What this calculator works out
Building a deck comes down to a handful of shopping-list numbers before anything gets ordered: how many boards, how many joists underneath them, how many screws, and whether to add a coat of oil. This tool takes the deck's length and width, a UK or US board size (or a custom one), an expansion gap and a joist spacing, and turns that into a straight materials list, with the working shown so you can check it against your own numbers.
It covers a UK garden deck built from 3.6m or 4.8m treated softwood board at 400mm joist centres, or an American deck built from 16ft 5/4-by-6 board at 16in on-centre framing, with every measurement carrying its own metric or imperial toggle so a length in metres and a board bought in feet sit side by side without converting anything by hand.
How the maths works
Boards run one direction across the whole deck: side by side over the width, with a "row" of one or more boards joined end to end covering the length. The width sets how many rows fit, since each row takes up the board's face width plus one expansion gap; the length sets how many boards each row needs, since a single board often doesn't reach the full length on its own.
Joists run the other way underneath, at right angles to the boards, spaced along the deck's length. This uses the same boundary logic as the Fence Calculator's post count: a joist at each end of the run, evenly spaced between, so the count is always one more than the number of full spacing intervals that fit.
That "+2" is a stated simplification, not a hidden extra: many builders double up the two outer joists that form the deck's perimeter frame, since those carry the fascia board and the end of every decking board, so this calculator adds two extra pieces on top of the spaced count. Real frames sometimes add noggins or blocking between joists too, particularly on a bigger deck, which isn't counted here.
Screws are worked out as a grid: every row crosses every joist once, and each crossing gets two screws, one either side of the board's centre line, the usual rule of thumb for fixing decking board. The total is then rounded up into whole boxes.
A worked example
Take a 4m by 3m UK deck using the wider 3.6m by 144mm board, a 6mm gap, and 5% wastage. Rows: 3m ÷ (144mm + 6mm) = 20 rows exactly. Boards per row: 4m ÷ 3.6m = 1.11, rounded up to 2 boards joined end to end. That's 20 × 2 = 40 boards before wastage, × 1.05 = 42 boards to buy. At 400mm joist centres, 4m ÷ 0.4 = 10, +1 = 11 joists, +2 for the doubled outer frame = 13 joist pieces, each 3m long (39m of joist timber). Screws: 20 × 11 × 2 = 440, rounded up to 3 boxes of 200. At a rough £13 a board, £9 a joist piece and £9 a screw box, materials come to (42 × 13) + (13 × 9) + (3 × 9) = £690, before oil, frame fixings or delivery.
The American equivalent: a 16ft by 12ft deck using 16ft by 5.5in (5/4-by-6) board and the same 5% wastage. The board length matches the deck length exactly, so each row is a single uncut board: 12ft ÷ (5.5in + a quarter-inch gap) gives 26 rows, × 1 board each = 26, × 1.05 = 28 boards to buy. At 16in (406mm) joist centres, 16ft ÷ 16in = 12, +1 = 13 joists, +2 for the doubled outer frame = 15 pieces, each 12ft long (180ft of joist timber). Screws: 26 × 13 × 2 = 676, rounded up to 4 boxes of 200. At a rough $11 a board, $14 a joist piece and $13 a screw box, materials come to (28 × 11) + (15 × 14) + (4 × 13) = $570, again before oil or delivery.
Choosing a UK or US board size
UK treated softwood decking board is fairly consistent between merchants on length, less so on face width. Wickes sells a 25 × 120mm standard board and a 27 × 144mm "Pro" board, both in 3.6m and 4.8m lengths; Travis Perkins sells the equivalent 26 × 120mm and 32 × 144mm finished sizes. Neither merchant stocks an exact 140mm board, so the wider preset here uses their real 144mm figure rather than round it to a number nobody sells. In the US, decking board is sold by nominal size: a 5/4-by-6 board is about 1in thick by 5.5in wide, and 16ft is a widely stocked length at major suppliers including Home Depot, this calculator's US default. A wider board needs fewer boards overall; a narrower one cups and twists less and gives a finer look.
