Siding Exterior Trim Calculator

Purpose and model boundary

This calculator estimates the siding and exterior trim to order for the outside walls of a one-storey house you draw. You draw the outline, the interior walls, the windows and the doors, set one wall height for the whole plan, mark which doors are in an outside wall, enter your siding product's piece length, width and exposure in the drawing's Materials panel, and press Lay siding: the drawing lays the siding on every outside wall and counts the pieces, and the spreadsheet returns the pieces and squares to order, the area to side, the J-channel round the openings, the outside corner posts and inside corners, and the starter strip, with each outside wall's start, length, gross area and pieces laid, and each room's openings. A gable and other wall area the plan cannot show is typed as one area and counted by area.

• Not included: soffit, fascia, drip cap, rake trim, utility trim, F-channel, house wrap, fasteners, pricing and labour.
• Each wall is assumed to run the full wall height, with no stepped foundations or split levels.
• The wall height is the sided height, from the base of the walls, where the starter strip runs, up to the eaves. It is not a room's ceiling height.
• Upper-storey openings stacked over lower ones cannot be drawn, so they are neither taken off nor trimmed. A wall taller than the drawing accepts is entered at the most it accepts, and the rest of its area typed as gable and other wall area.
• Horizontal siding only: lap siding, or panels that butt together without a lap. Vertical siding and board and batten are not laid.

Inputs and units

The drawing is the first input. The drawing tool writes the outline as its corner points, in feet under US and metres under Metric; one row per room; one row per wall side of every room; one row per window, always on an outside wall, because the drawing tool places windows on the outline only; and one row per door, each with its width and the room it opens into. When the siding is laid, it also writes the number of pieces laid on each outside wall, by the outline point the wall starts from. Up to five islands or columns inside the floor may be drawn as exclusions; they are floor features, and the siding ignores them. Each exclusion can carry a height, set by double-clicking it on the drawing (blank is full height); it is carried for the drawing and its 3D view only, and no figure reads it, since only the outside walls are sided.

The heights and the Exterior answers are set on the drawing or in the tables under it: one wall height for the whole plan, set with the drawing's Wall height button; each window's height and sill height; each door's height; and, for each door, Exterior: Yes for a door in an outside wall, No for a door between rooms. The drawing cannot tell an entrance door from an interior one, so every door needs an answer, written exactly Yes or No. Heights are in feet under US and metres under Metric, never inches. The spreadsheet takes a wall height from 6 to 20 ft (1.80 to 6.00 m), a window at least 1 ft (0.30 m) tall on a sill of zero or more, and a door at least 5 ft (1.50 m) tall, every opening below the wall; these are the drawing tool's ranges, not building-code limits. On the page the drawing accepts the wall heights valid in both unit systems, 6 to about 19.68 ft (1.83 to 6.00 m), so a unit switch never leaves the wall out of range; in the same way it asks for a window at least 1 ft (0.3048 m) and a door at least 5 ft (1.524 m) tall in both systems, so in Metric it refuses a 0.30 m window or a 1.50 m door that the spreadsheet alone would take. A new door starts at 6 ft 8 in (2.03 m), no higher than the wall, and a new window at 4 ft (1.20 m) on a 3 ft (0.90 m) sill, the sill lowered if needed to fit the window under the wall. A new door's Exterior starts blank.

Your siding product is entered in the drawing's Materials panel, above its Lay siding button, in inches under US and centimetres under Metric:

• Siding piece length: the length of one piece as it runs along the wall, 12 in (30 cm) by default.
• Siding piece width: the full height of one piece, including the part that laps under the course above, 7 in (22.5 cm).
• Exposure: the height of each piece left showing once it is hung, which is the course spacing, 6 in (20 cm): from 20% of the piece's width up to its full width. Enter the full width for panels that butt together without a lap.

The panel shows no counts: the pieces laid on each outside wall are returned in the results, under Siding to order and Each outline wall, once Calculate is pressed.

The inputs beside the drawing are:

• Siding waste, 10%.
• Gable and other wall area, 0: the triangles of the gable ends above the wall height (half the width times the rise, for each gable), dormer walls, and any wall height above what the drawing accepts.

