Vinyl siding calculator

The trim is half the order

A siding job is not panels. It is panels plus a receiving channel at every edge they terminate into, and the panel count tells you nothing about how much of that you need. CertainTeed's own measuring instructions list four separate accessory takeoffs alongside the siding: starter strip along the base of the building, J-channel around every door and window, under the eaves and at the rake edges, outside cornerpost down every outside corner, and inside cornerpost down every inside one.

On a modest 40 × 30 ft single-storey house that adds up fast:

• J-channel — a 3 × 5 ft window is 16 ft of perimeter, and ten of them is 160 ft before you have touched a door. Add the eave run and the rakes and 230 ft is normal. At 12 ft 6 in a piece that is 19 pieces, or 21 with waste on.
• Corner posts — four outside corners on a 10 ft wall is 40 ft, and posts come in 10 ft lengths (Georgia-Pacific's is 3 in × 120 in). You cannot splice a corner neatly, so a 9 ft wall still consumes a full post.
• Starter strip — the whole base perimeter, 140 ft here.

None of that appears in a calculator that returns squares and stops, and every piece of it is on the critical path: you cannot hang the first panel until the starter strip and the corners are up.

Squares, boxes, and why they are not the same

Siding is priced by the square — 100 sq ft — but stocked in two-square boxes. The commonest profile checks out exactly: a Double 4 panel shows two 4 in courses, so 8 in of exposure, in a 12 ft 6 in length. That is (8 ÷ 12) × 12.5 = 8.33 sq ft a panel, and a 24-panel carton is 200 sq ft on the nose.

Insulated and premium profiles often box as a single square, so a job that needs "16 squares" is 8 boxes of one product and 16 of another. Check the carton end before you convert.

Openings: deduct less than you think

Deduct garage doors, patio sliders and picture windows. Leave ordinary windows in the area. The panels cut around a normal window produce offcuts that are too short to use anywhere else, so the material genuinely is consumed — deduct them and you finish the last wall two boxes short.

Waste is CertainTeed's 10%, and note where they apply it: "to the totals for siding, soffit and accessories". Grossing up the panels and then ordering the trim exact is precisely backwards, because trim is what you run out of at 4pm on a Saturday — so the J-channel, corner post and starter figures here all carry the same waste percentage the panels do.

The pieces this does not count

Utility (undersill) trim at the top course and under windows, since the last panel is cut and has nothing to lock into. Window and door surround if you are not using plain J. Soffit and fascia if the eaves are being redone at the same time. And a box of the right nails — roofing nails long enough to hit the studs, driven to leave about 1/32 in of play so the panel can move.

Nor the water-resistive barrier behind all of it. IRC R703.2 requires an approved barrier over the studs or sheathing of every exterior wall, and it is a separate roll that has to be on before the starter strip goes up. Its area is your wall area, not the netted figure this page divides.

What you have to bring to this page yourself

The wall area is your takeoff, not a derived figure. There is no length, height or gable input, so nothing here can check the number you typed — including the instruction to add each gable as half its width times its height. The same holds for the J-channel run, the corner length and the starter run: they are four separate measurements you make, and the page never reconciles them against one another. Enter ten windows' worth of J-channel and no opening area, or the reverse, and it will answer just as confidently.

The panel count is an area division, not a course layout. Real siding waste is decided by how the wall height divides by the exposure — 8 in on a Double 4 — and by where the openings fall within the courses. A wall that ends mid-course is ripped along its whole length, and that rip goes in the skip. None of that geometry exists here; the waste percentage is carrying it, which is the reason a cut-up elevation with dormers and bays wants 15% rather than 10%.

The trim counts pool their offcuts, and corners genuinely do not. Corner posts are total corner length divided by post length, so four 6 ft corners come back as three posts rather than the four you will actually open — because a post is not spliced partway up a wall, as the section above says. Where your wall height is well under the post length, count one post per corner per storey by hand and override the figure. J-channel pooling is fairer, since the short pieces around a window do get used up, but on a long eave every joint is a lap, so read that piece count as a floor rather than an answer.

The panel arithmetic describes a horizontal lap profile. Everything worked out above — the 8 in exposure, the 12 ft 6 in length, the 24-panel carton — is a Double 4 running in courses. Vertical siding, shakes and scallops are handled here by exactly the same area-over-box-coverage division, so the only place that difference reaches the answer is the coverage field. Nothing on this page knows that a shake course is laid to its own exposure, or that vertical panels are cut to the wall height rather than run in courses, or that both carry a different trim list from the one counted here.

Starter strip is returned in feet, not pieces. Round it up to whatever stock length your supplier carries.