How to use the siding calculator
- Add one row for each wall and each gable. Enter the width and height of a wall, or the base and rise of a gable triangle, and the total area of windows and doors on that surface.
- Enter the piece length and exposure of the siding you plan to buy. These start as example values and set how much wall one piece covers.
- Keep the 10% waste or change it. Open More options to enter the number of corners, the corner height, trim around openings and the trim length.
- Read the squares, the number of siding pieces and the trim pieces, then copy the shopping list or add a price per square.
Formula
Wall area = width x height
Gable area = base x rise / 2
Net area = sum of wall and gable areas - openings
Squares = net area / 100 (one square covers 100 square feet)
Area with waste = net area x (1 + waste percent / 100)
Piece coverage = piece length in feet x exposure in inches / 12
Pieces = area with waste / piece coverage, rounded up
Trim pieces = trim length x (1 + waste) / trim piece length, rounded up
A square is the unit siding and roofing are priced and sold in. The exposure is the height of the visible face of each course. Starter strip length is the total width of the wall rows, and corner posts use the corner count times the corner height.
Worked example: a 40 by 30 foot house with 9 foot walls and two gables
The four walls are 40 x 9, 40 x 9, 30 x 9 and 30 x 9, which add up to 1,260 sq ft. Each gable is 30 x 8 / 2 = 120 sq ft, so the two gables add 240 sq ft. The gross area is 1,500 sq ft.
Windows and doors on the four walls take out 60 + 45 + 20 + 20 = 145 sq ft, leaving 1,355 sq ft, or 13.55 squares.
With 10% waste the area is 1,490.5 sq ft, or 14.91 squares. A 12 ft piece with 8 inch exposure covers 12 x 8 / 12 = 8 sq ft, so you need 1,490.5 / 8 = 186.3, rounded up to 187 pieces.
The starter strip runs 140 ft along the four walls, which needs 154 / 12.5 = 12.3, so 13 trim pieces, and four corner posts of 9 ft total 36 ft, which needs 4 pieces.
Measuring walls and gables
Measure the width and height of each wall from the bottom of the siding to the top, including the part above the roof line on a gable end. Gable triangles are measured by their base across the bottom and their rise from the base to the peak, and the tool halves the product for you. If a gable has a window or vent, enter its area as an opening.
Measure windows and doors by their outside size, including trim, and add them together for each wall. Small openings such as vents and light fixtures are often ignored, but they cost nothing to include and make the estimate a little tighter.
Squares, pieces and boxes
Siding is priced by the square but sold in many forms: panels, boards, shingles and cartons. This tool gives both the squares and the number of pieces for the size you enter, so you can match either unit to your quote. Cartons hold a set number of pieces, and the number on the carton usually states the square footage it covers, so divide your need by that figure to count cartons.
The example piece is a 12 foot length with an 8 inch exposure. Real products differ, and some include an overlap or nail hem in their length, so enter the coverage the maker states in place of the example when it is available.
Trim and accessories
Starter strips, corner posts, J-channel and window trim are usually sold in lengths of about a dozen feet. Enter your trim piece length, count the corners, and the tool turns linear feet into pieces. Soffit, fascia, house wrap, flashing and fasteners are separate, so ask your supplier for a full materials list before you order.
Assumptions and limits
- Siding runs horizontally and every piece is cut from standard lengths. Vertical siding and shingles use a different coverage rule.
- Piece length, exposure and trim length are example values. Use the figures on your product.
- Waste is a single percentage for siding and trim. Gables and walls with many windows may need more.
- Starter strip length equals the total width of the wall rows. Soffit and fascia are not included.
- Local code, house wrap and fastener requirements are not covered. Follow the manufacturer instructions.
Frequently asked questions
How many squares of siding do I need?
Add the area of every wall and gable, subtract windows and doors, and divide by 100 square feet. Then add a waste allowance. A net area of 1,355 square feet is 13.55 squares.
What is a square of siding?
A square covers 100 square feet of wall. Siding and roofing are often priced per square, which makes it easy to compare quotes.
How do I measure a gable for siding?
Measure the base of the triangle across the bottom and the rise from the base to the peak. The area is the base times the rise divided by 2.
How much extra siding should I buy?
Ten percent is a common starting point for a simple house, and complex rooflines and many openings can call for more. The tool lets you pick the percentage.
Does this calculator work for vinyl, fiber cement and wood siding?
It works for any lap siding if you enter the real piece length and exposure. Other products may sell by the box, so convert your area using the coverage shown on the product.