Piece sizes are in inches under US and centimetres under Metric, and the gable area in square feet or square metres. One piece covers its length times its exposure: the default piece covers 12 × 6 in = 0.5 sq ft, and 30 × 20 cm = 0.06 m² under Metric. Each unit system keeps its own product and gable area, and their defaults are not conversions of each other. The default piece is small, chosen so that each piece covers the worked examples' figure, so it gives many pieces; enter your product's, for example 150 in for a 12 ft 6 in piece. The piece count is in pieces of the size you enter.

On a switch of system the page converts the wall height exactly (8 ft is 2.4384 m). Each unit system keeps its own opening heights; after a switch, and when a worked example loads, an opening with no height in the current system gets the drawing's default heights, and one with its own keeps them. A door's Exterior answer is not a length, so it carries across a switch.

Governing relationships

The spreadsheet measures the outline from its points: the perimeter is the sum of its sides. A corner is where the outline turns, so a break point on a straight wall, within the drawing tool's straightness tolerance of 0.00001 in, is not one. Outside and inside corners are told apart by the direction of the turn against the direction of the whole outline, so drawing the house either way round gives the same corners. An outline that turns straight back on itself is refused. Each outside wall runs from one corner to the next.

Gross wall area = perimeter × wall height

Window and exterior door area = Σ width × height over every window and every door marked Yes; a door marked No is not taken off

Net wall area = gross wall area − window and exterior door area

Area to side = net wall area + gable and other wall area, and siding squares = area to side ÷ 100 sq ft (9.290304 m²)

Area one piece covers = piece length × exposure

Pieces to buy = (pieces laid + gable and other wall area ÷ area one piece covers) × (1 + waste), rounded up

Squares to order = pieces to buy × area one piece covers ÷ 100 sq ft (9.290304 m²), rounded up

J-channel = Σ 2 × (width + height) over the windows + Σ (width + 2 × height) over the exterior doors

Outside corner post length = outside corners × wall height, and the inside corners likewise

Starter strip = perimeter, door openings included

How the drawing lays the siding

The drawing makes the one count the spreadsheet does not: the pieces laid on each outside wall.

• It lays one wall at a time, from corner to corner, the full wall height.
• Courses run from the base up. The bottom course starts with a whole piece at the left end of the wall, seen from outside, and the joints are offset by half a piece from course to course.
• Each piece shows its exposure, and the rest of its width laps behind the course above.
• The top course is cut to the wall top. In this count a cut piece keeps its full lap, so two short cuts share a piece only when both, laps included, fit in its width. That is the drawing's counting rule: where the wall height is not a whole number of exposures, it can count a few more pieces than strips ripped to the wall top would need.
• Windows, and doors marked Yes, are cut out at their real heights and sills.
• Offcuts are reused on the same wall, never carried to another.
• The count is of whole pieces, cut pieces included, before waste.

These rules are fixed: the wall faces are not drawn on the plan, so the start, the direction and the offset cannot be changed. The drawing tool checks at most 10,000 piece positions on a wall and 50,000 in the plan; a small piece on a large house is refused on the drawing with its message, and no count is sent.

What the spreadsheet does with the counts: it checks that every outside wall has one whole-number count, from 0 to 10,000, listed once by the outline point it starts from, and that the pieces laid cover at least the net wall area at the given exposure, allowing for the drawing tool's contact tolerance. The drawing tool leaves out a piece that would meet the wall only along a sliver no wider than its contact tolerance, 0.00001 in (0.0000254 cm), so a wall may be covered short of its area by at most one such sliver along the longer of the piece length and the exposure for each of the 10,000 positions it examines: for the default 12 in piece, 10,000 × 0.00001 × 12 ÷ 144 = 0.0083 sq ft a wall. Then it adds the gable pieces and the waste, once.

Every count is rounded up after rounding to six decimals, so floating-point noise cannot add one: 840 × 1.10 = 924.0000000000001 buys 924. Nothing else is rounded, except that a height, the exposure, the opening widths and the enclosed area are compared with their limits after rounding to six decimals; the 3D view checks the raw value, so it may refuse a height within about 0.0000005 of a limit that the spreadsheet accepts. The shown figures are the display of the full values.

