The siding calculator above turns a house's dimensions into an order: how many squares of cladding, how many individual panels at your exposure and panel length, how many courses will run up each wall, how much starter strip goes round the base, and how much trim the corners and openings consume. It deducts window and door openings, applies a waste factor and prices the result.
Arb Digital publishes this as part of a free construction estimating set. It exists as a separate tool from a plain area calculation because siding is not sold or installed by area alone. It comes in panels of a fixed length that cover a fixed exposure, and the accessories — starter, corner posts, J-channel, trim — are counted in linear feet, not square feet. Order only by area and you will have cladding on site with nothing to terminate it against.
What This Siding Calculator Does
It works out the gross wall area from the perimeter and the eaves height, adds gable and dormer area, deducts the openings, and expresses the result in squares — the trade unit of 100 square feet that siding is quoted in. It then converts that area into a panel count using the exposure and the panel length you supply, because coverage per panel is exposure times panel length, not panel width times panel length.
Alongside that it counts the accessories. Starter strip runs the full perimeter at the base. Corner posts run the full wall height at each outside corner. Trim runs around every opening. Those three lines are frequently the ones that get forgotten in a first estimate and then arrive as a second delivery.
This is deliberately different from our wall area calculator, which returns area only with no notion of exposure, panels or accessories, and from the board and batten calculator, which handles a vertical cladding system where boards and battens are counted along the wall rather than up it. If you only need the surface area of a simple rectangle, the square footage calculator is quicker.
How to Use It
- Measure the perimeter, not each wall. Walking the outside of the building with a wheel gives the perimeter directly, and multiplying by the eaves height gives the rectangular part of the wall area in one step.
- Add gables and dormers separately. A gable is half its base times its height. Dormers add both cheeks and a face.
- Enter the openings. Count them and give an average area. Also give the trim length around each, which for a typical window is roughly its perimeter.
- Use the manufacturer's exposure, not the panel width. Exposure is the visible face after lapping, and it is always less than the width.
- Read the courses figure. If the wall height does not divide neatly by the exposure, the last course has to be ripped, and that changes how the job looks at the top.
The Formula / How It's Calculated
Gross area is perimeter × wall height + gable area. Net area is gross minus the total opening area. Squares are net area divided by 100. Coverage per panel is panel length × exposure ÷ 12 in imperial, which is what makes the exposure the driving variable rather than the panel width.
Worked example with the loaded defaults. A 160 foot perimeter at 9 feet high is 1,440 square feet, plus 120 square feet of gable gives 1,560. Twelve openings at 15 square feet each is 180, so the net area is 1,380 square feet, which is 13.80 squares. Add ten percent waste and that is 15.18 squares, rounded up to 16 squares to order.
On the panel side, a 12 foot panel at 7 inch exposure covers 12 × 7 ÷ 12 = 7 square feet. The waste-adjusted area of 1,518 square feet divided by 7 gives 216.9, so 217 panels. Courses up a 9 foot wall are 9 × 12 ÷ 7 = 15.4, which means 16 courses with the last one cut down. Starter strip is the full 160 foot perimeter. Trim is four corners at 9 feet each, 36 feet, plus twelve openings at 16 feet each, 192 feet, for 228 feet in total.
Exposure Is the Number That Drives Everything
A siding panel is wider than the face you see, because each course laps over the one below to shed water. The visible face is the exposure, and it is a manufacturer's specification, not an installer's choice. A panel that looks like eight inch cladding may have a seven inch exposure, and using eight in the calculation under-orders the job by more than twelve percent.
Exposure also controls the appearance in a way area never shows. Divide the wall height by the exposure and you rarely get a whole number, so one course somewhere has to be narrower. Where that cut course sits is a design decision: at the top under the soffit it is barely visible, at the bottom it can look deliberate, and in the middle it looks like a mistake. Experienced installers adjust the starter height slightly, within the manufacturer's tolerance, so that the courses land cleanly at window heads and sills across the whole elevation.
On a multi-storey elevation the same logic applies across floors. Getting a course line to align with the top of a window on one wall and the bottom of a window on another is what separates a considered job from a cheap one, and it is worked out before the first starter strip goes on.
Openings: Deduct Them, But Not Blindly
The instinct is to subtract every square foot of glass and door. In practice the panel that crosses an opening gets cut and the offcut is usually the wrong length for anywhere else, so a full deduction over-optimises. Two conventions are common in the trade. The first is to deduct only openings above a threshold size, treating small windows as solid wall. The second is to deduct everything and carry a higher waste factor to compensate.
Either approach works as long as you use it consistently. What does not work is deducting fully and also using the minimum waste factor, which is how estimators end up two squares short. Note also that openings increase trim, not just reduce area: every window adds J-channel or trim around all four sides, and on a house with many small windows the trim line can rival the cladding in cost.
Accessories Are Half the Job
Starter strip runs the entire base perimeter and it is what sets the angle of every course above it. If the starter is not level, every course repeats the error and it becomes visible against the soffit at the top. Outside corner posts run the full wall height at each corner. Inside corners, where two walls meet in a re-entrant angle, need their own trim. J-channel goes around openings and where cladding meets a soffit or a roof line. Utility trim, undersill trim and finish trim handle the terminations at the top and under windows.
