How much decking material should I buy?

People type this into search every day: decking calculator how many boards.

The usual answer is the deck’s area. Area is the surface you walk on. The order is a count of rows, a gap between those rows, and a waste allowance for cuts. Framing, joist size, and the ledger are a different drawing.

This page is a board-count estimate. It is not a structural design. Have the framing checked before you build. The calculator says the same thing.

Rows across the width, then waste along the run

The decking material calculator assumes the boards run along the length you type.

rows = ceiling of (width in centimetres ÷ (board face + gap))

linear metres = rows × length × (1 + waste)

Fasteners are ceiling of (rows × length ÷ 0.4 × 2). That is two fasteners every 0.4 m along each row. 0.4 m is 15.7 in, close to a 16 in spacing. The formula uses the row count and the length. The waste metres do not get a second set of screws.

On a US display, linear metres become feet and the surface becomes square feet. The inputs on the card stay in metres and centimetres.

The American Wood Council’s DCA 6 deck guide, written to the 2015 IRC, says to space decking boards approximately 1/8 in apart, and to attach decking to each joist with two 8d threaded nails or two #8 screws. Joist spacing in its Table 2 is 12, 16, or 24 in on center, and the allowable span depends on species and joist size. A 1/8 in gap is about 0.32 cm. The calculator’s default gap is 0.5 cm, about 0.2 in. Example 6 shows a width where that difference changes the row count. DCA 6 is a prescriptive guide for wood decks. Local approval still applies. The 12% waste figure is a planning allowance you type.

The board feet post is the volume of the framing lumber, which this tool does not count. The concrete post is the piers or the slab under the posts. The flooring post is the indoor cousin: area, then waste, then packs.

Example 1: the published 6 m by 4 m deck

Here are the calculator defaults:

• length: 6 m (the board run)
• width: 4 m
• board face: 14 cm, about 5.5 in
• gap: 0.5 cm
• waste: 12%

Module: 14 + 0.5 = 14.5 cm

Rows: ceiling of 400 ÷ 14.5 = ceiling of 27.59 = 28

Before waste: 28 × 6 = 168 m, which displays as 551.2 ft

With 12% waste: 168 × 1.12 = 188.16 m, which displays as 617.3 ft

Surface: 6 × 4 = 24 m², which displays as 258.3 ft²

Fasteners: ceiling of 28 × 6 ÷ 0.4 × 2 = 840

The waste is 20.16 m, displayed as 617.3 − 551.2 = 66.1 ft. It is 66.1 ÷ 617.3 = 10.7% of the order you are shown, because 12% was applied to the 168 m and the feet are rounded. The other 89.3% is the 28 full rows.

28 × 14.5 cm = 406 cm, which is 6 cm past the 4 m width. The formula counts a gap on every row, including the last. Dropping the last gap is a field adjustment the tool does not make.

Example 2: a 2.4 m by 1.2 m landing

• length: 2.4 m
• width: 1.2 m
• board face: 14 cm
• gap: 0.5 cm
• waste: 10%

Rows: ceiling of 120 ÷ 14.5 = ceiling of 8.28 = 9

Before waste: 9 × 2.4 = 21.6 m

With 10% waste: 21.6 × 1.10 = 23.76 m, displayed as 78 ft

Surface: 2.88 m², displayed as 31 ft²

Fasteners: 9 × 2.4 ÷ 0.4 × 2 = 108

Nine rows on a 1.2 m width looks high until you do the division. 8 × 14.5 cm = 116 cm, which is 4 cm short of 120 cm, so the ceiling adds a ninth row. That ninth row is most of why a small landing does not scale down from the 6 m deck in a straight line.

Example 3: an 8 m by 5 m deck at 15% waste

• length: 8 m
• width: 5 m
• board face: 14 cm
• gap: 0.5 cm
• waste: 15%

Rows: ceiling of 500 ÷ 14.5 = ceiling of 34.48 = 35

Before waste: 35 × 8 = 280 m

With 15% waste: 280 × 1.15 = 322 m, displayed as 1,056.4 ft

Surface: 40 m², displayed as 430.6 ft²

Fasteners: 35 × 8 ÷ 0.4 × 2 = 1,400

The waste here is 42 m, about 137.8 ft, on top of the 280 m of rows. Fasteners stay at 1,400 either way. If you buy the offcuts, the tool does not add screws for them.

Example 4: what if I change only the waste

Keep the 6 m by 4 m deck, the 14 cm face, and the 0.5 cm gap. The row count stays 28. Fasteners stay 840. Only the linear order moves.

From 0% to the 12% default you add 66.1 ft. From 12% to 20% you add another 661.4 − 617.3 = 44.1 ft. The surface stays 258.3 ft² through every row of this table. Waste is not area. It is extra length for cuts, ends, and a board you scrap.

Example 5: what if I change the board face

Keep 6 m by 4 m, a 0.5 cm gap, and 12% waste.

