Siding is not spread across a wall. It is stacked up it, one course lapping the one below, and the number that runs the whole job is the exposure: how much of each piece still shows once the next course covers the rest. Divide the wall height by that exposure and you almost never land on a whole number. What is left over is a real thing, the ripped top course under the eave, and no amount of square footage will tell you how wide it is. This siding calculator works in courses first and squares second, which is the order the job is actually laid out in.
The wall height decides the layout
Take a nine foot wall, which is 108 inches from the bottom of the starter strip to the underside of the eave, and hang Double 5 vinyl on it. Each course shows five inches. That is 21.6 courses, so you install 22, and the last one gets ripped down to three inches. It is a sliver, it sits at eye level with the soffit, and it is the first thing a siding contractor looks at when they walk up to a house.
Knowing the number in advance is what lets you do something about it. You can shift the starter strip down, you can split the difference by ripping the bottom course instead, or on the right material you can change the exposure so the problem disappears. What you cannot do is discover it on the last day.
A piece is not a course
Vinyl profiles are named for how many courses ride on one panel. Double 5 means two five inch courses on a single ten inch panel, so one piece covers two rows of the wall. This is where most siding counts go wrong: the layout runs on the exposure, the order runs on the face of the whole piece, and they are different numbers. Set the courses per piece to one for a plank and to two or three for a vinyl panel, and both stay correct. The Vinyl Siding Institute’s installation manual is the free industry reference for these profiles, listing the single, double, triple, vertical and Dutch lap families the names come from.
Pieces, squares and cartons
Siding is sold by the square, which is 100 square feet of finished wall. One piece covers its face width times its length, so the pieces in a square fall straight out of the profile:
| Profile | Face | Length | One piece covers | Pieces in a square |
|---|---|---|---|---|
| Double 4 vinyl | 8 in | 12 ft 6 in | 8.33 sq ft | 12 |
| Double 5 vinyl | 10 in | 12 ft 6 in | 10.42 sq ft | 9.6 |
| Triple 3 vinyl | 9 in | 12 ft 6 in | 9.38 sq ft | 10.67 |
| 8.25 in fiber cement at 7 in | 7 in | 12 ft | 7 sq ft | 14.29 |
| 7.25 in fiber cement at 6 in | 6 in | 12 ft | 6 sq ft | 16.67 |
That first row is worth checking against a real carton. A Double 4 box holds 24 panels of 8 inches by 150 inches, which is 200 square feet exactly, or two squares. The arithmetic is not an approximation, it is how the box was designed.
Because siding comes in whole cartons, the last part of the order is a rounding step, and on a small job it is not a small one. A ten foot wall on a shed needs under a square of siding and still leaves with two.
Fiber cement lets you move the exposure
Vinyl locks the exposure into the panel, so the rip is unavoidable. Lap siding does not. James Hardie requires a minimum overlap of 1.25 inches between courses, which sets a ceiling on exposure rather than a fixed value: an 8.25 inch plank tops out at 7 inches, a 7.25 inch plank at 6, a 12 inch plank at 10.75. Anything below the ceiling is allowed, it just uses more material.
That turns the layout into a choice. Switch on the adjustable exposure and the calculator looks for the course count whose exposure sits inside your range and lands closest to the one you wanted. The same 108 inch wall in 8.25 inch plank comes out at 16 even courses of 6.75 inches, a quarter inch tighter than the maximum and with nothing ripped at the top.
The trim is bought by the foot
None of the accessories are priced by the square, and forgetting them is the usual reason for a second trip. Starter strip runs the foot of every wall. Utility trim closes the top course under the eave, so it follows the same run. Corner posts are the number of outside corners times the wall height. J channel goes around every window and door, which means four sides of each opening, not two.
On a 30 by 40 house with nine foot walls, two doors and ten windows, that is 140 feet of starter, 140 feet of utility trim, 36 feet of corner post and 200 feet of J channel. The J channel is the one that surprises people.
How much to add for waste
CertainTeed suggests adding 10 percent to siding, soffit and accessories when estimating a large project, and that is a sensible default. A plain rectangle with few openings runs nearer 5 percent. Gables, dormers and a lot of windows push it higher, because every cut leaves an offcut too short to use somewhere else. Deducting your windows lowers the material but not the labor, which is exactly what the waste allowance is there to absorb.
Siding calculator FAQ
Divide the sided height by the exposure and round up. A nine foot wall is 108 inches, so Double 5 at five inches a course takes 22 courses. The calculator works this out from the wall height and the exposure, and tells you how much of the top course will show.
Because the wall height almost never divides evenly by the exposure. A 108 inch wall in Double 5 takes 21 full courses of five inches and leaves three, so the top course is ripped to three inches. Vinyl locks the exposure, so the only choice is where the rip lands. Fiber cement and lap siding set a maximum rather than a fixed figure, so the courses can be closed up until they divide evenly.
Add up every wall face and multiply by the sided height. A plain 30 by 40 house is 140 feet of run at nine feet, or 1,260 square feet. A gable is a triangle, so work it out on its own as half the base times the height and add that in. Then take off the doors and windows.
Seven inches is the maximum. James Hardie requires a minimum overlap of 1.25 inches between courses, and 8.25 less 1.25 is 7. You can go tighter to make the courses come out even on the wall height, which uses slightly more material but removes the ripped course at the top.
All of the trim is bought by the linear foot, off the shape of the building rather than its area. Starter strip runs the foot of every wall and utility trim closes the top course, so both follow the wall run. Corner posts are the outside corners times the wall height. J channel is all four sides of every opening. On a 30 by 40 house with nine foot walls, two doors and ten windows, that is 140, 140, 36 and 200 feet.