Decking calculator — boards, joists and screws

Enter the deck dimensions, choose board width, gap and joist spacing — and get the linear footage of decking, number of boards, joists and screws, with waste included. Works in metric and US units.

m
m
mm

4–6 mm works well for drainage and wood movement.

%

10% covers cutting waste and culled boards.

600 mm spacing requires at least 28 mm decking — use 400 mm for thinner boards or diagonal layouts.

Decking incl. waste
176 linear m
≈ 42 boards à 4.2 m
Board rows
32 pcs
Joists
8 pcs
40 linear m à 5 m
Deck screws
≈ 550 pcs
2 screws per joist crossing

Decking is often sold in random lengths of 3.6–4.8 m — plan the joints to land on a joist. Check the joist size against the span table from your supplier.

The calculation is an estimate and does not replace engineering design or the supplier’s instructions. Always check against project documents, product data sheets and applicable codes.

How to work out the decking quantity

Every board row takes up the board width plus the gap: a 5 1/2 in board with a 1/4 in gap makes 5 3/4 inches per row (or 120 + 5 = 125 mm in metric). The number of rows is the deck width divided by that pitch, and the total run is rows × deck length. Add roughly 10% waste — decking is sold in stock lengths, not every offcut is reusable, and a few boards usually get culled for wane, cupping or cracks.

Plan the joints before you drive the first screw: a butt joint must always land centered on a joist, and the deck looks best when joints are staggered between rows instead of lining up in a straight seam across the surface.

The substructure decides the result

A flat, solid joist frame is the difference between a deck that lasts 25 years and one that starts to bounce after five. Joist size, spacing and span belong together: tighter spacing or a deeper joist buys you a longer clear span, but the numbers come from a span table — check your local code or supplier data rather than guessing, especially for elevated decks or heavy point loads like a hot tub.

Give the frame a slight fall away from the house (about 1:100) so water drains off, and consider capping the joist tops with a protective tape or EPDM strip. It shields the top edge from water standing in the screw holes — the most common starting point for rot in an otherwise sound deck.

Frequently asked questions

How many deck boards do I need?
Each board row takes up the board width plus the gap. With a 5 1/2 in board and a 1/4 in gap, each row covers 5 3/4 inches — divide the deck width by that to get the number of rows, then multiply by the deck length for total linear footage. In metric: a 120 mm board with a 5 mm gap covers 125 mm per row. Add about 10% for cutting waste and culled boards.
What joist spacing should I use under decking?
Standard 1-inch or 28 mm decking laid perpendicular to the joists works on 24 in (600 mm) centers, but many builders prefer 16 in (400 mm) for a stiffer feel. Thinner boards, composite decking and diagonal layouts need the tighter spacing — 16 in or 400 mm on center. Always check the decking manufacturer’s span recommendation.
How big should the gap between deck boards be?
Plan for 1/8–1/4 in (4–6 mm). The gap lets rainwater drain and gives the wood room to move with moisture — fresh pressure-treated lumber often shrinks a little as it dries out. Use spacers so the gaps stay even across the whole deck.
What screws should I use for pressure-treated decking?
Use stainless steel screws or screws with a coating rated for treated lumber — the preservatives in pressure-treated wood are corrosive and will eat ordinary zinc-plated screws. Plan on 2 screws per board at every joist crossing; the calculator counts them for you and rounds up to whole boxes.
How far can deck joists span?
It depends on the joist size, spacing, and wood species — a 2×6 on 16 in centers typically spans around 8–9 ft, a 2×8 around 11–12 ft. Rules of thumb are no substitute for a span table: always check your local code tables or the supplier’s data, especially for elevated decks or point loads like a hot tub.

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