How We Calculate Siding and Trim

CladCount adds up rectangles and triangles in square feet, takes off windows and doors, adds waste and divides by what one piece of siding covers. Lengths are kept in feet as exact numbers, and nothing is rounded until a figure is shown: areas and linear feet to 1 decimal, squares to 2, pieces and cartons up to whole ones. Where a published estimating guide gives the rule, it is quoted with a link. Where we had to choose between sources, the rule is marked Our rule.

Squares

Siding is ordered in squares. The installation manual of the Polymeric Exterior Products Association, formerly the Vinyl Siding Institute (VSI), defines it: Every 100 square feet (9.29 square meters) is called a “square” for ordering purposes. (VSI manual). Ply Gem gives the step: Divide the total square footage by 100 to find the total number of squares. (Ply Gem). CladCount shows squares to two decimals and does not round them up; the pieces and cartons carry the rounding.

Walls and gables

A wall is its width times its height. VSI: The area to be sided can be determined by measuring the height and width of the house, including windows (Fig. 11). (VSI manual); CertainTeed: Measure height (excluding gables). Measure width (CertainTeed). The house block counts two walls of length times wall height and two of width times wall height, and puts the gable ends on the two width walls.

A gable is a triangle, half the base times the height. LP: For triangle areas, measure from the base of the triangle to the top and multiply that number by half of the base length. (LP). If you know the roof pitch instead of the gable height, the height is half the base times the rise per foot: BYU-Idaho's estimating text puts it as Slope is the ratio of Rise/Run. In construction, it is measured in inches per foot (BYU-Idaho). The field is labelled in 12, rise per 12 in of run, because the word pitch also has an older meaning, rise divided by span; Swanson Tool Co. records it as Pitch equals rise divided by span (Swanson).

Worked: a 24 ft gable at 6 in 12 has a run of 12 ft and rises 6 ft, so it covers 72.0 sq ft. Its sloped edge, the rake, is 13.42 ft per side; Swanson's rafter table prints 13.42 for a 12 ft run at 6/12 (Swanson). The rake length is what J-channel along the gable needs (see trim below).

Windows and doors: three published rules

The sources do not agree on what to do with openings.

Deduct them. The Western Red Cedar Lumber Association (WRCLA): Subtract square footage of openings (windows, doors) to determine wall area for siding. (WRCLA); the Cedar Shake & Shingle Bureau: Remember to deduct window and door areas from coverage calculations. (CSSB); LP: Subtract the uncovered areas’ square footage from the total amount you calculated in step four. (LP).
Do not deduct them. VSI: Windows and doors are not usually deducted. Including them will provide an allowance factor for waste. (VSI manual); CertainTeed measures width (including doors and windows). (CertainTeed).
Deduct half of the large ones only. The VSI estimating worksheet has a line Large areas not to be covered: (garage doors/sliding doors) x 0.50 = (VSI worksheet).

Our ruleThe switch Subtract windows and doors is on by default, as in the three sources that deduct and in the question most people ask. Switched off, openings stay in the total and a line says so. The half-of-large rule is not a third mode; How to measure a house for siding works one house under all three rules.

Worked example (WRCLA's own): Length x height = 160 square feet Door = 20 square feet Window = 20 square feet Area for siding = 120 square feet Add 10% for trim loss= 120 + 12 = 132 square feet (WRCLA). The calculator, with a 20 × 8 ft wall, a 2.5 × 8 ft door and a 4 × 5 ft window, gives 160.0, 40.0 of openings, 120.0 net and 132.0 sq ft to order. With the switch off it gives 176.0.

On the house used across this site (open it in the calculator), the openings are 162.0 sq ft: subtracted, the order is 13.01 squares; kept, it is 14.80 squares and 178 panels instead of 157.

An opening as large as the walls and gables together is an error with the switch on. LP's allowance, When in doubt, account for 15 square feet for windows and 20 square feet for doors. (LP), sets the default window at 3 by 5 ft; the default door, 3 by 7 ft with its trim, is our assumption.

