Decking Boards Calculator — Boards, Joists, Beams & Posts

Order too few decking boards and the job stops a row short of the edge. Order too many and you have paid for a pile of timber to store. The counts in this guide — boards, joists, beams, posts and screws — all come from a handful of short formulas, and this post works through each one in plain language, with a full example at the end. If you would rather skip the arithmetic, our deck calculator runs the same maths for you. And if you are budgeting at the same time, read this alongside our guide to how much it costs to build a deck, which prices the quantities this post counts.

The board maths: a board covers its width plus the gap

Here is the one idea everything else rests on: a board does not cover its own width. It covers its width plus the gap beside it. A 5.5 in board laid with a 1/4 in gap covers 5.75 in of deck width, because the space next to the board is part of the pattern that spans the deck. Forget the gap and every row lands a quarter inch early, the error grows row by row, and the count comes up short at the far edge — as well as leaving no room for drainage or for timber to expand.

The formula is short: boards = deck width ÷ coverage width, rounded up. Work in one unit: a 12 ft deck width is 144 in, and 144 ÷ 5.75 is just over 25, so you need 26 board-rows to cross it. Then check the other direction: the board length must cover the length of the deck, or you will be joining boards end to end and should count the extra pieces that takes. Finally, add waste for cuts, defects and the board you measure twice and still cut once, wrong: allow 10% for a straight layout and 15% for a diagonal one — the same defaults as the site’s deck calculator uses.

Joists: spacing first, then the count

Joists are the parallel timbers under the boards, and their spacing is set by what the boards can span without feeling spongy. The standard is 16 in on centre — measured from the centre of one joist to the centre of the next — for most timber boards laid straight. Use 12 in on centre for diagonal layouts, because a board crossing at 45 degrees spans further between joists, and for many composite boards, whose warranties often insist on 12 in. Check the maker of your boards before you build the frame, not after.

The count follows the fence-post rule: joists = deck length ÷ spacing + 1. The plus one closes the far end of the frame. For a deck whose joists are spaced along a 16 ft length, that is 192 in ÷ 16 in = 12 spaces, plus 1, giving 13 joists. If the length does not divide neatly by the spacing, round the number of spaces up first, then add the end joist.

Beams and posts: the structure underneath

Joists do not reach the ground; they rest on beams. Beams run under the joists, at right angles to them, and are commonly built as doubled 2×8 or 2×10 timbers bolted together, so the pair acts as one deep beam. How many beams you need, and how far apart they can sit, depends on the joist span between them — short spans may need one beam at the outer edge with the house ledger carrying the inner ends, while deep decks need a beam in the middle too.

Beams rest on posts. Posts are 4×4 at a minimum, and 6×6 is the better choice for tall decks, where a slender post looks and feels underdone. Space posts roughly every 6–8 ft along each beam, and stand every post on its own concrete footing, dug below the frost line so freezing ground cannot lift it. Exact spans and spacings depend on lumber species, grade and your local code, so use span tables for the final layout. This post gives quantities for estimating, NOT a structural design.

Screws: two at every crossing

Face-fixed boards take about 2 screws at every board–joist crossing, one into each side of the board. That makes the screw count a simple product: boards × joists × 2. A board crossing 13 joists takes 26 screws, and the total climbs quickly into the hundreds, which is why screws are sold by the tub, not the handful. If your boards fix with hidden fasteners instead, read the screw count as the number of fixing points and buy clip packs to the manufacturer’s stated coverage.

Worked example: a 16 × 12 ft deck

Take a 16 × 12 ft deck, with 5.5 in boards laid along the 12 ft direction, a 1/4 in gap between boards, and joists at 16 in on centre. Working through the formulas above — board coverage with the gap included, joists at length ÷ spacing + 1, screws at two per crossing, and fascia boards around the exposed edges — the order comes out as:

• Boards: 39 boards of 12 ft.
• Joists: 13 joists.
• Screws: 39 boards × 13 joists × 2 screws = about 1,014 screws.
• Fascia boards: 5 fascia boards to face the outside edges.

Those are the numbers to take to the supplier for this deck — then run your own dimensions through the deck calculator, because a different width, board length, gap or pattern changes every one of them.

UK and AU terminology: the same maths in millimetres

In the UK, decking boards are typically 120–145 mm wide, in softwood or composite, and Australian boards are often narrower still. Nothing about the method changes: the maths is identical in millimetres. A 140 mm board with a 5 mm gap covers 145 mm of deck width, a 3,600 mm wide deck needs 3,600 ÷ 145, rounded up, in board-rows, and joist spacings of 400 mm and 300 mm play the roles of 16 in and 12 in. Measure your actual board, add your actual gap, and divide — in any unit you like, as long as it is the same unit throughout.

Frequently asked questions