Siding Calculator
Estimate how much siding your walls need. Enter each wall run, add gable triangles, count the windows and doors to subtract, then add a waste allowance. Get the net area, the number of "squares" (100 ft2 each), the panels and boxes to order, and an optional total cost.
Formula
Worked example
A 9 ft x 40 ft wall is 360 sq ft. Six 12 sq ft windows and one 21 sq ft door remove 93 sq ft, leaving 267 sq ft net. Add 10% waste to 294 sq ft, and 294 / 100 is about 2.9 squares to order.
How the siding estimate is worked out
The amount of siding a wall needs starts from the gross wall area. For a flat-topped wall that is simply the wall height multiplied by its total length. If the wall rises into a gable under a roof pitch, the triangular area is added on top: one half of the gable base multiplied by its rise, repeated for each gable end. From that gross figure you subtract the area of every window and door, because siding does not cover those openings, paying for it there would only mean cutting it away. The result is the net area to be covered. A waste allowance is then added on top, because real installations lose material to angled cuts around openings and gables, to the overlap and starter courses, and to occasional breakage. Finally the net area with waste is divided by 100 square feet to convert it into "squares", the unit siding is sold and priced in throughout the trade.
Counting windows, doors and gables accurately
Rather than guessing one lump area for all openings, this calculator lets you count windows and doors separately and give an average size for each, which is faster and more accurate. A typical 3 x 4 foot window is 12 square feet, a standard 3 x 7 foot entry door is 21 square feet, and a single 9 x 7 foot garage door is about 63 square feet. For gable ends, set how many you have, then enter the base width and the vertical rise from the eave line up to the peak; the triangle area is half the base times the rise. Measure each wall run separately and add the lengths together before entering the total length so an entire elevation is captured in one figure. Be generous rather than precise on very small openings; rounding them down slightly is offset by the waste allowance, while large picture windows, patio doors and garage doors should always be measured carefully because they remove meaningful area.
Panels, boxes, waste and ordering by the square
A ten percent waste allowance suits most straightforward rectangular walls. Raise it toward fifteen percent when a wall has many windows and doors, several gables or dormers, diagonal cuts, or a lap siding with a short exposure that produces more offcuts. Because siding is sold by the square, and often in boxes that cover a fixed number of squares or panels, you can optionally enter the exposed coverage of one panel and how many panels come in a box. The calculator then converts the area into the number of panels and rounds up to whole boxes so your order matches what the supplier actually ships. Turn on the cost estimate and enter an installed price per square to get a planning budget; vinyl typically runs a few hundred dollars per square installed, with fiber cement and wood costing more. Keeping one spare bundle from the same production run guards against color drift if you ever need to repair or extend the wall later.
Common opening sizes to subtract
| Opening | Typical size | Area (sq ft) |
|---|---|---|
| Standard window | 3 x 4 ft | 12 |
| Large window | 3 x 5 ft | 15 |
| Picture / patio door | 6 x 7 ft | 42 |
| Entry door | 3 x 7 ft | 21 |
| Garage door (single) | 9 x 7 ft | 63 |
Approximate areas for typical residential windows and doors. Measure your own for accuracy.
Frequently asked questions
What is a "square" of siding?
A square is the trade unit for siding equal to 100 square feet of finished coverage (about 9.29 square metres). Siding is sold and priced by the square, and boxes are usually labelled with how many squares they cover, so converting your area to squares makes ordering and comparing prices straightforward.
How do I include gable ends in the estimate?
Set the number of gable ends, then enter the base width and the vertical rise from the eave line up to the roof peak. Each gable adds a triangle equal to half the base times the rise. The calculator adds those triangles to the rectangular wall area before subtracting openings, so a two-storey house with two gables is captured in one figure.
Should I really subtract windows and doors?
Yes, especially large ones. Subtracting openings avoids paying for siding you would only cut away. Some installers leave small openings in as a built-in safety margin, but for walls with large picture windows, patio doors or a garage door, subtracting them prevents meaningful over-ordering. The waste allowance still covers your offcuts.
How much waste should I add for siding?
Ten percent is a good default for simple rectangular walls. Increase it toward fifteen percent for walls with many openings, gables, dormers or diagonal cuts, and for lap or shingle styles with short exposures that generate more offcuts. Buying a little extra from the same batch also protects future repairs against color drift.
How much does siding cost to install?
Turn on the cost estimate and enter an installed price per square. Vinyl siding typically runs a few hundred dollars per square (100 sq ft) installed, while fiber cement, engineered wood and natural wood cost more. Multiply your squares by that price for a planning budget, then confirm with local quotes because labor and material prices vary by region.