Siding Calculator
This siding calculator works out how many squares and boxes a house needs across walls, gables and openings, and what it costs at your supplier price per square.
Wall Area
Gable Ends
The triangle above the wall line. This is the area most take-offs forget.
Siding Cost
How to use the Siding Calculator
Add up the perimeter of every wall you are siding and enter it once, with the wall height measured from the bottom of the siding to the soffit. Deduct openings in square feet, a standard window is about 15 sq ft and an entry door about 21.
Then do the gables separately and add them in. A gable is a triangle sitting above the wall line, and it is the part almost every rough take-off leaves out. Enter the width of the wall it sits on and the roof pitch, and the height is worked out for you.
Everything is reported in squares, because that is the unit siding is sold and quoted in. One square is 100 square feet.
Siding formula
wall area = perimeter × height − openingsgable area = (width × gable height) ÷ 2gable height = (width ÷ 2) × (pitch ÷ 12)squares = square feet ÷ 100boxes = squares ÷ squares per box
A gable is a triangle, so it is half of width times height, not width times height. Using the full rectangle overstates a 30 ft gable at 6/12 by 112 sq ft.
| Siding | Sold as | Coverage | Typical waste |
|---|---|---|---|
| Vinyl lap | Box | 2 squares (200 sq ft) | 10% |
| Fiber cement lap | Piece, 12 ft | Varies by exposure | 10% |
| Wood lap | Linear foot | Varies by exposure | 15% |
| Engineered wood lap | Piece, 16 ft | Varies by exposure | 10% |
| Panel / board and batten | 4 × 8 sheet | 32 sq ft | 15% |
Worked example
A 40 × 30 ft house, 9 ft walls, eight windows and two doors, gable roof at 6/12:
- Perimeter: 2 × (40 + 30) = 140 ft
- Wall area: 140 × 9 = 1,260 sq ft
- Openings: 8 windows × 15 + 2 doors × 21 = 162 sq ft deducted
- Net walls: 1,260 − 162 = 1,098 sq ft
- Gable height: (30 ÷ 2) × (6 ÷ 12) = 7.5 ft
- Gable area: (30 × 7.5) ÷ 2 = 112.5 sq ft each, × 2 = 225 sq ft
- Total: 1,098 + 225 = 1,323 sq ft = 13.23 squares
- Plus 10% = 14.56 squares → 15 squares, which is 8 boxes of vinyl
Leaving the gables out would have ordered 12.1 squares, nearly three squares short.
What the result means
A square is 100 square feet. It is the unit siding, roofing and stucco are all quoted in, and it exists so that trades can talk about a house without carrying three-digit numbers around. Vinyl is boxed in two-square boxes, so a job is almost always an even number of squares.
Deducting openings is a judgement call. Some estimators deduct nothing under 10 sq ft, on the grounds that the offcut round a small window is unusable anyway. Deducting everything and then adding 10% waste lands in roughly the same place; deducting everything and adding nothing does not.
Trim is a separate order. Corner posts, J-channel, starter strip, undersill and soffit are sold by the piece and are not included in a square count.
Common mistakes
- Forgetting the gables. The single most common shortfall. Two gables on an average house are one to three squares.
- Treating the gable as a rectangle. It is a triangle, half the area. Overstating it wastes a whole box.
- Measuring wall height floor to ceiling. Siding runs from the bottom edge, usually below the floor line, up to the soffit. That is often a foot more than the ceiling height.
- Ordering the bare calculated figure. Vinyl comes in whole boxes and dye lots vary. Order the full boxes and keep the offcuts for repairs.
- Leaving out trim. Corner posts and channel are a separate line and can be a fifth of the material cost.
Frequently asked questions
How many squares of siding do I need?
Multiply the perimeter of the house by the wall height, subtract the window and door area, add the gable triangles, then divide by 100. A 40 by 30 foot house with 9 foot walls and two 6/12 gables works out at about 13.2 squares before waste.
What is a square of siding?
One hundred square feet. It is the standard quoting unit for siding and roofing. Vinyl siding is boxed in two-square boxes covering 200 square feet, so material is usually ordered in even squares.
How do I calculate the area of a gable?
Half the base times the height. The height comes from the pitch: half the width times the rise over 12. A 30 foot wide gable at 6/12 is 7.5 feet tall, so its area is 30 times 7.5 divided by 2, which is 112.5 square feet.
How much waste should I allow for siding?
Ten percent on plain walls, fifteen on gables and anything with lots of corners, dormers or angled cuts. Board and batten and panel siding waste more than lap because a sheet cannot be pieced.
Should I subtract windows and doors?
Yes, but keep the waste allowance. Many estimators ignore openings smaller than about 10 square feet because the material cut around them cannot be reused. Deducting everything and then adding ten percent waste gives a similar answer.
How many boxes of vinyl siding is a square?
Half a box. A standard carton of vinyl lap siding covers two squares, or 200 square feet. Fifteen squares is therefore eight boxes once you round up to whole cartons.
Sources and assumptions
A square is defined as 100 square feet, the standard quoting unit for siding and roofing in the United States.
Vinyl carton coverage of two squares is the common industry packaging; confirm with your supplier, as some profiles box differently.
Gable height is derived from roof pitch as half the span times the rise over 12, which assumes a symmetrical gable. Saltbox, shed and unequal-pitch roofs need each side measured separately.
Opening areas are user-entered. Trim, corner posts, channel, starter strip, soffit and fasteners are not included.