Deck Board Calculator

What this deck board calculator does

Decking is sold by the board, but a deck is measured in feet, and the conversion is where orders go wrong. Three details drive the count: how wide the boards really are, how much gap you leave between them, and which board length you buy.

This deck board calculator takes all three. Enter the deck length and width, say which way the boards run, and choose the board width and length. It returns the number of boards, the leftover after cutting, the joists the boards cross, and an estimate of fasteners. It reuses offcuts where they fit, compares every common board length, and draws the boards with the joists beneath. It works for pressure-treated lumber, cedar, and composite, because the arithmetic is the same. If you need the deck area first, use the square footage calculator. For the footings, see the concrete bag calculator and the concrete yard calculator.

How to calculate deck boards

You can check the count by hand in three steps:

1. Count the rows. Boards stack across the deck. Each board takes up its width plus one gap, so rows = (deck width in inches + gap) ÷ (board width + gap), rounded up.
2. Work out the pieces per row. Divide the length the boards run by the board length. A 14 foot run takes one 16 foot board with 2 feet left over. A 16 foot run on 12 foot boards takes one full board plus a 4 foot piece.
3. Add extra. Bad ends, miscuts, and damaged boards happen, so add 5 to 10 percent and round up to whole boards.

A rough check is deck area ÷ (board width + gap, in feet) ÷ board length, but only the row method shows what happens at the ends of the boards, so the calculator uses it. It also shows the deck area in square feet so you can see what you are covering.

How wide are deck boards?

Lumber is sold by nominal size, which is larger than the actual size after milling. The table shows the common profiles.

Composite and PVC boards are engineered, so each brand sets its own width and thickness. Many are about 5.5 inches wide, which is why the calculator defaults to 5.5. Use the actual width from the product data, not the nominal size, and choose the custom option if your board differs. The method is the same for wood and composite, including Trex, as long as you enter the actual width and follow that maker’s gap rules.

Gap between boards

The gap changes the row count more than most people expect. On a 12 foot wide deck of 5.5 inch boards, a 3/16 inch gap gives 26 rows, while boards laid tight with no gap would need 27. A deck board spacing calculator that ignores the gap overcounts, which is why the gap is part of every row calculation here.

The right gap depends on the material and the weather when you install. Fully dry wood boards are commonly installed with about 1/8 inch between them, while wet or green boards are often laid close together because they shrink as they dry. Composite and PVC boards expand and contract with temperature, so manufacturers give gaps by product, often about 3/16 to 1/4 inch, and some give a table by installation temperature. The default here is 3/16 inch. Read your board’s installation guide, such as the Deckorators grooved decking guide, and enter the gap it specifies. Boards also need an end gap where two boards meet, which this calculator does not subtract.

Choosing a board length

Deck boards usually come in 8, 10, 12, 16, and 20 foot lengths, and the length changes both the board count and the waste. The chart under the calculator counts the boards at each length for your deck, reusing offcuts where a piece fits, and marks the length that leaves the least behind.

Here is how it plays out. When the boards run 16 feet, 16 foot boards fit exactly. On 12 foot boards, every row needs one full board plus a 4 foot piece, and each 12 foot board yields three 4 foot pieces. A 12 × 16 foot deck therefore takes 26 full boards plus 9 for the short pieces, 35 in all, against 26 on 16 foot boards. Fewer joints is also a practical benefit, since every board end has to land on a joist. Long boards are harder to carry and can cost more per foot, so use the chart as a guide and ask your supplier about price and availability.

Which way should the boards run? Boards run across the joists, so the direction you pick sets the joist direction. Running boards along the longer side usually gives fewer end joints, but decide it together with the frame design. The calculator lets you pick either.

One caution: reusing offcuts assumes a piece can be used wherever it fits. Real end joints have to land on joists and are usually staggered. For a safer count, set the calculator to not reuse offcuts, and it assumes each row needs its own pieces.

Picture-frame borders and diagonals

A picture-frame border runs a row of boards around the edge of the deck, usually with mitered corners, to hide the board ends. The calculator takes one board width plus a gap off each side, counts the field boards inside, and counts the border pieces on each side separately. Borders need extra framing to support the mitered ends, so plan the substructure with them in mind. Corner cuts waste some board, which the extra allowance covers.

Diagonal layouts, with boards at 45 degrees, use more material because every board is cut at an angle at both ends. For a diagonal layout, the calculator switches to a footage method: it divides the total board length needed by the board length and adds your extra allowance, which it sets to 15 percent. Many composite makers also call for closer joist spacing on diagonal boards, so check the installation guide.