The expansion gap, and why it's there
Timber decking moves with moisture: it swells when wet and shrinks back as it dries, across its width more than along its length. Leave no gap and a run of boards fitted snugly on a dry day can cup, bow or lift after rain, with nowhere for that extra width to go. UK guidance generally sits in a 5-8mm range for pressure-treated softwood, with the Building Research Establishment's commonly cited minimum for hardwood decking put at 6mm, this calculator's default. Composite decking usually wants less, typically 3-5mm, since it moves less than solid timber; check the fitting guide for the specific product, as warranties can be tied to it.
Joist spacing for timber and composite
400mm centres is the commonly recommended spacing for standard 25-32mm timber decking board laid straight across the joists, tight enough that the board doesn't spring underfoot. Composite and PVC brands typically ask for the same or tighter, often a maximum 16in (406mm) on-centre for a straight layout and closer to 12in (300mm) for a diagonal or herringbone one, since the board crosses fewer joists per run at an angle. In the US, 16in on-centre framing is the standard baseline regardless of decking material. Thicker board (38mm and up) can usually go to wider centres; the safest source for an exact figure is the manufacturer's own span table.
Fixings: how many screws
Two screws per board, at every joist it crosses, is the standard fixing rule, driven a little in from each edge rather than dead centre so the board can still move as it swells and shrinks. This calculator treats every row-and-joist crossing as one of those pairs and rounds the total up to whole boxes, since a part-used box is still useful next time. Budget a little above the raw total: where two board ends meet over a joist, both usually get their own pair, which this grid-based estimate doesn't add on top.
Decking oil, if you want it
Oil isn't compulsory: pressure-treated softwood is already protected against rot without it, and plenty of decks are left bare or weather to a silvery grey. Oiling mainly buys back colour and a water-shedding surface, useful on a deck with heavy foot traffic or a shadier spot. Coverage varies by product and by how thirsty the timber is: data sheets for oil-based products commonly quote 6-10 m²/L per coat on already-sealed wood, but new, untreated softwood on its first coat soaks up more. This calculator defaults to a conservative 5 m²/L per coat across two coats, editable against your own product's data sheet, rounded up to the nearest half-litre.
Questions people ask
How many decking boards do I need?
Divide the deck's width by the board's face width plus the gap, rounded up, for rows; divide the length by the board's own length, rounded up, for boards per row; multiply and add a wastage allowance. A 4m by 3m deck in 3.6m by 144mm board with a 6mm gap comes to 20 rows of 2 boards each, 42 boards with 5% wastage.
How big a gap should I leave between decking boards, and why?
Around 5-8mm for treated softwood, with 6mm a common middle figure and this calculator's default. Timber swells when wet and shrinks as it dries, mostly across its width, so a gap gives it somewhere to move without cupping, bowing or locking the boards up tight. Composite decking usually wants less, typically 3-5mm; check the specific product's fitting guide.
What joist spacing should I use, and is composite decking different?
400mm centres is the usual recommendation for standard timber board laid straight across the joists. Composite and PVC brands often ask for the same 16in (406mm) maximum for a straight layout, tightening to around 12in (300mm) for a diagonal or herringbone pattern, since the board crosses fewer joists per run at an angle. Always check the specific manufacturer's span table for an exact figure.
Do I need to oil my deck?
No, pressure-treated softwood is already protected against rot on its own. Oiling mainly restores colour and sheds water more readily, which matters most on a deck with heavy use or a damp, shaded spot. Left untreated, timber decking weathers to a silvery grey rather than rotting away.
How many screws does decking need?
Two screws per board at every joist it crosses is the standard rule, driven in a little from each edge rather than dead centre. Where two board ends meet over the same joist, both pieces usually get their own pair rather than sharing one, so it's worth buying a little above a strict grid-based total.
Can I lay decking boards in a different direction or pattern?
Yes, diagonal, herringbone and picture-frame border layouts are all common, but they use more board per square metre because of the angled cuts involved, and diagonal layouts need tighter joist spacing for the same stiffness. This calculator assumes the standard straight layout; add extra wastage above 10% for something more elaborate.
Board sizes here are checked against current listings at Wickes and Travis Perkins for the UK, and Home Depot for the US 5/4×6 board. Gap, joist spacing and screw guidance follow commonly published UK and US decking advice, including Building Research Establishment guidance on hardwood decking gaps; oil coverage follows the range quoted across manufacturer data sheets. Real figures vary by supplier, timber and product, so treat this as a planning estimate, round up when buying, and check the manufacturer's span table and fitting guide for a structural build.