Clapboard and Lap Board Siding Calculator

A clapboard siding calculator returns the courses and stock boards a lap siding wall needs. It divides wall height by exposure, which is board width minus overlap, to count courses. It then multiplies the course count by wall length, adds waste, and divides by stock board length, rounding up.

Last reviewed September 2026Method & sources Report an error

Not to scale.

Stock Boards to Buy

Includes a 10% waste allowance.

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Lap siding boards28 boards

Breakdown
Stock Boards to Buy	28
Courses (Rows) Needed	20
Total Linear Feet (with waste)	440 ft

How to Calculate Clapboard and Lap Board Siding

Each course of lap siding overlaps the one below it, so only part of the board's width is actually visible. That visible portion, the exposure, is board width minus the overlap amount. Dividing wall height by exposure and rounding up gives the number of courses needed to reach the top of the wall. Each course runs the full wall length, so total linear footage is courses times wall length. A waste allowance is added for cuts and joints. Then the total is divided by the stock board length and rounded up to whole boards.

Example: 20 ft wall, 10 ft tall, 7.25 in. (1x8) boards, 1.25 in. overlap

A 20 ft wall, 10 ft tall, 7.25 in. (1x8) boards, 1.25 in. overlap, 16 ft stock length, 10% waste: 1. Exposure: 7.25 − 1.25 = 6.0 in. 2. Wall height: 10 × 12 = 120 in. 3. Courses needed: 120 ÷ 6.0 = 20 (divides evenly) 4. Raw linear feet: 20 courses × 20 ft = 400 ft 5. With 10% waste: 400 × 1.10 = 440 ft 6. Boards to buy: 440 ÷ 16 = 27.5, rounded up to 28 boards

Clapboard and Lap Board Siding: Compare Scenarios

Scenario	Stock Boards to Buy	Courses (Rows) Needed	Total Linear Feet (with waste)
Small (10×8 ft wall)	11	16	176 ft
Medium (20×10 ft wall)	28	20	440 ft
Large (40×14 ft wall)	77	28	1,232 ft

What Is Clapboard and Lap Board Siding?

Clapboard (also called lap or bevel siding) is horizontal siding installed so each course overlaps the one beneath it, shedding water down and off the wall. This calculator converts a wall's dimensions and a board's width and overlap into the number of courses and stock boards needed, the same exposure-based approach shingle and shake siding also use.

Clapboard and Lap Board Siding: Pros and Cons

Pros

Uses exposure (board width minus overlap) rather than raw board width, correctly reflecting that only part of each course is visible once installed
Converts courses directly to stock boards needed, accounting for both wall length and stock board length in one calculation
Works for either 1x6 or 1x8 board widths with any overlap amount, rather than assuming one fixed reveal

Cons

Doesn't subtract door and window openings from wall length. For a wall with several openings, consider calculating each wall section between openings separately for better accuracy.
Doesn't account for corner boards or trim, which are typically a separate material list item for a lap siding installation
Assumes uniform overlap across all courses. A design with a varying reveal (wider at the bottom, narrower higher up, for example) needs each band calculated separately.

Clapboard and Lap Board Siding: What a Pro Checks

Check your specific siding product's installation instructions for its minimum overlap. 1 to 1.5 in. is common for wood clapboard, but the right figure shifts with the manufacturer's spec and your local wind exposure.
Calculate a wall with several openings section by section instead of one flat length. This estimate doesn't subtract door and window widths on its own.
Budget corner boards and trim as their own line item. They finish a lap siding wall's edges but aren't part of this course-and-board count.

Frequently Asked Questions

How many courses of lap siding do I need?

Divide your wall height by the exposure (board width minus overlap) and round up. A 10 ft wall with 7.25 in. boards and a 1.25 in. overlap (6 in. exposure) needs exactly 20 courses.

What is exposure in lap siding?

Exposure is the portion of each board that's actually visible once installed. It equals the board's width minus however much the course above it overlaps. A wider exposure means fewer, more visible courses. A narrower exposure means more courses with a tighter shadow line.

How much overlap should lap siding have?

1 to 1.5 in. is a commonly used overlap range for traditional wood clapboard, though it can vary by manufacturer specification and local climate/wind exposure. Check your specific product's installation instructions for its recommended minimum overlap.

How many boards do I need to buy for a wall?

Multiply the number of courses by the wall's length to get total linear footage, add a waste allowance, then divide by the stock board length you're buying and round up. A 20 ft × 10 ft wall in the example above needs 28 stock boards at 16 ft each.

Does a wider board mean fewer courses?

Yes, for the same overlap amount. A wider board has more exposure (width minus overlap), so fewer courses are needed to cover the same wall height. Switching from 1x6 to 1x8 boards at the same 1.25 in. overlap increases exposure from 4.25 in. to 6 in., reducing the course count.

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