Joists and fasteners

Boards span from joist to joist, and the spacing is set by the board, the layout, and your local code. Many 5/4 and composite boards need joists no more than 16 inches apart on center when laid straight, and some makers call for 12 inches on center when boards run at 45 degrees. Manufacturers such as TimberTech publish joist spacing guidance by product and layout. Always follow the specification for your own board.

The calculator estimates the joist count as the length the boards run divided by the spacing, plus one. It estimates fasteners from the places where a board crosses a joist: the deck area divided by the joist spacing and the board width plus gap gives the crossings, which it multiplies by two for face screws or one for hidden clips, then adds 5 percent. At 16 inch spacing that is roughly 3 to 3.5 screws per square foot of deck. These are ordering estimates, not a structural design. The joists, beams, ledger, footings, railings, and stairs all need to meet your local code, which often means a permit and an inspection.

Deck board cost

If you enter a price per board, the calculator multiplies it by the boards to buy, which gives the cost of the decking surface only. A whole deck also needs framing, footings, fasteners, railings, stairs, stain or sealer, permits, and labor. Board prices vary a great deal by material, length, and region, so no figure is built in. Ask your supplier for the price of the exact board and length you plan to use.

Worked examples

These use the calculator’s defaults: a 5.5 inch board, a 3/16 inch gap, 16 inch joist spacing, and 10 percent extra for straight layouts.

Example 1: a 12 × 16 foot deck, boards along the length

Rows: (144 + 0.1875) ÷ (5.5 + 0.1875) = 25.35, so 26 rows. With 16 foot boards each row is exactly one board, so you need 26, and with 10 percent extra, 29. That is 464 feet of board with 48 feet left over, about 10 percent. On 12 foot boards, each row needs a 12 foot board and a 4 foot piece, so 35 boards, or 39 with extra. Joists at 16 inches on center: 192 ÷ 16 + 1 = 13. Fasteners: 192 square feet gives about 304 joist crossings, so about 639 screws or 320 hidden clips.

Example 2: a 10 × 14 foot deck with a picture-frame border

The boards run along 14 feet, with a border one board wide. After the border, the field is about 13.05 feet by 9.05 feet, which is 20 rows. On 16 foot boards, each field row and each border side needs a board of its own, because the 2 to 3 feet left on each is too short to reuse, so you buy 27 boards and have about 123 feet left over. On 12 foot boards you buy 30 and have about 51 feet left over, so the shorter boards waste less even though you buy more of them. A plain area division would not show that.

Example 3: a 12 × 12 foot deck laid on the diagonal

The deck area is 144 square feet, so the board length needed is 144 ÷ (5.6875 ÷ 12) = 303.8 feet. With 15 percent extra that is 349.4 feet, which is 29.1 boards of 12 feet, so buy 30. Joists at 12 inches on center for the diagonal layout: 144 ÷ 12 + 1 = 13.

Common mistakes to avoid

• Using the nominal width. A 2 × 6 or 5/4 × 6 board is 5.5 inches wide, not 6, and the difference adds up across the deck.
• Ignoring the gap. It changes the row count, and the rows decide the board count.
• Buying one length by habit. The wrong length can double the waste, so compare lengths.
• Counting offcuts that cannot be used. Short pieces and board ends that miss a joist do not help.
• Forgetting the border and diagonals. They need more boards and extra framing.
• Mixing units. Entering meters while the unit is set to feet changes the answer by a factor of about three. Select the unit first.

What the calculator assumes

The calculator runs in your browser, so your measurements are not sent anywhere. It converts the deck to feet, counts the rows from the board width and gap, works out the pieces per row, applies your extra allowance, and rounds up to whole boards. Joists and fasteners come from the spacing you pick. It assumes:

• A rectangular deck with boards parallel to one edge, or diagonal if you choose that layout.
• The board width, gap, and board length are the values you enter. The calculator does not know your product.
• Offcuts are reused where they fit if you leave that option on. Real layouts must put board ends on joists, so use the no-reuse setting for a safer count.
• Board end gaps, breaker boards, and mitered corner waste beyond the extra allowance are not modeled.
• The joist count and fastener figures are estimates for ordering, not a design for your framing.
• The cost covers decking boards only, at the price you enter.

Frequently asked questions

Sources and further reading

The row and board math is basic geometry. For joist spacing by product and layout, see TimberTech’s deck joist spacing guide. For gap and fastener instructions for grooved boards, see the Deckorators installation guide. Gaps, joist spacing, and fasteners differ between products, so follow the instructions for the board you buy and check your local building code for the frame. See also our Disclaimer.