Decking is not just square footage divided by a board — the gap between boards and the length of the stock you buy both change the count. This deck board calculator handles that. Enter the deck length in the direction the boards run, the deck width across the boards, the board face width and the gap in inches, the length of board you are buying, and a waste percent. It returns the number of rows across the deck, the total decking in linear feet, and how many boards that is at your chosen length. Boards run the length; rows stack across the width. Enter the deck the way the boards actually lie, and figure a wrapped or multi-level deck one section at a time.
Deck Board Calculator: Boards and Linear Feet
Calculate how many deck boards a deck needs from its length and width, board face width, gap, and board length, with a waste allowance. Returns rows, linear feet, and board count.
Deck Board Calculator
Enter the deck length and width, the board face width and gap in inches, the board length, and a waste percent. Returns rows, total decking in linear feet, and board count.
Rows26 rows
Total decking563.2 linear ft
Deck boards35 16ft boards
Estimates for planning only. Check the result against your plans and a supplier quote before you order.
How to Use the Calculator
Enter the deck length in the direction the deck boards run, and the deck width across them. Enter the board face width — a nominal five-and-a-half-inch board for most composite and five-quarter lumber — and the gap you plan to hold, usually an eighth to a quarter inch. Enter the length of board you will buy, commonly sixteen feet, and a waste percent. The calculator figures how many rows fit across the width, multiplies by the run for total linear feet, adds waste, and divides by board length for a board count. Round the board count up when you order.
The Formula
Each row of decking takes up its face width plus the gap. Rows across the deck equal the deck width in inches divided by that face-plus-gap. Multiply rows by the deck length for the total linear feet of decking, add waste, then divide by the board length for the number of boards. A twenty-by-twelve deck with five-and-a-half-inch boards, an eighth-inch gap, and ten percent waste needs about twenty-six rows, roughly five hundred sixty linear feet, or about thirty-five sixteen-foot boards.
Why the Gap Matters
The gap between boards is small but it adds up across a wide deck. A twelve-foot width at a five-and-a-half-inch face is about twenty-six boards if you butt them tight, but the gap spreads that same coverage over slightly fewer rows, and getting the gap wrong in your estimate throws the row count off. Composite decking usually wants a specified gap for drainage and expansion, and pressure-treated lumber is often laid tight because it shrinks as it dries. Enter the gap you will actually hold on site, and if the manufacturer specifies one for their board, use that number.
Board Length and Waste
Board length changes both waste and seams. A deck longer than your board forces butt joints, and every joint needs to land on a joist and creates offcuts. Buying a board length that divides cleanly into your run cuts waste; buying random lengths increases it. The ten-percent default covers normal cutting and a few bad boards. Bump it to fifteen or twenty percent for a diagonal layout, a picture-frame border, or a deck with lots of angles, because each of those multiplies the cuts. Order to the higher board count so a miscut does not leave a gap in the field.
What This Does Not Include
This calculator sizes the deck boards only. It does not count the fasteners, which for hidden-clip systems run roughly one clip per board per joist, or the joist tape, fascia, stair treads, and railing that finish the job. It also assumes a simple rectangle; a deck with a bump-out, a wrap, or multiple levels should be broken into rectangles and totaled. Size the structural framing and footings separately — decking is the surface, not the frame underneath it. Use this number for the field of the deck and add the trim and hardware on their own lines.
Decking Reference
| Reference | Planning value | Field note |
|---|---|---|
| Rows | width / (face + gap) | In inches |
| Common face | 5.5 in | Nominal 6-in composite or 5/4 board |
| Gap | 1/8 to 1/4 in | Per manufacturer for composite |
| 20x12 deck | ~35 boards | 16-ft boards, 10% waste |
| Waste | 10-20% | Higher for diagonal or picture-frame |
FAQ
How many deck boards do I need? Divide the deck width by the board face plus the gap for the number of rows, multiply rows by the deck length for total linear feet, add waste, and divide by your board length. A twenty-by-twelve deck is about thirty-five sixteen-foot boards with ten percent waste.
What gap should I leave between deck boards? Usually an eighth to a quarter inch. Composite decking has a manufacturer-specified gap for drainage and expansion, so follow it. Pressure-treated lumber is often laid tight because it shrinks as it dries.
How much waste should I add for decking? Ten percent covers a straight layout. Add fifteen to twenty percent for a diagonal pattern, a picture-frame border, or a deck with many angles, because each of those multiplies the cuts and offcuts.
Does this include fasteners and railing? No. It counts the deck boards only. Fasteners, joist tape, fascia, stair treads, and railing are separate, and the framing and footings under the deck are sized on their own.