Waste

The waste percentage is added to the area: order area = net area × (1 + waste ÷ 100). The default is 10 %, the figure of three sources: * Add a factor of 10 percent to all material estimates to allow for waste. (VSI worksheet), Add 10% for trim loss. (WRCLA) and you may want to add a waster allowance of 10 percent to the totals for siding, soffit and accessories. (CertainTeed). LP says Add 5-10% extra siding to account for mistakes. (LP). The range is 0 to 50 %. CertainTeed also measures a gable's height at its center and adds a foot (Measure height at center (add 1' to allow for waste)., CertainTeed); CladCount uses one percentage for everything and does not add that foot.

Pieces: rounding up

One piece covers its exposure (the part of its width left showing, in inches) divided by 12, times its length. Pieces = order area ÷ that coverage, rounded up. This is what LP's SmartSide lap coverage chart prints: all 90 of its 90 lap cells equal the round-up, and only 45 equal rounding to the nearest piece (LP chart). LP adds: Disclaimer: The above Tables do not account for waste factors. Remember to include your expected jobsite waste. (LP chart), so the waste goes on the area first, then the division.

Our ruleRound up. The 2016 fiber-cement lap chart in James Hardie's installation instructions (Number of 12 ft. planks, does not include waste, fiber-cement chart) rounds to the nearest plank instead: 160 of its 180 cells equal the nearest whole plank, 126 equal the round-up. We round up because a short count leaves a gap on the wall, and LP rounds up. Worked: 200 sq ft of a 6 in exposure in 12 ft planks is 33.33 planks, so CladCount says 34; that chart prints 33 (fiber-cement chart).

The same chart's column for the 9-1/2 in board at an 8-1/4 in exposure repeats the column of the 9-1/4 in board at 8 in: all 20 of its 20 rows print the same numbers (fiber-cement chart). Our counts for the 8 in column match the chart in 20 of 20 rows; Lap siding coverage sets the columns side by side.

Ply Gem prints per-square counts without panel lengths: Double 4″ siding: 12 pieces = 1 square, Double 5″ siding: 10 pieces = 1 square, Double 4-1/2″ siding: 11 pieces = 1 square (Ply Gem). Ply Gem prints 11 per square without a panel length; at a 12' 1" panel the round-up gives 12 — check the chart for your product.

Linear feet of siding = order area × 12 ÷ exposure. WRCLA's bevel table prints this factor as 12 ÷ exposed face rounded to two decimals (2.67 for a 4-1/2 in face), though its heading reads Linear Feet Factor = Exposed Face Width/12. (WRCLA), and its worked example says 132 x 2.67 = 357.4 linear feet, which is 352.4 by arithmetic. CladCount uses the exact factor, not the rounded one.

Cartons = pieces ÷ pieces per carton, rounded up, shown only when you fill in pieces per carton. Carton squares are nominal: Style Crest labels both Length 12' - 6" | 24 Panels/Ctn. | 2 Sq./Ctn. and Length 12' - 1" | 22 Panels/Ctn. as two squares (Style Crest), though the second covers less than 200 sq ft; What is a square of siding works each carton out.

Profile chips

Each chip sets the exposure, the piece length and the pieces per carton. Their sources:

Profile chips and where their values come from
Chip	Exposure	Length	Per carton	Source
Vinyl double 4"	8 in	12' 6"	24	Alside catalog: Double 4" exposure, 12' 6" panel length, 24 panels per carton; CertainTeed catalog: Panels/Ctn.: 24
Vinyl double 5"	10 in	12'	20	Alside catalog: Double 5" exposure, 12' 0" panel length, 20 panels per carton
Vinyl Dutch lap 4-1/2"	9 in	12' 1"	22	Style Crest: Length 12' - 1" \| 22 Panels/Ctn.
Steel double 4"	8 in	12' 6"	12	Alside catalog, steel: 12 pieces per carton
Fiber cement lap 8-1/4" (7" exposure)	7 in	12'	—	2016 fiber-cement lap chart, 8-1/4 in board at 7 in exposure, 12 ft planks
Engineered wood lap 8" (6-7/8" reveal)	6.875 in	16'	—	LP SmartSide chart, 16 ft lap at a 6-7/8 in reveal
Cedar bevel 1x8 (6-1/2" exposed face)	6.5 in	12'	—	WRCLA bevel table: 8 in nominal, 6-1/2 in exposed face; the 12 ft length is our assumption (cedar comes in mixed lengths)

