By Tim King · Updated 16 September 2026

Decking Calculator

Boards, Lineal Metres, Joists and Screws

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Units

Deck length, along the boards (metres)

Deck width, across the boards (metres)

Board width (e.g. 90mm)

Gap between boards (e.g. 5mm)

Board length as sold (e.g. 4.8 m)

Waste allowance (%, 10 if left blank)

Result

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How the Decking Calculator works

This decking calculator gives boards, joists and screws for a deck. Enter the deck size, the board width and the gap you will leave between boards, and the length the boards come in, and the page works out the rows of boards, the lineal metres to order, how many boards that is, a joist count at 450mm centres and a screw estimate. A 10% waste allowance is added unless you change it.

Decking calculator: rows, then lineal metres, then boards

Decking is estimated differently from tiles because boards run in one direction. The deck width across the boards, divided by the board width plus the gap, gives the number of rows. Each row is as long as the deck, so rows × deck length is the total lineal length of board. Add the waste allowance, then divide by the length the boards are sold in and round up to get the number of boards to buy. Timber yards quote and sell by the lineal metre, so the lineal figure is the one to ask for; the board count is what fits on the trailer.

The gap matters more than it looks. A 90mm board with a 5mm gap needs 32 rows for a 3 m wide deck; the same board laid tight needs 34. Hardwood is usually gapped at 4 to 6mm to let it drain and move; composite boards have a gap specified by the manufacturer, often 6mm.

The assumed waste allowance is 10%

Waste on a deck comes from cutting boards to length, from rejecting bowed or split pieces, and from staggering joints so that they do not line up. Ten per cent is the usual figure for a simple rectangle where the deck length is close to a multiple of the board length. If the deck length is awkward — say a 4 m deck built from 4.8 m boards, which wastes 800mm of every board unless you can use the off-cuts elsewhere — raise it to 15 or 20%. Picture-frame borders, angled boards and decks with steps or cut-outs also want 15% or more.

Joists and screws

Joists are estimated at 450mm centres, which is the common spacing for hardwood and most composite decking on a domestic deck; check the board manufacturer's span table, because some composites want 400mm or less and some hardwoods can go to 600mm. The count is one joist per 450mm along the deck length plus one for the end, so the row of boards is supported at both ends. The screw estimate is two screws at every place a board crosses a joist, which is the standard fixing for face-screwed timber decking. Hidden-clip systems for composite boards use one clip per crossing instead, so halve the number for clips.

What is not included

Bearers, posts, footings, stirrups, ledger fixings and stairs depend on the height of the deck and the ground beneath it and are outside this estimate. Oil or stain for a timber deck can be worked out from the deck area row using the coverage rate on the tin. Fixings are sold in boxes of a few hundred, so round the screw figure up to the next box.

Decking boards by deck size

140 mm boards with a 5 mm gap, sold in 5.4 m lengths, with 10% waste.

Boards, joists and screws by deck size

Deck size	Area	Boards	Joists	Screws
2 × 2 m	4.00 m²	6	5	about 140
3 × 2 m	6.00 m²	9	7	about 196
3 × 3 m	9.00 m²	13	7	about 294
4 × 2 m	8.00 m²	12	9	about 252
4 × 3 m	12.00 m²	18	9	about 378
4 × 4 m	16.00 m²	23	9	about 504
5 × 3 m	15.00 m²	22	12	about 504
5 × 4 m	20.00 m²	29	12	about 672
5 × 5 m	25.00 m²	36	12	about 840
6 × 3 m	18.00 m²	26	14	about 588
6 × 4 m	24.00 m²	35	14	about 784
6 × 5 m	30.00 m²	43	14	about 980
6 × 6 m	36.00 m²	52	14	about 1176
7 × 4 m	28.00 m²	40	16	about 896
7 × 5 m	35.00 m²	50	16	about 1120
8 × 4 m	32.00 m²	46	18	about 1008
8 × 5 m	40.00 m²	58	18	about 1260
8 × 6 m	48.00 m²	69	18	about 1512
10 × 4 m	40.00 m²	58	23	about 1288
10 × 5 m	50.00 m²	72	23	about 1610
10 × 6 m	60.00 m²	86	23	about 1932

Decking formula and worked examples

The formula: rows of boards = deck width ÷ (board width + gap), rounded up; lineal metres = rows × deck length × (1 + waste %); boards = lineal ÷ board length, rounded up; joists = deck length ÷ 450 mm + 1; screws = rows × joists × 2.

First example. A 4 m × 3 m deck in 140 mm boards with a 5 mm gap needs 3 ÷ 0.145 = 21 rows. With 10% waste that is 21 × 4 × 1.1 = 92.4 m of board, or 26 lengths at 3.6 m, laid over 9 joists with about 378 screws.

Second example. A 16 ft × 12 ft deck in 5.5 in boards with a ¼ in gap needs 26 rows, 457.6 ft of board with 10% waste, 29 boards at 16 ft, 11 joists at 18 in centres and about 572 screws.

How many deck boards do I need? Work out the rows across the width first, multiply by the length, add waste and divide by the board length. Rounding the rows up, not down, is what stops you running a board short at the edge.

FAQ

How many decking boards do I need for a 4 by 3 metre deck?

With 90mm boards and a 5mm gap, a 3 m width is 32 rows. Boards running the 4 m length give 128 lineal metres before waste, or about 141 m with the default 10%. In 4.8 m lengths that is 30 boards, though a 4 m deck built from 4.8 m boards wastes 800mm each unless the off-cuts are used elsewhere.

What waste allowance does this calculator use?

Ten per cent by default, added to the lineal length before dividing into boards. Raise it to 15 or 20% when the deck length does not divide neatly into the board length, when boards are laid at an angle or with a picture-frame border, or for hardwood where some boards will be rejected for bowing or splits.

What gap should I leave between decking boards?

Four to six millimetres for hardwood and treated pine, so the boards can drain and move with the weather without buckling. Composite boards use the gap the manufacturer specifies, commonly around 6mm. Boards laid tight when wet will open up as they dry; boards laid tight when dry may buckle when wet.

How far apart should deck joists be?

Four hundred and fifty millimetres between centres is the common spacing for hardwood and most composite decking, and it is the spacing this page uses. Some composite boards and thinner timber need 400mm or less, and thicker hardwood can span 600mm. Always check the span table for the board you are buying.

How many screws does a deck need?

Two at every point where a board crosses a joist, which is what this page counts. A 4 by 3 metre deck with 32 rows and 9 joists is around 576 screws. Hidden clip systems use one clip per crossing, so roughly half that number, plus the starter and end clips the system specifies.

Can I use feet and inches?

Yes. Switch to imperial and enter the deck size in feet, including feet and inches such as 12'6", with the board width and gap in inches and the board length in feet. The lineal length is shown in feet and joists are spaced at 18in centres, the imperial equivalent of 450mm.