The surface is 258.3 ft² in all three. A narrower face needs more rows to cross 4 m, so you buy more linear board and more fasteners. Going from the 14 cm default to 9 cm adds 948 − 617.3 = 330.7 ft and 1290 − 840 = 450 fasteners. Going to 18 cm saves 617.3 − 485 = 132.3 ft and 180 fasteners.

Check the 9 cm row if you want the working. Module: 9.5 cm. Rows: ceiling of 400 ÷ 9.5 = ceiling of 42.11 = 43. Linear: 43 × 6 × 1.12 = 288.96 m, displayed as 948 ft. Fasteners: 43 × 6 ÷ 0.4 × 2 = 1,290.

Example 6: the gap, and a width that lands on a round number

Two widths, same 6 m run, 14 cm face, 12% waste.

On the 4 m default, a 0.5 cm gap and a 0.32 cm gap (about 1/8 in, the DCA 6 spacing) both round to 28 rows and 617.3 ft. 400 ÷ 14.32 = 27.93, and 400 ÷ 14.5 = 27.59. Both ceilings are 28. On this width the gap you type does not change the order.

On a 2.9 m width, 290 cm is exactly 20 modules of 14.5 cm.

• gap 0.5 cm: 20 rows, 440.9 ft, 600 fasteners, surface 187.3 ft²
• gap 0.32 cm: ceiling of 290 ÷ 14.32 = ceiling of 20.25 = 21 rows, 463 ft, 630 fasteners, same surface

The tighter gap needs another row, because 20 × 14.32 cm = 286.4 cm, which is short of 290 cm. A zero gap on this same 2.9 m width also needs 21 rows (290 ÷ 14 = 20.71) and the same 463 ft. Butting the boards, and spacing them at 1/8 in, happen to share a row count here. The 0.5 cm default is the one that lands exactly.

Set the 4 m deck’s gap to 0 and the module shrinks to 14 cm. Rows: ceiling of 400 ÷ 14 = ceiling of 28.57 = 29. Linear: 29 × 6 × 1.12 = 194.88 m, displayed as 639.4 ft. Fasteners: 870. That is one more row than the gapped default, and 639.4 − 617.3 = 22.1 ft more board. A gap can reduce the row count when the wider module still covers the width after the ceiling. Leave the gap the decking product calls for. DCA 6’s approximately 1/8 in is the wood-deck figure cited above. Type the gap you will actually leave.

Example 7: the common mistake, order the area

The default surface is 24 m², or 258.3 ft². Dividing that area by a 0.14 m face, and then adding 12% waste:

24 ÷ 0.14 = 171.43 m before waste

171.43 × 1.12 = 192 m, about 629.9 ft

The tool’s order is 617.3 ft. The area method is about 12.6 ft long on this deck. It skipped two things the formula does on purpose. It ignored the 0.5 cm gap, and it did not round up to a whole row before multiplying by the length.

The other version of the same miss is to skip the ceiling and use the raw 27.59 rows: 27.59 × 6 × 1.12 = 185.38 m, about 608.2 ft. That is 9.1 ft short of 617.3 ft. You cannot buy 0.59 of a row that stops 6 cm before the edge. The ceiling is the 28th row.

Skipping waste entirely is the larger miss. You would order 551.2 ft instead of 617.3 ft and arrive 66.1 ft short of the default allowance.

Example 8: what if the boards run the other way

The tool always treats length as the direction the boards run. To ask about the other direction, swap the two dimensions.

A deck that is 6 m one way and 3.2 m the other, with a 14 cm face, a 0.5 cm gap, and 12% waste:

Boards along the 6 m side (length 6, width 3.2):

Rows: ceiling of 320 ÷ 14.5 = ceiling of 22.07 = 23

Linear: 23 × 6 × 1.12 = 154.56 m, displayed as 507.1 ft

Fasteners: 23 × 6 ÷ 0.4 × 2 = 690

Surface: 206.7 ft²

Boards along the 3.2 m side (length 3.2, width 6):

Rows: ceiling of 600 ÷ 14.5 = ceiling of 41.38 = 42

Linear: 42 × 3.2 × 1.12 = 150.53 m, displayed as 493.9 ft

Fasteners: 42 × 3.2 ÷ 0.4 × 2 = 672

Same surface, 206.7 ft². The short run uses 507.1 − 493.9 = 13.2 ft less board and 690 − 672 = 18 fewer fasteners, and it uses 42 rows instead of 23. On the square-ish 6 m by 4 m default, swapping the dimensions happens to land on the same 617.3 ft and the same 840 fasteners, because 42 × 4 equals 28 × 6 before waste. The 3.2 m side is the one that shows the ceiling changing the order.

Picture frames, breaker boards, and a diagonal lay are outside the formula. DCA 6 allows decking from square to the joists up to 45 degrees, and it wants each piece to bear on at least three supports. A diagonal run is longer than either of these two swaps. This tool does not price it.

The orders next to each other

Open the decking material calculator with the board run in the length box, the face width and the gap you will leave, and a waste percentage you are willing to carry home. Then take the row count to the board feet calculator for the joists, and the fence material calculator if the same project includes a rail. Joist span, ledger bolts, and footings still belong to the structural check, not to this linear total.