Trim

Starter strip runs around the base: VSI, To estimate the amount of starter strip required, measure the linear feet around the entire base of the house. (VSI manual); CertainTeed, Starter strip: Measure along base of building. (CertainTeed). Door widths are not taken off.
J-channel goes around windows on four sides and doors on three: BYU-Idaho, Note that the bottom length of door openings will most likely not include J channel (BYU-Idaho); and along the gable rakes. CertainTeed lists more places, measure around doors and windows, under eaves, at rake edges of gables (CertainTeed); J-channel under the eaves is not counted.
Corner posts: Outside cornerpost: Measure length of outside corners. and Inside cornerpost: Measure length of inside corners. (CertainTeed). The corner height follows the wall height until you type another one.

Our ruleWaste applies to starter strip and J-channel before they are divided into pieces, as VSI and CertainTeed apply it to the totals for siding, soffit and accessories. (CertainTeed). Each corner gets whole posts: posts per corner = corner height ÷ post length, rounded up, with no waste. No source gives an overlap for joining two pieces of trim, so none is added. Default stock lengths: starter and J-channel 12 ft 6 in, corner posts 10 ft (CertainTeed, Alside); outside posts are also sold 12 ft long. All four can be changed.

Worked: the house above, 40 × 30 ft, 8 ft walls, two gables at 6 in 12, 8 windows and 2 doors, vinyl double 4 in, 10 % waste, as the calculator's Show the math prints it:

Walls: 2 × 40' × 8' + 2 × 30' × 8' = 1,120.0 sq ft
Gables: pitch 6 in 12 → height 15' × 6 ÷ 12 = 7.5'; 2 × ½ × 30' × 7.5' = 225.0 sq ft
Openings: 8 × 3' × 5' + 2 × 3' × 7' = 162.0 sq ft
Net: 1,120.0 + 225.0 − 162.0 = 1,183.0 sq ft
To order: 1,183.0 × 1.10 = 1,301.3 sq ft = 13.01 squares
Pieces: 8" × 12' 6" = 8.33 sq ft each; 1,301.3 ÷ 8.33 = 156.2 → 157
Cartons: 157 ÷ 24 = 6.5 → 7
Linear feet: 1,301.3 × 12 ÷ 8 = 1,952.0 ft
Starter: 140.0 ft × 1.10 ÷ 12' 6" = 12.3 → 13
J-channel around openings: 8 × 2 × (3' + 5') + 2 × (3' + 2 × 7') = 162.0 ft
Rake: √(15² + 7.5²) = 16.77 ft per side; 2 sides × 2 = 67.1 ft
J-channel: openings 162.0 + rakes 67.1 = 229.1 ft × 1.10 ÷ 12' 6" = 20.2 → 21
Outside corners: 4 × 8' = 32.0 ft; 8' ÷ 10' = 0.8 → 1 post each = 4

Limits

The tool adds up the rectangles and triangles you enter. A garage that covers part of a house wall, a porch roof or a chimney is not detected: enter the visible wall pieces instead.
Trim lengths have no splice overlap and no allowance for cutting runs into short pieces beyond the waste percentage; corner posts are whole posts per corner.
Under-eave J-channel, soffit, fascia, F-channel and undersill trim are not counted.
Carton coverage is nominal; check your product's carton label.
Gambrel and other shapes: split them into rectangles and triangles. VSI gives a gambrel end as ½ (A + B) x C + ½ B x D (VSI manual).
Shingles, shakes and vertical siding (board-and-batten panels, metal panels): use the squares. Bundles and panels depend on the product's chart and, for vertical panels, on how many fit across each wall.

Sources

All retrieved and checked on 2026-11-15.

Changelog

2026-11-15: v1 — launch: walls, gables by height or roof pitch, windows and doors by size with the subtract switch, waste on area and trim, squares, pieces rounded up as the LP SmartSide chart does, cartons, starter strip, J-channel around openings and gable rakes, corner posts per corner; sources checked 2026-11-15.

CladCount counts material only. It does not cover installation, fastening, wind or fire ratings, building codes or permits — follow the manufacturer's installation instructions and your local building department.