How much shiplap do you need?
Enter the walls, pick the board, get linear feet and the number of boards to buy.
A ceiling is a surface with no doors or windows. Each door takes off 21 ft², each window 15 ft².
What one board adds to the wall: its overall width minus the lap, plus any gap that shows. Not the nominal width.
10% is the allowance in the WWPA coverage table and on Pioneer Millworks' spec sheet.
How much shiplap do I need?
Take the wall area minus the doors and windows, divide by the width one board covers, and add extra for cuts. A 12 × 8 ft wall with one window is 96 − 15 = 81 square feet. With 1×6 shiplap that covers 4¾ inches per board, 81 × 12 ÷ 4.75 = 204.6 linear feet; with 10% extra that is 225.1 feet, or 29 boards of 8 feet (28.1 rounded up). Use the table of lengths below the answer to compare 6, 8, 10, 12 and 16 foot boards, and the linear feet calculator to convert between square feet and linear feet.
Which width do I use: nominal, actual or face?
None of those: use the coverage, the distance one board adds to the wall after the overlap. Montana Timber Products' cedar specification lists a 6-inch nominal board 5¼ inches wide overall; with its ½ inch lap the board covers 4¾ inches, and the same coverage holds whether or not the joint is left open as a reveal. Pioneer Millworks lists a 5 inch face for its Douglas fir shiplap, and Edge's Timeless 1×6 board is 5.375 inches wide with a 4.625 inch exposed face and a ⅛ inch nickel gap. Mills differ by a quarter inch or more, and a quarter inch on a 4¾ inch board is about 5% more boards, so ask your mill for the coverage and use the custom option if it is not in the list.
How much extra shiplap should I buy?
The Western Wood Products Association adds 10% to the square footage for trim and waste before using its coverage factors, and notes that the factors do not apply to diagonal installations. Pioneer Millworks asks for at least 10% extra material on its sheet for grading, trimming and installation. The calculator starts at 10%; raise it for a layout with many cuts, short boards or a diagonal pattern, where more of each board ends up in the scrap.
Do I subtract doors and windows?
Yes. The calculator takes 21 square feet off for each door (3 × 7 ft) and 15 square feet for each window (3 × 5 ft), the same figures as our paint calculator, and never more than the surface itself. Count a double door or a big picture window as two, or split the wall into separate surfaces around an odd opening. A ceiling is a surface with no doors or windows.
How many rows will horizontal boards take?
Divide the wall height by the coverage and round up. An 8 foot wall is 96 inches, and 96 ÷ 4.75 = 20.2, so 20 full rows and a 21st that is only 1 inch wide; the table shows this for each surface. That last row has to be ripped to fit, so work out the row layout before you start cutting.
Sources: Montana Timber Products, “Standard Siding Profile Specifications for Western Red Cedar” (June 5, 2025); Pioneer Millworks, Vertical Grain Shiplap spec sheet (April 2025); Western Wood Products Association, “Standard Patterns” (Coverage Estimator); Edge, Timeless Collection FAQ. Converting board feet and lengths: linear feet calculator; vertical slats: board and batten calculator; finishing: paint calculator; the wall behind: drywall calculator. How we check our calculators.