Clapboard and Lap Siding Calculator

How clapboard coverage is counted

Lap siding, also called clapboard or bevel siding, covers a wall one course at a time, and each course hides part of the board below it. That hidden strip is the overlap, and the strip you can see is the exposure, sometimes called the reveal. The Vinyl Siding Institute installation manual treats the exposure as the number that drives the layout, because the wall is divided into courses by exposure, not by the full width of the board.

The coverage of one board follows a simple relation:

Coverage per board = exposure in inches x board length in feet / 12, and the answer comes out in square feet. A board installed with a 6 inch exposure over a 12 foot length covers 6 x 12 / 12 = 6 square feet. The same physical board at a 7 inch exposure covers 7 x 12 / 12 = 7 square feet, one full square foot more, even though nothing about the board itself changed.

This is the point people trip on: the exposure and the board width are two different numbers. A nominal 1x6 cedar board dresses down to about 5.5 inches at the mill, and after it laps the course below by roughly 1.5 inches, only about 4 inches shows. Ordering by the nominal width instead of the installed exposure is the fastest way to end up short on material, so measure the exposure on the wall or set it by your overlap plan before you count boards.

Worked example: siding a 40 by 10 foot wall

Suppose one side of a garage measures 40 feet wide and 10 feet tall, with a 3 x 7 foot door and one 3 x 4 foot window. Here is the full count, step by step:

• Gross wall area: 40 x 10 = 400 square feet
• Openings: 3 x 7 = 21 square feet, plus 3 x 4 = 12 square feet, so 21 + 12 = 33 square feet
• Net area to cover: 400 - 33 = 367 square feet
• Coverage per board at a 6 inch exposure and 12 foot length: 6 x 12 / 12 = 6 square feet
• Boards before waste: 367 / 6 = 61.17 boards
• With 10 percent waste: 61.17 x 1.10 = 67.28, so order 68 boards

Courses matter too. A 10 foot wall is 120 inches tall, and at a 6 inch exposure that is 120 / 6 = 20 courses. If the exposure drops to 5 inches, the count becomes 120 / 5 = 24 courses, and each board covers only 5 x 12 / 12 = 5 square feet. The tighter layout takes 367 / 5 = 73.4 boards before waste, about 20 percent more material for the same wall, which is exactly why the exposure number belongs in your estimate before you shop.

Exposure, overlap and board width: three numbers, one plan

The three numbers are locked together by one relation:

Board width = exposure + overlap. Work it in either direction: if you bought 5.5 inch actual-width cedar and want a 1.5 inch overlap, the exposure is 5.5 - 1.5 = 4.0 inches. If your layout needs a 4 inch exposure for a clean course count, the same board laps 1.5 inches. Manufacturer guidance, from the VSI manual and fiber cement makers like James Hardie, puts typical overlap for horizontal lap siding at 1 to 1.5 inches on wood clapboard, while vinyl panels lap about 1 inch along their length and lock courses by profile instead of a nail-on overlap.

The last column gives square feet per 12 foot board at the smaller and larger exposure, since exposure in inches is square feet per 12 foot board. A 1x6 with a 1.5 inch overlap covers 4.0 x 12 / 12 = 4.0 square feet per board, and with a 1.0 inch overlap the exposure is 4.5 inches, so it covers 4.5 x 12 / 12 = 4.5 square feet. Wider exposure means fewer courses and fewer boards, but the manufacturer sets the minimum overlap for weather protection, so never stretch exposure by shrinking overlap below spec.

Deducting doors and windows the right way

Openings reduce the board count, but not as much as their full area, because you still cut and fit boards around every window and door. A common field adjustment is to deduct only 70 to 80 percent of the opening area for small windows, where offcuts rarely line up with anything useful. On the worked example, deducting the full 33 square feet gave 68 boards; deducting only 75 percent of it removes 33 x 0.75 = 24.75 square feet instead, so the net area is 400 - 24.75 = 375.25 square feet and the count is 375.25 / 6 x 1.10 = 68.79, still 69 boards.