Calculation sequence

1. The spreadsheet checks the drawing, the tables, the heights, the Exterior answers, the openings against the walls, the siding inputs and the laid counts, in the order the status list below gives.
2. It measures the perimeter and the corners from the outline points, and each outside wall's start, length and gross area.
3. It takes off the windows and the doors marked Yes, adds the gable and other wall area, and reads the pieces laid on each wall.
4. It works out the pieces and squares to order, the J-channel, the corners and the starter strip.

The default plan works through as follows: one 20 by 15 ft room, 70 ft of outline at 8 ft, is 70 × 8 = 560 sq ft of wall. The drawing lays 320 + 240 + 320 + 240 = 1,120 pieces, exactly 560 ÷ 0.5, because 8 ft is 16 whole 6 in courses and every wall is a whole number of 12 in pieces. 1,120 × 1.10 = 1,232 pieces to buy; 1,232 × 0.5 ÷ 100 = 6.16, so 7 squares to order; 5.60 squares of wall; 4 outside corner posts of 8 ft; and 70 ft of starter strip.

Worked example 11.4.7.1 takes off two 3 by 4 ft windows and one exterior door, 3 ft by 6 ft 8 in, from 560 sq ft, leaving 516 sq ft. The drawing lays 1,033 pieces, one more than 516 ÷ 0.5, because the course over the door head is cut; 1,033 × 1.10 = 1,136.3 → 1,137 pieces to buy, and 5.685 → 6 squares to order. Its J-channel is 2 × (3 + 4) × 2 + (3 + 2 × 6.6667) = 44.33 ft. Worked example 11.4.7.3, in Metric, lays 30 by 22.5 cm pieces at 20 cm on 2.50 m walls: 2.50 m is 12½ courses, so each cut of the top course keeps its 2.5 cm lap and is 12.5 cm high, and two such cuts do not fit one 22.5 cm piece; it lays 719 pieces, 791 to buy and 6 squares to order, on 40.72 m² of net wall.

Outputs and interpretation

• Siding to order: the pieces to buy, the squares to order, the pieces laid, the siding squares, the net wall area and the area to side.
• Trim: the J-channel, the starter strip, the outside corner posts and their length, and the inside corners and their length.
• How the walls are counted: the gross wall area and the window and exterior door area.
• Each outline wall: one row per outside wall with its start point, its length, its gross area, whether the corner at its end is outside or inside, and the pieces laid on it. The area is gross because the drawing's rows do not tie an opening to a wall. The pieces laid are the drawing's count for that wall, with no waste added.
• Openings by room: one row per room with its windows, their area, its exterior doors, their area, and the J-channel for them, by the room each window is in or each door opens into.

The headings carry the units of the system the drawing was sent in. The floor plan's own areas are not shown: floor area is not a siding quantity.

A square is 100 sq ft (9.290304 m²) in both systems. Siding squares is the area to side in squares; squares to order is the pieces to buy, in whole squares.

A laid count is the drawing's own: it is usually higher than the area divided by the area one piece covers, because the pieces cut at the openings, the corners and the wall top are counted whole. It equals that figure when every wall is a whole number of pieces long and of courses high with nothing cut out, as in the default (1,120 = 560 ÷ 0.5), and the spreadsheet refuses counts that cover less than the net wall area, allowing for the drawing tool's contact tolerance.

Openings are taken off in full, as the worked examples do. Leaving windows and doors in the area is sometimes used as a waste allowance; here the pieces cut at the openings, the corners and the wall top are counted by the drawing, so the waste percentage covers the rest: breakage, mistakes and offcuts too short to use. The default is 10%, the worked examples' figure. Gables and dormer walls are not drawn: their typed area is divided by the area one piece covers, so it carries no cut count of its own, and the waste percentage covers it. The starter strip follows the whole base, door openings included.

The 3D view draws the walls you drew at the wall height, with the door and window openings cut at their sills and heights, and no roof. While the siding is laid, it draws the courses on the outside face of every wall as joint lines, clipped at the openings; while it is not laid or is out of date, none are drawn and its note says to press Lay siding; above the drawing tool's 3D limit none are drawn, and its note says the counts are not affected. The siding shows in 3D only: the plan draws no siding, because a wall face is not on the plan. It is for checking the drawing; it shows no figures, sends nothing and changes nothing.

