around corners, windows, and eaves, plus the odd damaged piece. Ten percent is the standard default for a straightforward house with rectangular walls. Raise it to 15 percent for a house with lots of gables, angles, or dormers, where every board seems to need a cut. The calculator adds the waste to the net area and then divides by 100 to get the number of squares, always rounding up, because a supplier will not sell you a fraction of a square.
From squares, the calculator works out boxes and pieces for the siding type you picked. Vinyl lap siding usually comes in boxes covering 200 square feet, or 2 squares, with 24 pieces per box. Fiber cement and wood lap are commonly sold in boxes covering 100 square feet, with 10 pieces per box for fiber cement and 12 for wood lap. Metal siding is usually boxed at 200 square feet with 16 pieces. These are typical pack sizes, so check the coverage printed on the product you are buying and adjust the square coverage field if your boxes differ.
Measuring the walls is the part that takes the most care. Measure the length of each wall at its base and the height from the foundation to the top of the wall, not to the peak of the roof. For a gable end, measure the rectangular part and then add the triangle above it, which is half the base times the height of the triangle. Measure windows and doors by their outside trim dimensions, since the siding has to be cut around the trim, not just the glass. If your house has more than four walls, measure the extra walls separately and add their area into one of the wall rows.
Do not forget the extras when you order. Siding jobs also need starter strips along the bottom, J-channel around windows and doors, corner posts, and soffit and fascia material for the eaves. This calculator covers the field siding only, so budget separately for trim pieces. And order the whole job from one batch if you can, because dye lots vary slightly between production runs and two batches of the same color can look different side by side on a wall.
Worked example
A house has two walls of 40 feet by 10 feet and two walls of 30 feet by 10 feet, with 8 windows measuring 3 by 5 feet and 2 doors measuring 3 by 7 feet. The siding is vinyl lap and the waste allowance is 10 percent.
Step 1: Find the wall area — (40 × 10) + (40 × 10) + (30 × 10) + (30 × 10) = 1,400 sq ft.
Step 2: Find the openings — windows 8 × 3 × 5 = 120 sq ft, doors 2 × 3 × 7 = 42 sq ft, total 162 sq ft.
Step 3: Net area — 1,400 − 162 = 1,238 sq ft.
Step 4: Add waste — 1,238 × 1.10 = 1,361.80 sq ft to cover.
Step 5: Squares — 1,361.80 ÷ 100 = 13.62, rounded up to 14 squares.
Step 6: Boxes and pieces — 1,361.80 ÷ 200 = 6.81, rounded up to 7 boxes, and 7 × 24 = 168 pieces.
The job needs 14 squares of vinyl siding, which is 7 boxes and 168 pieces. Ordering 6 boxes would leave the last stretch of wall bare, so the rounded-up seventh box is the waste allowance in physical form.
FAQ
How many square feet are in a square of siding?
One square of siding covers 100 square feet of wall. It is a trade unit, not a shape: a square can be any combination of boards that adds up to 100 square feet of coverage. The calculator divides your net wall area, plus waste, by 100 to find how many squares you need.
How much waste should I allow for siding?
Allow 10 percent for a simple house with straight rectangular walls and few openings. Allow 15 percent for a house with gables, dormers, or lots of windows, where more boards have to be cut. The default in the calculator is 10 percent, and you can change it to match your house.
Do I subtract windows and doors from the siding area?
Yes. Windows and doors do not get sided, so their area comes off the wall total before you work out squares. Measure them by their outside trim dimensions. Some installers skip the subtraction on small jobs as a built-in waste allowance, but subtracting and then adding a waste percentage is more accurate.
How many boxes of vinyl siding do I need?
It depends on the box size. A typical vinyl lap box covers 200 square feet, or 2 squares, so divide your total square footage including waste by 200 and round up. The calculator does this for you and also shows the piece count, which is 24 pieces per box for the vinyl lap option.
What is the difference between vinyl, fiber cement, and wood siding?
Vinyl lap is the cheapest and easiest to maintain, but it can crack in extreme cold and fade over time. Fiber cement is heavier, holds paint well, and resists fire and insects. Wood lap looks the most natural but needs regular painting or staining. Metal siding sits between vinyl and fiber cement on price and is very durable.