Siding Calculator

A square is 100 square feet, and a box usually is not

Siding is the last corner of the building trade that still runs on squares. One square is 100 square feet of finished wall, and it is the unit your supplier quotes, your contractor bids, and your delivery gets counted in. Divide your square footage by 100 and you have the number everyone else in the conversation is using. A 1,200 square foot wall total is 12 squares, and nobody selling siding will call it anything else.

A box is a different thing. Most vinyl siding ships two squares to the box, so the box count is half the square count — but one-square boxes exist, and the only reliable figure is the one printed on the carton. Fiber cement skips boxes entirely and is sold by the plank, which is why the piece counts in the chart above matter more than any box figure for a Hardie order.

The table below runs net wall area through both conversions at a 10 percent waste allowance. Read it as an ordering guide rather than a quote: the box column assumes two squares per box, and the moment your carton says otherwise the column is wrong.

Measure the walls, then add the gables everyone forgets

Walls are the easy half: length times height, one elevation at a time. Measure to the bottom of the soffit rather than to the roofline, and treat a walkout basement or a wall that steps down a slope as its own area instead of averaging its height. The number you want at this stage is gross area — openings come out later, and framing never comes out at all.

Gables are where estimates go wrong. A gable is a triangle, so its area is half the base times the rise: base measured eave to eave, rise measured from the eave line straight up to the peak. Two gables on an ordinary house add somewhere between 100 and 300 square feet, which is a box or two of vinyl and enough to stop a job on a Saturday afternoon. This calculator takes rectangles, so enter a gable with its full base as the length and half its rise as the height — the arithmetic lands on the same number.

Dormers, bay windows, and porch gables each get the same treatment: measure them, add them, and do not try to eyeball them as part of the wall behind. Steep gable rakes also drive up waste, because every course dies into an angled cut and the offcut is rarely long enough to start the next course.

Windows and doors: deduct them or absorb them, never both

There are two defensible ways to handle openings and they are not compatible. The Vinyl Siding Institute's method is to leave ordinary windows and doors in the gross area, on the reasoning that the material saved on the opening is spent cutting around it — the deduction and the waste cancel out. The estimator's method is to subtract every opening and then add a separate waste percentage on top. Both land in roughly the same place.

What does not work is doing half of each. Deduct all the openings, then also skip the waste allowance because you already 'took some off', and the order arrives short by the margin you needed for cuts. That mistake is common enough to be worth naming: the deduction and the waste factor are doing the same job, so keep one of them.

Large openings are the exception that everybody deducts. A two-car garage door is over 100 square feet of wall that gets no siding at all, and no waste allowance is meant to swallow that. The practical rule most estimators land on is to deduct anything over about 30 square feet, leave the rest in, and add waste to the result.

Exposure, not width, is what actually covers the wall

Every piece of lap siding is wider than the wall it covers, because the course above it laps over the top edge. The part left showing is the exposure, and it is the only width that counts in an estimate. Fiber cement laps 1.25 inches, so an 8.25 inch plank has a 7 inch exposure and covers 7 square feet over its 12 foot length — not the 8.25 square feet its width suggests. Estimating on width instead of exposure runs about 15 percent short, and it is the most expensive arithmetic mistake on this page.

The fiber cement rows in the chart above are James Hardie's HardiePlank dimensions, because it is the product most of this market is actually buying and the one whose figures people cross-check against. Every width in the range takes the same 1.25 inch lap, which is why the exposures come out as clean whole numbers: 5.25 gives 4, 6.25 gives 5, 8.25 gives 7, and so on up to the 12 inch plank at 10.75. Other fiber cement brands are dimensionally similar but not identical, so read the exposure off your own product sheet before converting.

One naming point worth settling, because it sends people to the wrong product page: HardiePlank is lap siding, sold in 12 foot planks, and it is what almost everyone means when they say hardie board. HardiePanel is a different thing — 4 by 8 foot sheets for board-and-batten and vertical applications, estimated by dividing wall area by 32 square feet per sheet rather than by exposure. If your walls are going vertical, the exposure chart does not apply to you and the sheet count does.

Vinyl names itself after its exposure, which removes the trap but hides the panel. A double 4 panel is one panel moulded to look like two 4 inch courses, so it exposes 8 inches per panel, and a double 5 exposes 10. Panel lengths of 12 ft and 12 ft 6 in are both common and change coverage by 4 percent, so check the length before converting piece counts.

Exposure also sets the course count, which is what you need to know before the first piece goes on. Divide wall height in inches by exposure in inches and round up. The last course at the top is almost always a rip, and a good installer will shave the exposure slightly across the whole wall to make the courses land evenly rather than finish with a two-inch sliver under the soffit — that even-coursing adjustment slightly increases material, which is another reason not to order to the exact number.

Waste factor, and why fiber cement wants more of it

Ten percent is the floor, and it only holds for a simple rectangular house with square corners and few openings. Every corner is a cut, every opening is two or three cuts, and every gable rake turns a full-length piece into a wedge with an unusable offcut. Add corners, dormers, and gables and the realistic figure moves to 15 percent; on a cut-up elevation with steep rakes and a lot of windows, 20 percent is not padding.

Fiber cement runs a point or two higher than vinyl at the same complexity. It is brittle, it does not tolerate a bad cut, and offcuts are less reusable because the ends are factory-sealed. It is also heavy enough that a mis-cut plank is a real loss of time as well as material.

Order the waste with the job rather than after it. Siding is dyed in production lots and colour drifts between runs, so a box bought three weeks later can be visibly different in daylight on the same wall. Buying one extra box beyond the calculation and storing it flat is the cheapest insurance available on this kind of project, and it is the only way to have a matching piece the day something gets hit.

What the installed cost is actually built from

Siding is quoted per square, which is why getting the square count right is most of the work of understanding a bid. Material and labour both scale with squares, and a quote that arrives as a single number can be checked by asking what the per-square rate is and multiplying it by the figure this page gave you. If the two are far apart, one of you has measured different walls — usually the gables.

The rest of the bid does not scale with squares at all. Trim runs on linear feet of corners, openings, and roof edge, so a house with the same wall area but more windows costs more to trim. Tear-off and disposal depend on how many layers come off and what they contain, and a second storey adds staging that has nothing to do with area. This is why two houses of identical square footage get quotes that are not close.

Real prices move with region, season, and product line far too much to be worth printing here — a number that was right last year in one metro is misleading everywhere else. Take your square count to two or three local contractors and compare their per-square rates against each other. The estimate that matters is the one written against your walls.