All of that is measured in linear feet and priced separately, and it is why a siding quote is never simply a square count times a rate. Fasteners are their own consideration: most panel systems are designed to hang rather than be clamped, with the nail driven in the centre of the slot and left slightly loose so the panel can move. Overdriving fasteners is one of the most common installation faults, because it prevents that movement and causes buckling in hot weather.
Expansion, Substrate and What Sits Behind the Cladding
Polymer cladding expands and contracts significantly with temperature — a long panel can change measurably in length between a cold morning and a hot afternoon. That is why panels are cut short of their receiving channels, why fasteners are not driven tight, and why installation instructions specify a gap that varies with the temperature at the time of fixing. Ignoring that produces waves along the wall that no amount of refixing will remove. The Polymeric Exterior Products Association publishes the installation guidance and installer certification programme for this category of cladding.
What sits behind matters as much as the cladding itself. Cladding is a rain screen and a first line of defence, not the weather barrier: the water-resistive barrier, the flashing at openings and the drainage plane behind are what actually keep water out of the wall. The Building America Solution Center, run by the US Department of Energy, documents how those layers are detailed. Requirements for water-resistive barriers and flashing sit in the building code adopted where you build, published for much of the United States through the ICC Digital Codes library, and local amendments differ.
If insulation is going on at the same time, our insulation calculator handles the material quantity, and for sheathing behind the cladding the plywood sheet calculator covers panel counts.
Waste, Colour Batches and Ordering Practicalities
Ten percent is a reasonable starting waste factor for a plain rectangular house with a modest number of openings. Raise it where there are many gables, dormers, bay windows or angled walls, all of which produce cuts that cannot be reused. Lower it only if the elevations are long, plain and the panel length divides neatly into the wall length.
Two ordering points are worth knowing. Colour is produced in batches, and batches can differ slightly, so ordering everything at once and checking the batch codes on delivery avoids a visible line halfway up an elevation. And ordering a little extra for future repairs is genuinely worthwhile, because a discontinued colour cannot be matched later. A single spare box costs far less than re-cladding an elevation. For finish comparisons against a painted surface, the paint calculator covers coating quantities on the same wall areas.
Arb Digital builds free calculators and interactive tools that earn search traffic for exterior, remodelling and construction businesses. Browse the full library, or tell us what your customers keep asking you to work out.
Browse Free Tools Talk To Arb DigitalCommon Mistakes to Avoid
- Using panel width instead of exposure — the lap means the visible face is always narrower, and the error under-orders the whole job.
- Forgetting the accessories — starter, corner posts, J-channel and finish trim are linear items and can approach the cladding in cost.
- Deducting every opening and keeping the minimum waste factor — pick one convention, because doing both leaves you short.
- Nailing panels tight — polymer cladding must be free to move with temperature, and overdriven fasteners cause buckling.
- Ignoring the course layout — where the cut course lands, and whether course lines meet window heads, is decided before the starter goes on.
Related Free Tools From Arb Digital
Pair this with the wall area calculator for net areas, the board and batten calculator for vertical cladding, the plywood sheet calculator for sheathing, the insulation calculator for what goes behind and the paint calculator for coated finishes. The full free online tools hub lists every calculator we publish.
Frequently Asked Questions
It depends entirely on the perimeter and height. In the worked example here, a 160 foot perimeter at 9 feet with 120 square feet of gable and twelve openings gives 1,380 square feet net, which is 13.8 squares, or 16 squares once a ten percent waste factor is added.
A square is 100 square feet of coverage. It is the unit siding is quoted, sold and delivered in across North America, which is why the calculator reports squares alongside the raw area.
Exposure is the visible face height of each course after the lap. It is always less than the panel width and it is set by the manufacturer. Coverage per panel is exposure times panel length, so using the width instead under-orders the job.
Deduct the larger ones. Small windows are often treated as solid wall because the panel crossing them is cut anyway and the offcut is rarely usable. Whichever convention you choose, pair a full deduction with a higher waste factor.
A triangular gable is half its base times its vertical height. Add the two gables of a simple gabled house together, and add dormer cheeks and faces separately.
Because polymer cladding expands and contracts noticeably with temperature. Fasteners are driven in the centre of the slot and left slightly loose so panels can move. Nailing tight causes waves and buckling in hot weather.
Starter strip around the base, corner posts at every outside corner, inside corner trim where walls meet in a re-entrant angle, J-channel around openings and at soffits, and finish or undersill trim at terminations. All are measured in linear feet.
It is worth doing. Colour is produced in batches that can vary slightly, and a discontinued colour cannot be matched later. Ordering everything at once and keeping a small surplus avoids a visible mismatch on a future repair.
This tool produces material estimates only. Water-resistive barriers, flashing, fastening and installation requirements are governed by the manufacturer's instructions and your local building code, and must be confirmed by a qualified installer.
