Deck Board Calculator

About the Deck Board Calculator

Decking is sold by the board, but a deck is measured in rows. Converting one into the other is where most ordering mistakes happen: a deck that is 14 feet wide does not take 14 feet divided by the board width, because every board also has a small gap beside it for drainage. This calculator handles that conversion. Enter the deck length and width, the length and width of the boards you plan to buy, the gap you will leave between them, and a waste percentage, and it returns the number of rows, the total linear footage of decking, and the number of boards to purchase.

The math is done in two stages. First it works out how many rows fit across the deck by dividing the deck width by board width plus gap, and it always rounds that row count up, because the last row is a real board even if it has to be ripped narrower to fit. Then it multiplies the rows by the deck length to get linear feet, divides by the length of one board, adds your waste allowance, and rounds up again. Because both steps round up, the answer errs on the side of having enough material.

Getting the count close matters because decking is one of the larger material costs in an outdoor project, and boards from different production runs can vary slightly in color and profile, especially with composites. Buying everything in one order, plus a small reserve, helps the finished surface look uniform. Homeowners, contractors pricing a bid, and anyone comparing board lengths or widths for a design will find the number a useful starting point before talking to a supplier.

When to Use It

How to Use It, Step by Step

1. Choose your unit system. U.S. mode uses feet for the deck and board length and inches for the board width and gap, while Metric uses meters for lengths and centimeters for width and gap.
2. Enter the Deck length as the dimension the boards will run along. This calculator assumes each board runs the full length of the deck, so if your boards run the other way, swap the length and width entries.
3. Enter the Deck width as the dimension across the rows, measured to the outside edge of the finished surface, including any overhang past the framing.
4. Set the Board length to the stock length you actually plan to buy. Common choices are 8, 12, 16, and 20 ft, and picking a length that matches or exceeds the deck length reduces cuts and joints.
5. Enter the Board width as the actual width of the board, not the nominal size. A board sold as a 1 by 6 or a 5/4 by 6 is normally about 5.5 inches wide.
6. Fill in the Gap between boards. Check the board maker's guidance, since the right spacing depends on the material, how dry it is at installation, and whether you use hidden fasteners with built-in spacing.
7. Adjust the Waste factor. The default 10 percent suits a straight layout, while diagonal patterns, picture-frame borders, and decks with many angles need more.
8. Calculate and read the results, then add stairs, fascia, rail posts, and fasteners separately, because the tool counts only the deck surface boards.

Understanding Your Results

Rows across deck width tells you how many boards sit side by side from one edge to the other. A 14 ft wide deck using 5.5 inch boards and a 3/16 inch gap works out to 30 rows, since 168 inches divided by 5.6875 is 29.5 and the count rounds up. If your last row will be very narrow, say under an inch or two, consider adjusting the first board's position or ripping both outer boards to be equal widths for a balanced look.

Linear feet of decking is the rows times the deck length, which is the total run of board you need before waste. Boards to buy divides that by your board length and then multiplies by the waste factor. The two are different views of the same plan: linear footage helps when a supplier prices boards per foot, while the board count is what you take to the lumber yard. Both are rounded to whole numbers.

A quick sanity check is to compare the deck area with the board area. A 20 by 14 ft deck is 280 sq ft, and a 16 ft board of 5.5 inches covers about 7.3 sq ft, which suggests a count in the high thirties before the waste allowance. Anything wildly different usually means a unit slip, such as entering board width in feet. If the figure looks right, round up your purchase for the sake of future repairs.

More Worked Examples

Tips for Better Results

• Choose a board length that equals or slightly exceeds the deck length wherever you can. The calculator treats offcuts as reusable, but a 2 ft leftover is of no use on a run that needs 10 ft, so matching length avoids hidden waste.
• Measure the actual width of your chosen board with a tape before you enter it. Many boards are sold with a nominal size name, and the real width is often a half inch smaller than the name suggests.
• Ask the supplier about the gap for your specific material. Pressure-treated boards that are still wet from the yard shrink as they dry, so installers usually place them nearly tight, while dry or kiln-dried boards and most composites need a wider planned gap.
• Set the first and last rows by dry-laying boards before fastening. Adjusting the starting overhang or ripping the two outer boards to equal widths avoids a thin sliver along one edge.
• Raise the waste factor for diagonal layouts and picture-frame borders. Angle cuts leave triangular offcuts that are hard to reuse, and many installers allow noticeably more than the straight-lay default for those patterns.
• Buy one or two extra boards beyond your calculated total and keep them from the same batch. Matching a color or profile years later can be difficult, especially with weathered wood or discontinued composite lines.

Common Mistakes to Avoid

• Entering the nominal board size, such as 6 inches, instead of the real width of about 5.5 inches. The error adds up over many rows and can leave you a row short.
• Forgetting that boards run the length you entered. If your boards will run across the short dimension, swap length and width before you calculate.
• Ignoring joist layout. Board ends have to meet over a joist, so a plan that leaves a board end hanging between joists needs extra framing or a different board length.
• Using a zero gap for every material. Even if boards will shrink or expand, a planned gap keeps water draining, so follow the product's installation instructions.
• Leaving stairs, risers, fascia, and picture-frame borders out of the budget. Those parts need separate board quantities that this calculator does not include.

Assumptions and Limits

• The tool works from linear footage, so it assumes every offcut is usable. In practice, butt joints must land on joists and some pieces are too short to use, so real waste can be higher than the percentage you enter.
• Rows are computed from deck width alone, with no deduction for stairs, cutouts around posts, or a picture-frame border, all of which change the board count.
• Gap, board width, and board length are treated as exact. Real boards vary a little in width and length, and composites can expand and contract with temperature, so follow the manufacturer's spacing guidance.
• This is a materials estimate, not a structural design. Joist size, span, footing depth, ledger attachment, and railing requirements are governed by local codes, so confirm those details with your building department or a qualified contractor.

Key Terms

Decking surface

The finished top layer of boards you walk on, as opposed to the joists and beams underneath that support it.

Joist

A horizontal framing member beneath the deck boards. Boards are fastened to joists, and board ends should meet over one.

Nominal size

The rounded name a lumber product is sold under, such as 1 by 6. Actual dimensions are smaller, so always enter the real measurement.

Butt joint

The place where two board ends meet end to end in a row. It should be centered on a joist and staggered from neighboring rows.

Picture frame

A border of boards laid around the perimeter, usually perpendicular or mitered at the corners, which adds a finished edge but uses extra material.

Rip

To cut a board lengthwise to make it narrower, usually to fit the last row to the exact remaining space.