Coverage width is not sheet width, and the difference is the whole count
Ribbed steel panel is sold by a nominal width and installed at a smaller one, because one rib laps the next. A panel described as 38 inches wide commonly covers 36. On a 50 ft wall that is the difference between sixteen runs and seventeen. One run does not sound like much, but it is one run on every wall of the building, so a 50 by 30 shed comes out four runs short — and short is the direction that costs you a second delivery.
This calculator asks for the coverage figure and nothing else, so the answer is only as good as what the supplier told you. Get it from the quote or the product sheet, not from a tape across a sheet in the yard. The corrugated panel coverage and purlin calculator works the other way round and derives coverage from the corrugation pitch and the number of corrugations you lap, which is worth doing if all you have is the profile.
The gable triangle wastes a sixth of what you buy and there is nothing to do about it
A 30 ft wide building at 4 in 12 has a 5 ft rise, so each gable is a triangle 30 ft across and 5 ft tall, 75 square feet, 150 for both ends. Cover it in 36 inch panel and each half gable takes five runs, cut to the rake at 1, 2, 3, 4 and 5 feet. That is 15 lineal feet per half, 60 feet across both gables, and 60 feet of 36 inch panel is 180 square feet of purchased material for 150 square feet of triangle.
The 16.7 percent that comes off in rake cuts is not a mistake and it is not avoidable. A triangle covered in rectangles has to waste something. Where the half gable divides evenly by the coverage width into a whole number of runs, the waste works out at exactly one over that number plus one: five runs gives a sixth, six runs gives a seventh, eight runs gives a ninth. So a wider building wastes a smaller share, right up until the width stops dividing evenly.
| Width at 4 in 12 | Runs per half gable | Triangle, both ends | Panel bought | Cut off |
|---|---|---|---|---|
| 24 ft | 4, even | 96 sq ft | 120 sq ft | 20.0% |
| 27 ft | 4.5, rounded to 5 | 122 sq ft | 174 sq ft | 30.2% |
| 30 ft | 5, even | 150 sq ft | 180 sq ft | 16.7% |
| 36 ft | 6, even | 216 sq ft | 252 sq ft | 14.3% |
| 40 ft | 6.67, rounded to 7 | 267 sq ft | 332 sq ft | 19.7% |
| 48 ft | 8, even | 384 sq ft | 432 sq ft | 11.1% |
The two odd rows are the interesting ones. A 27 ft building throws away 30 percent of its gable panel, worse than a building three feet narrower, because the half gable is four and a half runs wide and the half run at the ridge still gets bought as a whole one at full length. A 40 ft building does the same thing less severely. If the gable width is a free choice at design stage, landing the half gable on a whole number of coverage widths is worth a few percent of the panel bill; if it is not free, at least know the figure before ordering.
Pitch does not move the percentage at all. Doubling the rise doubles both the triangle and the panel bought for it, so a 40 ft gable wastes 19.7 percent at 3 in 12 and 19.7 percent at 12 in 12. What pitch changes is the absolute quantity, and it changes it fast: the same building goes from 200 square feet of gable at 3 in 12 to 800 at 12 in 12.
The offcut that pays for the wainscot, until it does not
On the default numbers — 14 ft eave, 3 ft wainscot, 2 inch lap — the upper panel is 11.17 ft long. Cut that from a 16 ft stock panel and 4.83 ft falls off, which is one full 3 ft wainscot piece with 1.83 ft to spare. There are 54 runs on the rectangular walls, so on paper the offcuts supply the entire wainscot band and the wainscot costs nothing.
On paper. A wainscot exists because it is a different colour from the wall above it, or a heavier gauge, or both — that is the point of the detail. The moment you order two colours, every one of those 54 offcuts is scrap and the wainscot is 54 more pieces to buy. The ledger is still worth printing, because it tells you exactly what the colour change costs: 162 lineal feet of panel you already own and are throwing away, plus 162 feet you have to order. If the wainscot is the same colour and you are only breaking the wall for a heavier gauge at the bottom, take the offcut. If it is a colour change, do not plan around it.
