Clapboard and Lap Board Siding Calculator

Find the number of clapboards needed using our siding board calculator. The calculator will also determine the overlap and exposure of each board, find the total number of boards needed, and is perfect for estimating cedar wood siding or Hardie fiber lap siding. Optionally enter the number of doors and windows to refine your estimate.

Get the dimensions for a wall by measuring the length and height
Get the dimensions for a gable by measuring the width of the gable and the height of the peak

Results:



How to Estimate Siding Boards Needed

Find the number of siding clapboards needed using a few simple calculations

Cedar wood siding and Hardie fiber board siding is commonly available in a clapboard or lap board style and is often sold by the 12′ board. The boards overlap each other and are only partially exposed to the elements to keep the building water-tight. Estimate the number of boards needed for a project in a few simple steps.

Find the Overlap and Exposure of the Board

Board siding is installed from the bottom up, with each board overlapping each other by a small margin. Different materials and styles of siding have varying requirements for the minimum amount of overlap needed to keep the wall water-tight and maximize the durability of the siding. Often, the minimum amount of overlap is 1″ – 1 1/2“.

The exposure of the board is the amount of the board that is exposed to the elements. This can be found by subtracting the overlap of the boards from the actual board width. The exposure is what will be used to determine the number of courses needed for the wall, and ultimately to find the number of linear feet of siding needed.

Clapboard and lap siding is installed with one board overlapping the one below it.

Cedar Board Dimensions

Common cedar board sizes and their maximum exposure.
Nominal Actual Exposure
4″ 3.5″ 2″
6″ 5.5″ 4″
8″ 7.5″ 6″
10″ 9.5″ 8″
12″ 11.5″ 10″

Hardie Fiber Board Dimensions

Common Hardie board sizes and their maximum exposure.
Nominal Exposure
5.25″ 4″
6.25″ 5″
7.25″ 6″
8.25″ 7″
9.25″ 8″
12″ 10.75″
These dimensions are for average styles and materials only, refer to the siding manufacturer for exact specifications.

Estimate the Number of Courses of Siding

The number of courses of siding can be found by dividing the height of the wall in inches by the exposure of the board. Use our feet to inches conversion calculator if you know the height of the building in feet. Siding should be kept 6″ off the ground, account for that when measuring and estimating material.

For example, installing siding with a 4″ exposure on a wall that is 12ft high will take 36 courses of siding.

12′ × 12″ per ft = 144″
144 ÷ 4 = 36

It’s likely that a partial course will be needed if the exposure of the board does not divide evenly into the height of the building. In that case, re-calculate the exposure slightly so that each course is even and the top course does not end up being too short.

For example, installing siding with a 7″ exposure on a wall that is 12ft high will take 20.6 courses of siding. Adjust the exposure of the board to be 6.85″ to end up with an even 21 courses.

12′ × 12″ per ft = 144″
144 ÷ 7 = 20.6
144 ÷ 20.6 = 6.85

Estimate Linear Feet

Estimate the number of linear feet of siding needed by multiplying the width of the wall in feet by the number of courses. Boards are most commonly sold in 12′ lengths, though 16′ and 20′ lengths can often be ordered. Divide the linear footage by 12′ to find the number of boards needed to cover the wall.

For example, installing 21 courses of siding on a 20′ wide wall will require 420 linear feet of siding, or 35 boards.

21 × 20 = 420
420 ÷ 12 = 35

Learn more about how to calculate linear feet.

Estimating Gable Walls

Finding the number of boards needed for a gable is fairly simple. Simply multiply the height and width of the gable and estimate the linear footage like you would for a rectangle using the width and height. Divide in half to get the linear footage needed to cover a the triangular area.

For example, installing 21 courses of siding on a 20′ wide gable will require 210 linear feet of siding, or 18 boards.

21 × 20 = 420
420 ÷ 2 = 210
210 ÷ 12 = 17.5 – round up to 18

Consider ordering additional material for offcuts needed for the angle.

More Siding Installation Resources

Use our siding squares calculator to estimate vinyl siding square material needed. Our board and batten calculator can estimate board and batten siding materials. Learn more about the cost of siding installation in your area and find qualified installers near you.

More Construction Calculators