Validation and status logic

The status reads OK, CHECK with what to look at, or NOT VALID with what to fix. While it is NOT VALID every quantity reads zero and both tables are blank. The first problem found is reported, in this order:

1. NOT VALID: choose US or Metric
2. NOT VALID: draw at least three outline points
3. NOT VALID: every outline point needs both coordinates
4. NOT VALID: every point of exclusion 1 needs both coordinates
5. NOT VALID: every point of exclusion 2 needs both coordinates
6. NOT VALID: every point of exclusion 3 needs both coordinates
7. NOT VALID: every point of exclusion 4 needs both coordinates
8. NOT VALID: every point of exclusion 5 needs both coordinates
9. NOT VALID: each exclusion needs zero or at least three complete points
10. NOT VALID: outline and exclusion points must fill consecutive rows, with no blank row between them
11. NOT VALID: two outline points in a row are the same point
12. NOT VALID: the outline encloses no area
13. NOT VALID: the outline doubles back on itself
14. NOT VALID: room, wall, window and door rows must be complete, with lengths above zero and no negative area, and fill consecutive rows with no blank row between them
15. NOT VALID: every room must be listed once, and every wall, window and door must belong to a listed room
16. NOT VALID: wall, window and door IDs must each be listed once, and no ID or room ID may contain *, ? or ~ or begin with <, > or =
17. NOT VALID: the wall height must be from 6 to 20 ft (US) or from 1.80 to 6.00 m (Metric)
18. NOT VALID: every window needs a height of at least 1 ft (US) or 0.30 m (Metric)
19. NOT VALID: every window needs a sill height of zero or more
20. NOT VALID: every door needs a height of at least 5 ft (US) or 1.50 m (Metric)
21. NOT VALID: choose Yes or No under Exterior in the doors table for every door
22. NOT VALID: a window or door reaches above the wall height
23. NOT VALID: an exclusion height must be greater than zero; leave it blank for full height
24. NOT VALID: an exclusion height is given for an exclusion that is not drawn
25. NOT VALID: a window or exterior door is wider than the longest outline wall
26. NOT VALID: the windows and exterior doors are wider in total than the outline, or cover more than the wall area
27. NOT VALID: siding piece length and width must be greater than zero
28. NOT VALID: the exposure must be from 20% to 100% of the siding piece width
29. NOT VALID: enter a siding waste between 0% and 100%
30. NOT VALID: the gable and other wall area cannot be negative
31. NOT VALID: the siding layout must list every outline wall once, by its start point, in consecutive rows with no blank row between them: press Lay siding on the drawing
32. NOT VALID: siding counts must be whole numbers of 0 or more, with at most 10,000 pieces laid on a wall: press Lay siding on the drawing
33. NOT VALID: the siding laid has fewer pieces than the wall area needs: press Lay siding again
34. CHECK: some outline corners are not right angles; choose corner posts that suit those corners

NOT VALID means the drawing, the tables, the heights, the siding layout or the inputs do not describe walls this model can side, and the line says what to fix; items 31 to 33 also say what to press. CHECK means the answer computed but deserves a look: a corner of the outline is not square, so the corner post there must suit its angle; the count of posts is right. OK means neither applied.

The height limits are the drawing tool's own: the spreadsheet holds a direct caller to the drawing tool's range for each unit system (items 17 to 20 and 22), and the page narrows the wall and the openings to the ranges valid in both systems. The Exterior answer must be exactly Yes or No, with that case and no spaces, and two IDs that differ only in case count as one ID listed twice. An exclusion changes nothing here, even one drawn against an outside wall, so it raises no CHECK.

The counts are the drawing's: on the page Calculate lays first and sends only a laid layout, and before it reports the state the drawing reads the heights, the Exterior answers and the siding inputs again, so a value changed in a table always leads to a new lay. Items 31 to 33 refuse counts sent from elsewhere that miss a wall, are not whole numbers from 0 to 10,000, or cover less than the net wall area; item 33 catches a layout laid for a lower wall only when its pieces fall short of the taller wall's net area, so a layout with pieces to spare can still read OK. The spreadsheet checks the rows' completeness, IDs, heights, Exterior answers, opening widths and counts. It does not check that the outline never crosses itself, which wall a window or door is on, that a door marked exterior is in an outside wall, or that the counts are the drawing's own; the drawing ensures these on the page. A direct caller could send counts above any real layout of the drawn walls, which over-orders, or counts below the drawing's own that still cover the net wall area, which under-orders.

