How to use the siding calculator
- Measure around the house along every wall you are siding, and the wall height from the foundation to the eaves.
- Add the gables. Enter how many triangles there are under the roof ends, and the width and height of one.
- List windows and doors. Group the same sizes together with How many.
- Pick your product. This sets the exposure (how much of each piece shows) and the piece length.
- Count corners for corner posts, and add prices for a materials total.
The siding formulas
Walls = perimeter × wall height
Gables = number × width × height ÷ 2
Net = walls + gables − windows and doors
Squares = net × (1 + waste) ÷ 100
Pieces = area with waste ÷ (piece length × exposure ÷ 12)
Worked example
A 140 ft perimeter, 9 ft walls, two 30 × 8 ft gables, 10 windows and 2 doors
- Walls = 140 × 9 = 1,260 sq ft. Gables = 2 × 30 × 8 ÷ 2 = 240 sq ft.
- Openings = 10 × (3 × 4) + 2 × (3 × 7) = 162 sq ft.
- Net = 1,260 + 240 − 162 = 1,338 sq ft.
- With 10% waste = 1,472 sq ft = 14.7 squares.
- Fiber cement at 7 in exposure, 12 ft pieces: each covers 7 sq ft, so 1,472 ÷ 7 = 210.3, 211 pieces.
Coverage by product
| Product | Exposure | Piece | Covers per piece | Pieces per square |
|---|---|---|---|---|
| Fiber cement lap 8 1/4 in | 7 in | 12 ft | 7.0 sq ft | 14.3 |
| Vinyl double 4 | 8 in | 12 ft 6 in | 8.3 sq ft | 12.0 |
| Vinyl double 4.5 | 9 in | 12 ft 6 in | 9.4 sq ft | 10.7 |
| Vinyl double 5 | 10 in | 12 ft 6 in | 10.4 sq ft | 9.6 |
| Wood bevel 6 in | 5 in | 12 ft | 5.0 sq ft | 20.0 |
Exposures are typical. Your product's installation guide gives the exact figure.
Common mistakes to avoid
- Using the face width instead of the exposure. Lap siding overlaps, so only the exposure counts toward coverage.
- Forgetting gables and dormers. They are often 15 to 20% of the total.
- Too little waste on complex houses. Every angled cut wastes material.
- Skipping trim. J-channel, starter, corners, soffit and flashing are a large share of a siding order.
Frequently asked questions
How do I calculate siding for a house?
Multiply the distance around the house by the wall height, add each gable (width × height ÷ 2), subtract windows and doors, then add 10 to 15% for waste. Divide by 100 to get squares.
What is a square of siding?
A square is 100 square feet of wall covered. Vinyl siding is usually sold in boxes of 2 squares; fiber cement and wood are sold by the piece.
Should I subtract windows and doors?
Yes for large openings like doors, garage doors and big windows. Many installers skip subtracting small windows because the cut-offs are wasted anyway. Our waste setting covers that; lower it slightly if you subtract everything.
How many pieces of siding are in a square?
It depends on the exposure and length. A 12 ft piece of fiber cement at 7 in exposure covers 7 sq ft, so about 14.3 pieces per square. A 12 ft 6 in vinyl double 4 covers about 8.3 sq ft, so 12 pieces per square.
How much J-channel do I need?
Enough to go around the sides and top of every window, around the sides and top of every door, and along the gable rakes. The calculator adds all of these and divides by the trim length.
How much waste should I add for siding?
About 10% for a simple rectangular house and 15% or more for gables, dormers, many windows or a complex shape, because angled cuts waste more.