Siding Calculator
Convert wall area net of openings into squares of siding, pieces at your exposure, courses up the wall and corner trim — for lap, panel or shake siding.
Perimeter × height, plus gable triangles at half base × height each.
Windows and doors. Subtract them — on a typical house this is 10–15%.
The visible height of each course — not the width of the board.
At 6.5 in exposure each 12 ft piece covers only 6.5 sq ft — the overlap is not coverage, and that is the number people forget when they price by board width.
What this calculator does
Gross wall area and openings go in, along with the siding type and its exposure. Out comes net area, squares before and after waste, the square feet one piece actually covers, the number of pieces, how many courses the wall takes, corner trim in linear feet, and the cost at your price per square.
The coverage-per-piece figure is the one that surprises people, and it is worth understanding before you price anything.
Exposure is not width
This is the central idea in siding estimating and the source of most bad estimates.
An eight-inch lap board is eight inches wide. Installed, each course overlaps the one below it, so only about six and a half inches of that board is visible. That six and a half inches is the exposure, and it is the only part that covers anything.
So a 12-foot board with a 6.5-inch exposure covers 12 × (6.5/12) = 6.5 square feet, not the 8 square feet its width suggests. Estimate from width and you are short by nearly twenty percent — enough to stop a job in its third day.
Shakes and shingles work the same way, usually with a larger overlap proportionally. Panel siding is the exception: a 4 × 8 sheet covers its full 32 square feet, because the joints are butted and battened rather than lapped.
Measuring the walls
Rectangular walls are perimeter × height. Straightforward.
Gables are triangles: half the base times the height. A gable 30 feet wide rising 7 feet is 105 square feet, and a house with two of them has 210 square feet of siding people routinely forget until the truck arrives.
Dormers, shed roofs and bump-outs each need their own small calculation. Miss one and the shortfall is small; miss four and it is a square.
Openings come off. A 3 × 5 window is 15 square feet, a 3 × 7 door is 21. On a typical house the total lands somewhere between ten and fifteen percent of the gross wall area, and subtracting it is the difference between a good estimate and a generous one.
Squares, because that is how it is quoted
One square is 100 square feet. Both siding and roofing are sold and quoted this way, which makes contractor pricing legible once you know it: $310 per square is $3.10 per square foot, and a 1,600 square foot wall is 16 squares.
The unit exists because it makes labour and material commensurate — a crew talks about squares per day, and a supplier quotes squares delivered.
Waste, and the batch problem
Ten percent for a simple house. Fifteen for one with many corners, gables, dormers or a pattern that has to line up across openings.
But there is a second reason to over-order that has nothing to do with cutting: colour batching. Vinyl and fibre cement are both manufactured in production runs, and shade drifts measurably between them. Two pieces from different runs installed side by side will not match, and in raking afternoon light the difference is obvious.
Which means the leftover from your order is the only material that will ever match your house. Store a bundle in the garage. In eight years when a ladder goes through a panel, you will be extremely glad you did.
What goes on behind the siding matters more
Siding is a rain screen, not a seal. Wind-driven rain gets behind it — that is expected and normal, and every siding system is designed on the assumption that it happens.
What keeps the wall dry is the layer behind: the water-resistive barrier (house wrap or building paper), correctly lapped shingle-fashion so water runs down and out rather than in, and the flashing at every window head, every door, every deck ledger and every horizontal transition.
Nearly every wall rot failure traces back to that layer, not to the siding. Flashing details are worth more attention than colour selection, and they are not in this calculator because they are per-detail rather than per-square-foot.
What this leaves out
- Starter strip, J-channel, corner posts, soffit and fascia. All sold by the piece or the foot and all necessary.
- Flashing. Head flashing, kick-out flashing, step flashing at roof-wall intersections. Non-negotiable.
- House wrap and fasteners. The sheathing calculator counts wrap rolls and fasteners over the same area.
- Rain screen furring. Increasingly common and highly effective, adds material and changes trim depths.
- Removal and disposal of existing siding, including any asbestos-cement siding, which has its own handling rules entirely.
- Labour, which on siding is usually more than the material.
For a metal building envelope instead, the metal panel calculator handles ribbed panels and their coverage widths, and paint covers finishing what you already have.
How this is calculated
net area = gross wall area − openings squares = net area ÷ 100 lap & shake: coverage per piece = piece length × (exposure ÷ 12) panel: coverage per sheet = 32 sq ft courses = ceiling( wall height × 12 ÷ exposure )
Frequently asked questions
- How do I calculate how much siding I need?
- Multiply the perimeter of the house by the wall height, add half base times height for each gable triangle, subtract the window and door openings, and divide by 100 for squares. Then add waste — ten percent for a simple rectangle, more for a house with many corners, dormers or gables.
- What is a square of siding?
- One hundred square feet. Siding and roofing are both sold and quoted in squares, which is why a contractor's price per square looks alarming until you remember it covers a hundred square feet. A 1,600 square foot wall is 16 squares.
- What is siding exposure?
- The visible height of each course once it is installed — always less than the width of the board, because each course overlaps the one below. An eight-inch lap board typically has a six-and-a-half-inch exposure, so a quarter of what you buy is hidden. Calculating coverage from board width instead of exposure understates the order by twenty percent or more.
- Should I subtract windows and doors from siding?
- Yes. On a typical house openings are ten to fifteen percent of the wall area, and that is real money at siding prices. Subtract the full opening area and keep your waste allowance — cutting around openings is precisely where the waste goes.
- How much waste should I allow for siding?
- Ten percent for a simple rectangular house with a straightforward roof. Fifteen for many corners, dormers or gables, and more still for a pattern that has to align across openings. Also buy from one production run — vinyl and fibre cement both shift shade between batches, and a repair a year later will not match.