The bigger the openings, the more this matters. A wall of glass with 120 square feet of openings in a 400 square foot wall behaves differently from one small window. Measuring each opening and writing it into the estimate takes ten minutes, and it is the difference between a comfortable material order and a second trip to the supplier. Doors near a corner also generate long offcuts you can reuse on plain walls, which is one reason a modest 10 percent waste factor is realistic for straightforward elevations and 15 percent is wiser where openings are many or boards must be color-matched at joints.

How many boards come in a square and a carton

Siding is sold by the square, which is 100 square feet of coverage, and vinyl lap is boxed in cartons that hold 2 squares. With a 6 inch exposure, one square needs 100 / 6 = 16.67 linear courses of 12 foot boards, which works out to about 17 boards per square. A 2 square carton therefore covers 2 x 100 = 200 square feet of wall at that exposure, which is about 34 boards' worth.

The carton math matters because vinyl exposure is fixed by the panel profile. A D4 double-4 inch panel is locked to a 4 inch exposure, a D5 panel to 5 inches, and a Dutch lap or single 7 inch panel to its own reveal. You cannot choose a tighter exposure the way you can with wood clapboard, so the only lever on a vinyl job is the wall area and the waste factor. Wood and fiber cement are the opposite: the same 1x6 or 8.25 inch plank can be laid at whatever exposure the weather and the design allow, within the manufacturer's minimum overlap, which is why clapboard estimates always start from the exposure you plan to install.

Waste factors and cut planning

A 10 percent waste allowance is the standard starting point for a simple rectangular wall with boards longer than the wall, because the offcut from one course starts the next. Stretch that to 15 percent when walls have many openings, hips, bays or short courses that break over windows, and toward 20 percent on complex elevations or when boards arrive in random lengths that must be blended.

Two habits keep waste honest. First, lay out courses before cutting: snap a chalk line at the starter course and check that a full-height run of courses lands cleanly at the window heads and the frieze board, adjusting the starter strip by a half inch if needed. Second, stagger butt joints at least one stud bay between courses, which the VSI manual calls for on vinyl and good practice demands on wood, and use the longest offcuts where they will be hidden behind downspouts and trim. Neither habit changes the formula, but both decide whether the 68 boards from the worked example are enough or whether the last course is a patchwork.

From boards to a full material list

Boards are only the field. A complete clapboard order also carries the pieces that frame it:

• Starter strip, one length per foot of wall base, which tilts the first course to match the lap angle
• Corner boards or corner posts, one pair per outside corner, cut to full height
• J-channel or casing for every window and door, roughly twice the opening perimeter
• Frieze board at the soffit line and water table where a story breaks
• Fasteners: ring-shank or galvanized nails for wood, or the manufacturer-specified nail for fiber cement, spaced up to 16 inches apart on studs

Estimate fasteners by course count. Twenty courses on a 40 foot wall at 16 inch stud spacing is 40 x 12 / 16 = 30 stud positions per course, so 20 x 30 = 600 nails for the field, plus trim. The calculator on this page gives you the board count and courses in one pass; add the trim pieces from the actual tape-measure lengths of each edge and the order is complete.

Limits and good practice

This estimate assumes flat, rectangular walls measured at the face, no gables, and no bump-outs. Gables add triangular areas you can estimate as width x rise / 2, and each one should carry its own waste allowance. Tall single-course runs waste more than the standard factor because full boards must be cut at every opening sill and head.

Also keep exposure inside the manufacturer's limits. Wood clapboard manufacturers publish minimum overlap for weather resistance, vinyl is fixed by profile, and fiber cement planks have a maximum exposure printed on the installation sheet, commonly 7 inches for a 8.25 inch plank. Going beyond those limits risks wind-driven rain behind the wall, and no quantity of saved boards is worth that. When the numbers here and the product sheet disagree, the product sheet wins; use the calculator for quantity and budget, and the installation manual for the exposure you are allowed to set.

All calculations run in your browser and nothing you type is uploaded. Confirm the exposure limits with the siding manufacturer before ordering.