How much siding do you need?
Enter your walls and gables, get squares of siding instantly.
How do I calculate how much siding I need?
Find the wall area — the perimeter times the wall height — add the gables (width × height ÷ 2 each), subtract doors and windows, add about 10% and divide by 100 to get squares. A 40 by 30 ft house with 9 ft walls has 2 × (40 + 30) × 9 = 1,260 square feet of wall. Two gables 30 ft wide and 7.5 ft tall add 225, for 1,485. Minus 2 doors and 10 windows (192 square feet) it is 1,293; with 10% extra, 1,422 square feet, or 14.2 squares.
What is a square of siding?
A square is 100 square feet of wall, the same unit roofers use. Vinyl siding is often sold in cartons of 2 squares, but carton sizes vary by product — check the label and enter your own size in the table under the result.
How much extra siding should I order?
About 10% for a simple house, and 15% if it has many corners, gables and dormers, where every piece is cut at an angle. Trim, J-channel, corner posts and starter strip are bought by the linear foot and are not part of this count.
How do I find the height of a gable?
Take half the gable width and multiply it by the roof pitch over 12. A 30 ft wide gable under a 6/12 roof is 15 × 6 ÷ 12 = 7.5 ft tall at the peak. The roof pitch calculator finds the pitch if you do not know it.
How big are the doors and windows it subtracts?
Each door counts as 21 square feet (3 × 7 ft) and each window as 15 square feet (3 × 5 ft). For a garage door or a large picture window, enter that wall with the Area tab, with the opening already taken out.
How do I find the area of a gable?
A gable is a triangle, so its area is half the width times the height. A gable 30 ft wide under a 6/12 roof is 7.5 ft tall, so one gable is 30 × 7.5 ÷ 2 = 112.5 square feet. Treating it as a rectangle would double the figure.
Why are there different numbers of squares for the same house?
Because the waste allowance changes the order. The same 1,293 square feet of wall needs 14.2 squares with 10% extra and 14.9 squares with 15%. That is 8 and 8 cartons of 2 squares, so the choice of allowance can change the carton count.
How the count works
The calculator measures the wall area you enter as a house (perimeter times wall height), adds the gable triangles, subtracts the doors and windows you list, and then adds the waste percentage. The total is divided by 100 to give squares, the unit in which siding is priced and described, and then divided again by the size of the carton to give a purchase count, rounded up. Everything is area: the calculator does not know about panel lengths, exposure, or how many pieces a wall can be cut from, so a wall with many short runs may use more material than its area suggests. That is the reason for the waste allowance.
Worked example: 40 by 30 ft house, 9 ft walls
Perimeter is 2 × (40 + 30) = 140 ft, and the walls are 140 × 9 = 1,260 square feet. The two gables add 225. Two doors (21 square feet each) and ten windows (15 each) take out 192, leaving 1,293 square feet of wall. With 10% extra the order is 1,422 square feet, which is 14.2 squares or 8 cartons of 2 squares (1,600 square feet). The siding is the part this tool counts; the trim is separate.
What the count does not include
Corner posts, J-channel, starter strip, soffit, fascia and trim are sold by the linear foot or by the piece, so measure them separately along corners, windows, doors and the top and bottom edges of the walls. Fasteners, house wrap and flashing are also extra. Colors can differ slightly between production runs, so order all the siding for a wall, plus the allowance, at once rather than going back for a few more pieces later.
Common mistakes
Forgetting the gables or the second story is the first one; the second is measuring a house in feet and then entering the wall height in inches. Counting a garage door or a large picture window as a standard opening removes too little; enter those walls with the area tab, already net of the opening. Another is rounding the squares down: a house that needs 14.2 squares cannot be finished with 14. Finally, leaving the allowance at zero for a house with many corners, dormers and bay windows is a reliable way to run short.
Sources: the square (100 square feet) is the standard unit for roofing and siding, as used by manufacturers' installation guides; the door (21 square feet) and window (15 square feet) allowances are the same defaults as the paint calculator on this site; gable height from the roof pitch is the rise over 12 times half the width. Related: roof pitch calculator, paint calculator and roof calculator.