Step 1: Measure Your Walls
Siding estimation starts with accurate wall measurements. For each wall, measure the width and height. For gable ends (triangular areas above the eave line), measure the base width and the peak height, then calculate the triangle area (base × height / 2). Add all wall areas together for the gross wall area.
You can measure individual walls separately or calculate the total by multiplying the building perimeter by the average wall height. The per-wall method is more accurate for buildings with varying heights or complex shapes.
Step 2: Deduct Openings
Subtract the area of all windows and doors from the gross wall area. A standard window is approximately 15 square feet, and a standard door is approximately 20 square feet. Measure each opening for accuracy, especially large picture windows or sliding glass doors.
A common rule of thumb is to deduct 10-15% of gross wall area for openings on a typical home. However, this varies significantly — a wall with a large garage door might have 30% openings, while a back wall might have only 5%. Always measure rather than estimate.
Step 3: Choose Your Siding Type
Vinyl Siding
Vinyl siding is sold by the “square” (100 square feet) in cartons containing multiple panels. Standard panels are 12 feet long with an exposure height of about 8 inches (double 4-inch profile) or 8 inches (single 8-inch profile). Vinyl has the lowest waste factor (5-7%) because panels lock together with minimal cutting.
Fiber Cement Siding (HardiePlank)
Fiber cement siding is sold as individual planks, typically 12 feet long with 5.25 or 7.25-inch exposure. It is heavier and more durable than vinyl but has higher waste from cutting (8-12%). Each plank must be cut with a fiber cement blade, and cuts produce more waste than vinyl snap cuts.
Wood Lap Siding
Wood siding (cedar, redwood, pine) is measured in board-feet or linear feet depending on the profile. Standard lap siding has a 4-6 inch exposure with 1-inch overlap. Wood has the highest waste factor (10-15%) due to natural defects, knots, and precise fitting requirements.
Step 4: Calculate Accessories
Siding installations require several accessories that are easy to overlook:
- Starter strip: Installed along the bottom of the first course. Measure the building perimeter.
- J-channel: Installed around all windows, doors, and where siding meets other surfaces. Measure the perimeter of each opening.
- Inside/outside corner posts: One for each building corner, measured to wall height.
- Soffit and fascia: Often ordered separately but part of the exterior package.
- Utility trim: Used at the top of walls under soffits.
Waste Factors by Material
Every siding installation produces waste from cutting around openings, fitting at corners, and trimming at eave lines. Recommended waste factors:
- Vinyl: 5-7% for simple walls, 10% for complex facades with many openings.
- Fiber cement: 8-12% due to heavier cutting waste.
- Wood: 10-15% due to natural material variability and precision fitting.
Common Estimation Mistakes
- Forgetting gable end areas (triangular sections above the eave line).
- Not accounting for J-channel around every opening.
- Using gross wall area without deducting windows and doors.
- Ordering insufficient starter strip (need full perimeter).
- Ignoring the difference between panel face area and coverage area (overlap reduces effective coverage).
Calculate Your Siding
Use our siding calculator to estimate siding quantity from wall area with opening deductions. It supports vinyl (squares), fiber cement (boards), and wood (board-feet) with appropriate waste factors for each material type. For the framing behind your siding, check the stud wall framing calculator.