Home Material Calculator

Deck calculator

Enter the deck length and width, the way the boards run, and the board width, gap and length. You get the number of deck boards to buy, rounded up, plus the linear feet, rows and joists.

Last updated

Boards run along
The boards

5.5 in is a starting value. Measure your board or use the width its maker gives.

1/8 in is 0.125. Use the gap your board maker gives, or 0 for boards laid tight.

The spacing your boards allow depends on the board maker and your local code.

You need about

Enter the deck length and width. The result updates as you type.

Estimate only. Check the count with your lumber yard or deck builder before you buy.

For cuts, end trims and boards you reject. 10% is a planning estimate, not a published figure. Diagonal layouts cut more, so allow more.

Add a price to estimate cost

Estimates only. Board widths, gaps and joist spacing vary by maker. Check the count with your supplier, and follow the board maker's instructions and your local code for joist spacing and spans.

How many deck boards do I need?

Measure the side of the deck the boards run across. Divide it by the board width plus one gap, and round up: that is the number of rows. Multiply the rows by the length of each row to get the linear feet of board. Add an allowance for cuts, divide by the board length, and round up, because boards are sold whole.

A 12 x 10 ft deck with the boards running along the 12 ft side, in 5.5 in boards with a 1/8 in gap, needs 22 rows and 264 linear feet. In 12 ft boards that is 22 boards before waste, and 25 boards with a 10 percent allowance.

The formula

  1. Rows: the side the boards run across, in inches, divided by (board width + gap), rounded up to a whole row.
  2. Linear feet: rows x the length of each row.
  3. Boards: linear feet plus the waste allowance, divided by the board length, rounded up.
  4. Joists: joists run across the boards. The run length in inches divided by the joist spacing, rounded up, plus one, so there is a joist at each end and none are further apart than the spacing you picked.
  5. Deck area: length x width, in square feet.

Mixed units are fine. The calculator converts every measurement to feet first, using 1 in = 2.54 cm exactly.

Worked example: a 12 x 10 ft deck

The boards run along the 12 ft side, so the rows stack up across the 10 ft side. The boards are 5.5 in wide with a 1/8 in (0.125 in) gap, sold in 12 ft lengths, and the joists are 16 in on center.

  1. One board with its gap: 5.5 + 0.125 = 5.625 in.
  2. Rows: 10 ft is 120 in, and 120 ÷ 5.625 = 21.3, rounded up to 22 rows.
  3. Linear feet: 22 x 12 = 264 ft.
  4. Boards: 264 ÷ 12 = 22 boards with no allowance. With 10 percent, 264 x 1.10 = 290.4 ft, and 290.4 ÷ 12 = 24.2, rounded up to 25 boards.
  5. Joists: 12 ft is 144 in, and 144 ÷ 16 = 9, plus one = 10 joists, each spanning the 10 ft side.

Turn the boards the other way, along the 10 ft side, and the rows stack across 144 in instead: 144 ÷ 5.625 = 25.6, so 26 rows and 260 linear feet. The joists then sit along the 12 ft side: 120 ÷ 16 = 7.5, rounded up to 8, plus one = 9 joists.

Which way should the boards run?

Deck boards usually run across the joists, so the direction you pick sets both counts. Running them along the longer side means fewer, longer rows and fewer joists across the short side. Running them along the shorter side means more rows and more joists. Pick the direction your plan shows; the calculator works out both counts for either.

Board length and offcuts

The calculator adds up the total length of board and divides by the board length once, so it assumes offcuts are used in later rows. That works when your rows are longer than a board and the boards are joined end to end over a joist. When a row is shorter than a board, the offcut is often too short to reuse. In that case pick a board length close to the row length, such as 10 ft boards for 10 ft rows, or add more to the waste allowance.

Board width and gap

The calculator starts at a 5.5 in face width and a 1/8 in gap. Those are starting values you can change, not recommendations. Measure your board across its face, and use the gap the board maker gives for your material. Enter 0 for boards laid tight.

Joist spacing

The joist spacing menu offers 12, 16 and 24 in on center. The spacing your boards allow, and how far each joist can span, depend on the board maker and your local building code. This calculator counts joists. It does not size them or check spans.

Waste allowance

The calculator starts at 10 percent. That is a planning estimate, not a published figure. Diagonal and picture-frame layouts, decks with many corners, and short rows that leave unusable offcuts need more. Ask your supplier or builder for the figure for your design.

What the calculator leaves out

Common mistakes

Sources

Questions

How do I work out how many deck boards I need?

Divide the side the boards run across by the board width plus one gap, and round up to get the rows. Multiply the rows by the row length for the linear feet, add your waste allowance, divide by the board length and round up. A 12 x 10 ft deck in 5.5 in boards with a 1/8 in gap, running along the 12 ft side, is 22 rows and 264 linear feet: 22 boards of 12 ft, or 25 with 10 percent extra.

How do I calculate linear feet of decking?

Multiply the number of rows by the length of each row. Rows are the side the boards run across divided by the board width plus the gap, rounded up. For 22 rows of 12 ft, that is 264 linear feet.

How many joists do I need for a deck?

Joists run across the boards. Divide the length the boards run, in inches, by the joist spacing, round up, and add one so there is a joist at each end. A deck with 12 ft boards at 16 in on center needs 144 ÷ 16 = 9, plus one, so 10 joists. The spacing your boards allow and how far a joist can span depend on the board maker and your local code.

How much gap should I leave between deck boards?

It depends on the board material and the maker. Use the gap in the board maker's instructions. The calculator starts at 1/8 in, which is a starting value you can change, not a recommendation, and accepts 0 for boards laid tight.

How much extra decking should I buy?

The calculator starts at 10 percent, which is a planning estimate, not a published figure. Diagonal layouts, picture-frame borders and short rows that leave unusable offcuts need more. Use your supplier's or builder's figure if you have one.