Perimeter times height is where the error starts
Every siding estimate begins with a rectangle that does not exist. A house is a perimeter with corners, bump-outs, a garage that sticks forward, and two or more triangles at the top that nobody measures because you cannot reach them with a tape. The perimeter is easy to get wrong in the friendly direction: people walk the front, the back and one side, double the side, and lose the entire bay window. Walk it, write each side down, and add them.
A gable is the part that surprises people. The triangle above the wall plate has an area of half the span times its height, and the height comes from the pitch. For a span of 28 feet at 6/12, the peak is 7 feet above the plate and the triangle is 98 square feet. Two of them is nearly two hundred square feet, which on a modest house is more than a tenth of the whole job. This calculator works that out from the span and the pitch so you do not have to climb anything. If you do not know the pitch, the roof pitch calculator will get it from a rise and run measured off a rafter tail or a level held against the gable.
Whether to deduct the windows at all
There are three defensible policies and they give three different answers.
| Policy | What it assumes | Who uses it |
|---|---|---|
| Deduct nothing | The offcut from around an opening is scrap anyway | Fast bids, small houses, cut-up elevations |
| Deduct openings over about 30 sq ft | Small windows are absorbed by waste, garage doors are not | The common middle, and the default here |
| Deduct everything in full | You will use the offcuts elsewhere | Long plain walls, sheet products, careful crews |
The reason the middle policy exists is that a 3 by 5 window in the middle of a wall does not save you a plank. The course still runs across the wall and you cut a hole through it. What you save is a strip of material that mostly becomes scrap. A 16 by 7 garage door is different: that is 112 square feet you genuinely do not cover, and treating it as waste allowance means ordering a pallet you do not need.
Exposure, not board width
Lap siding is sold by the board but installed by the exposure. A nominal 8 inch board with a 1.25 inch lap shows 6.75 inches of face, and the coverage of a plank is its length times that exposure, not its length times its width. Get this wrong and you can be off by fifteen percent on the whole order. If you are matching existing siding, measure the exposure on the wall by putting a tape from the bottom edge of one course to the bottom edge of the next.
Sheet products behave differently again. A 4 by 8 sheet is 32 square feet, but sheets do not tolerate the offcut arithmetic that planks do, because a piece left over from a 9 foot wall is 1 foot tall and useless. Where the wall height is not a clean multiple of the sheet, count the sheets per wall rather than dividing total area, and treat the result here as a floor.
The parts of the job this does not count
Starter strip, J-channel, undersill, utility trim, drip cap, fasteners, house wrap, flashing at every horizontal transition, and the sealant that goes with them. On a vinyl job the accessories can be a third of the material cost, and they are the part that determines whether water gets behind the wall. The house wrap calculator handles the weather-resistive barrier and its tape, which goes on before any of this. What sits above the siding at the top of the wall is covered by the soffit and fascia calculator.
Two things are worth saying plainly about scope. Siding is a rain-management system, not a skin, and the details at penetrations and horizontal joints are what keep water out. And if the existing wall is coming off, look at what is behind it before you order anything, because rot found halfway through a re-side changes the job entirely. Signs of long-term water entry inside the house are worth chasing down first with the water leak guide.
Questions people ask
How many square feet is a square of siding?
One hundred. It is the same unit roofers use, and it exists because it makes coverage comparisons easy: a box of vinyl that covers two squares covers 200 square feet, and a bundle rated at a third of a square covers 33.3. The unit does not tell you anything about the product, only about how much wall one package will hide. Note that manufacturers state coverage after the lap is taken out, so you should not deduct the lap a second time.
Do I really need to add the gables separately?
Yes, unless you measured wall height to the peak, which almost nobody does. Wall height is normally measured to the soffit or the wall plate, and everything above that line on a gable end is extra. The triangle is half the span times the height above the plate, and the height above the plate is half the span times the pitch over twelve. For a 28 foot span at 6/12 that is 98 square feet per gable. On a hip roof there are no gables at all, which is why the field defaults are worth changing rather than accepting.
What waste percentage should I use?
Ten percent is the usual starting point for a plain rectangular house with a modest number of openings. Push it to fifteen when you have several corners, multiple gables, dormers, or a product that comes in fixed lengths that do not divide neatly into your wall runs. Push it higher for anything with a mitred corner treatment, because a bad mitre costs you both pieces. The asymmetry is the point: a few extra boxes cost money once, while running short means a second delivery and a colour or lot difference that shows on the wall.
Why does the trim number look so large?
Because trim runs around everything. Four corners on a nine foot wall is 36 feet before you have trimmed a single window, and twelve windows at 16 feet of perimeter each is another 192. That is normal. What the number does not include is cutting waste on the trim itself, which is significant because trim comes in fixed lengths and a corner post has to be one piece for the full height. Add ten to fifteen percent to the trim figure, and count the pieces per corner rather than dividing the total by the stick length.
Can I use this for fiber cement, wood or engineered plank?
The area arithmetic is identical for any lap product: coverage is length times exposure. What changes is everything around it. Fastener type and spacing, whether butt joints need a joint flashing behind them, clearance to the ground and to roof surfaces, and the treatment of cut ends are all manufacturer instructions and they vary by product and by region. Follow the printed installation instructions for the exact product, and expect your local building department to have its own requirements on top.