How Much Siding Do I Need?

Siding is estimated by area but bought by the box or the board, so every estimate is a two-step job: wall square footage to squares, then squares to boxes of vinyl (or to lap boards by their exposure). The one thing to get right first is that siding is about your wall area, not your floor area.

Skip the math

Enter your wall sizes - gable ends included - take off the doors and windows, and the siding calculator returns the net area, the squares, the boxes of vinyl (or the lap boards), plus the J-channel and corner posts. The rest of this guide is how it works, so you can check the number.

The quick formula

net wall area = sum of (wall length × height) − doors & windows
squares       = net wall area (sq ft) ÷ 100
vinyl boxes   = ceil( net area × waste ÷ box coverage )   (a carton ≈ 2 squares)
lap boards    = ceil( (net area ÷ exposure) × waste ÷ board length )

A square is 100 sq ft of finished wall - the same trade unit roofers use. Vinyl is normally boxed two squares (about 200 sq ft) to the carton. Add 10% waste for simple walls, 15% where there are lots of openings and corners, and round up.

Siding by wall area

Once you know your net wall area, this is the order at 2 squares per vinyl box with 10% waste:

For lap boards instead, divide the area by the exposure to get the lineal run - a 1500 sq ft wall at a 7 in (0.583 ft) exposure is about 2,570 lineal ft of board.

Siding by shed size

Sheds are simple boxes, so the wall area is easy. These use 8 ft walls, one door taken off, and exclude the gable ends (add a little for those):

What about by house size?

Floor area is a poor proxy for siding, because the wall area depends on the footprint shape, the number of storeys, and the wall height. A long, narrow house has far more wall than a square one of the same floor area, and a two-storey home has roughly twice the wall height on a smaller footprint. As a rough single-storey starting point, a 1500 sq ft home is around 10 to 12 squares and a 2000 sq ft home around 12 to 15 - but the only reliable way is to measure each wall. See how to measure for siding for the method.

Do not forget the trim

The box or board count is just the field. A full order also needs:

• Starter strip along the bottom of the walls to lock in the first course.
• J-channel around every door and window and along the eaves and soffits.
• Corner posts running the full wall height at each outside corner.
• Finish / undersill trim at the top of the walls.

The siding calculator gives the J-channel and corner-post runs from your openings and corners - trim is sold in fixed sticks, so round each up.

Worked example

A 30 ft × 9 ft front wall in vinyl, less two 4 ft × 4 ft windows, 10% waste:

1. Gross wall area: 30 × 9 = 270 sq ft.
2. Deduct the windows: 2 × (4 × 4) = 32 sq ft, so net = 238 sq ft.
3. Squares: 238 ÷ 100 = 2.38 squares.
4. Add 10% waste: 238 × 1.10 = 262 sq ft, ÷ 200 per box = 2 boxes.

Round every material up to whole units, keep the batch number, and hold a board or two back for repairs.