Openings do not reduce the run count
The area figures deduct the openings, because paint, insulation and cost per square foot all care about area. The run count does not, and that is deliberate. A 12 by 14 ft door in a 14 ft wall does happen to be full height, but a 12 by 10 door in the same wall leaves 4 ft of wall above it that still needs a panel run reaching from the eave down past the header, and the panel below the opening is a separate short piece. You end up with more pieces, not fewer, and the same number of runs across the wall.
Deduct runs by hand only where an opening goes wall-top to wall-bottom and its edges land on panel joints. Anything else and the deduction costs you more in short pieces than it saves in panel. The framing side of the same question — what the opening does to the girts behind the panel — is the door opening framing take-off.
Where this stops
This is walls. The roof is a different plane with different laps and its own purlin question: use the metal roof panel calculator when you have the coverage width, or the corrugated panel and purlin calculator when you have the profile. Trim here is given in lineal feet and stops there; converting a run into pieces with an overlap allowance is what the metal roof panel calculator does for trim and what the exterior trim and corner board calculator does for wood. The girt rows the panel screws into come from the post-frame bay layout calculator. For lapped wood siding rather than steel, the lap siding course layout calculator handles coursing, and for interior panelling the wainscot rail, stile and panel layout calculator is a different job entirely despite the shared word.
Questions people ask
Should I enter the panel width or the coverage width?
Coverage. Ribbed panel laps one rib onto the next, so the width you measure across a loose sheet is always more than the width it covers once installed. A common pairing is a sheet a little over 38 inches wide covering 36. If you enter 38 the calculator will tell you a 50 ft wall takes sixteen runs when it takes seventeen, and you will be one panel short on every wall. The coverage figure is on the supplier quote and the product sheet. If all you have is the profile, work coverage out from the corrugation pitch and the number of corrugations you lap, which is what the corrugated panel calculator on this site does.
Why is the gable waste the same at 4 in 12 and 6 in 12?
Because doubling the pitch doubles the triangle and doubles the panel you buy for it, so the ratio does not move — a 40 ft gable comes out at 19.7 percent waste at 3 in 12 and 19.7 percent at 12 in 12. What moves the percentage is the width, and specifically how the half gable divides by the coverage width. Where it divides evenly into n runs the waste is exactly one over n plus one: four runs on a 24 ft building is 20 percent, five runs on a 30 ft building is 16.7, eight runs on a 48 ft building is 11.1. Where it does not divide evenly the part run at the ridge is still bought whole, and the waste jumps — a 27 ft gable throws away 30 percent, worse than a narrower building.
Does the calculator include the roof?
No. It covers the four walls and the two gable triangles, which is the wall plane only. The roof is a separate take-off with its own slope length, end laps, purlin spacing, ridge and eave closures and a completely different fastening pattern, and two calculators on this site already do it: the metal roof panel calculator when your supplier has given you a coverage width, and the corrugated panel and purlin calculator when you have the corrugation profile and want coverage derived from it. Running the walls here and the roof there is deliberate — the two planes share almost no arithmetic beyond the slope factor.
What is the wainscot lap for?
The upper panel overlaps the top of the wainscot rather than butting to it, so water running down the wall crosses the joint on the outside. The lap adds to the length of the upper panel: a 14 ft wall with a 3 ft wainscot and a 2 inch lap needs upper panels 11 ft 2 in long, not 11 ft. Two inches sounds trivial and it is, until it pushes the upper panel past a stock length and forces you into the next size up on every run. That is exactly what the offcut section is for — it shows what the upper panel takes out of a stock length and what is left.
Why are the gable screws not counted?
Because the calculator does not know what the gable panel lands on. Below the eave the panel screws to girts, and you tell it how many girt rows there are. Above the eave the framing varies too much to guess: some gables are framed off the end truss with their own girts, some carry posts up to the rake, some are sheeted vertically onto a different member spacing altogether. Rather than invent a number, the page counts the field screws for the rectangular walls, counts the stitch screws down the side laps for the full wall height, and says plainly that the gable is not included.