Assumptions and limitations

• One storey, one wall height for the whole outline, and every wall the full height.
• Openings are taken off at their full width and height; their trim is J-channel only.
• Corner posts are counted, not sized: a corner that is not square needs a post that suits its angle.
• Gables and dormer walls are counted by area, with no cut count of their own.
• Offcuts are reused only on the wall they were cut on, and a cut piece keeps its full lap.
• The wall, window and door heights must each be within its range. The wall height converts with the units when you switch between US and Metric, and a door or window with no height in the new units gets the defaults.
• Nothing is saved between visits. Export keeps the drawing and whether the siding was laid, but not the heights, the Exterior answers or the siding inputs; after an import the siding is laid again with the page's own values when it can be, and otherwise reads out of date.

Restrictions and non-computing states

The spreadsheet computes nothing while the status is NOT VALID: every quantity reads zero and both tables are blank. Edits on the page stay local until Calculate is pressed, and Calculate lays the siding first whenever it is not laid or is out of date: after any change to the drawing, a height, an Exterior answer, the piece size, the exposure or the unit system.

Errors and warnings

Each NOT VALID line above names the input to fix. A height outside the drawing's limits, or a blank opening height, is refused, because the opening must still fit the wall. If the drawing cannot lay the siding, Calculate sends nothing and the drawing tool's own message says why: a door with no Exterior answer ("Answer Yes or No under Exterior in the doors table for D001 before laying."), a door marked Yes that is not in an outside wall ("D001 is marked as an exterior door but is not in an outside wall."), a piece size of zero, an exposure outside 20% to 100% of the piece width, or a piece too small for a wall or for the plan. A height typed in the windows or doors table below the drawing's minimum, or a blank sill height, is also refused when the siding is laid, but the message may not name the real cause: for a window it reads "This window would reach above the wall", and for a door under Metric it gives the minimum as 1.50 m, while the page asks for 1.524 m. If you see one of these, check that opening's height and sill height against the limits under Inputs and units.

Sources and further reading

• Vinyl Siding Institute, Inc., Installation Manual (copyright 2020; the PDF file is dated 2022). polymericexteriors.org. The industry association's manual: outside and inside corner posts (p. 8 and p. 16); wall area from height times width, gable and dormer areas from one half of the height times the width, and the note that leaving the windows and doors in the area gives an allowance for waste (p. 10); the starter strip measured around the entire base of the house (p. 10); the square of 100 sq ft for ordering (p. 10); the estimating guidance, outside and inside corner posts by length and 10% added for waste (p. 11); and J-channel at the gables and round openings (p. 20).
• National Institute of Standards and Technology, Guide for the Use of the International System of Units (SI), NIST Special Publication 811, Appendix B.8. nist.gov. The foot is exactly 0.3048 m and the square foot exactly 0.09290304 m², so a square of 100 sq ft is 9.290304 m²; the inch is exactly 2.54 cm.
• Eric W. Weisstein, "Polygon Area," MathWorld, Wolfram Research. mathworld.wolfram.com. The shoelace formula, and the sign of the signed area, which gives the order of the points and sorts outside corners from inside ones.

0.5 sq ft and 0.06 m² per piece are the worked examples' figures; the default piece (12 by 7 in at a 6 in exposure, and 30 by 22.5 cm at 20 cm) is the calculator's own, authored to cover exactly that, not a source's or a manufacturer's product. The way the drawing lays the pieces is the calculator's own method, described in the methodology, not a source's. 8 ft is the floor plan's default wall height. 10% is the worked examples' figure; the Vinyl Siding Institute's estimating guidance (p. 11) is cited as a source for adding a waste allowance. Use your own product's size and exposure and your supplier's guidance. These sources describe units, geometry and practice and certify no particular result.