Deck Calculator

Rows, then the boards you actually carry home

Rows: how many times the board pitch fits across the depth. Boards: how many pieces of the length your yard sells it takes to cut those rows. The default deck is 16 ft along the house by 12 ft out from it, boards 5.5 in wide with a 0.1875 in gap, so the pitch is 5.6875 in and the depth plus one gap is 144.1875 in. That division gives 25.352: 26 rows, the last one ripped narrow.

Each row is 16 ft long, so the deck wants 416 lineal feet of decking — still not an order. At 16 ft stock every row is one board, 26; at 8 ft stock every row takes two, 52; on a square 8 x 8 ft deck the 17 rows pair up two to a board, 9.

The gap belongs to your board, not to a calculator

Gapping is quoted here as what it is, one manufacturer's instruction for its own boards: the minimum required width-to-width gapping is 3/16" (5 mm), 3/8 in in heavily wooded areas, no gapping should ever exceed 1/2" (Trex, 2026 Decking Installation Guide). The 3/16 in minimum gap, the 3/8 in for heavily wooded areas and the 1/2 in ceiling are Trex's, for Trex composite boards, and they are shown with the product name. No open source was found for the gap of wood decking: ask your supplier. Closed tight at zero, the default deck takes 27 rows instead of 26.

The width in the form is a face width, not a nominal name. It opens on 5.5 in because that is the minimum dressed dry width of a nominal 6 in NIST PS 20-20, Table 3, where a nominal 4 reads 3.5 in. Those are standard minimums for softwood lumber, not the size of a product: a composite board carries its own width on its own sheet, and typing it in is the point of the field.

Forty-five degrees is geometry, not a percentage

Turn the boards to 45 degrees and each course becomes a chord across the rectangle, every piece carrying one extra board width because the cuts are measured long point to long point. That is 42 courses and 424.3 lineal feet here, against 26 rows and 416 ft straight — while the net is unchanged, 192 sq ft over a 5.6875 in pitch being 405.1 lineal feet either way. A 45 degree layout is computed, not surcharged. Net lineal footage is the same as a straight layout (deck area over the board pitch); what actually changes is the end triangles - one extra board width on every course - the longest piece, and how badly the pieces fit your stock length. All three are counted here, so no extra waste percentage is invented.

The longest piece hurts more than the footage. Straight, nothing exceeds 16 ft; at 45 degrees the longest course here is 17.43 ft, over a 16 ft board, so the sheet warns and the packing splits it: 27 boards at 16 ft, or 22 at 20 ft with nothing jointed.

Of the five pages we could read, one offers a 90 / 45 degree selector and tells its reader a diagonal layout adds another 10 to 15 % of waste. That is its sentence, not a figure this sheet uses: the triangles, the longest piece and the stock-length mismatch are counted.

Pieces, not lineal feet

Lineal feet divided by a board length is not a board count. The sheet packs the cut list longest first into the stock you chose, and a piece longer than the stock becomes whole boards plus a remainder. A 20 ft deck ordered in 12 ft boards takes 52 of them, each row a board and an offcut. The cut list is a length budget, not a joint layout. Pieces are packed into the stock length you chose, longest first; the tool does not place butt joints over joists, and they have to land on one.

Waste starts at zero. No open source for a decking waste percentage was found, and the percentages on the pages we read are those pages' own defaults, so this sheet asks for your number instead of printing one.

The frame under the boards

Joists are that division turned ninety degrees: length over spacing, rounded up, plus one for the joist at the far end. 192 in over 16 gives 13 joists of 12 ft, 156 lineal feet; at 24 in on center it is 9, at 12 in 17. Those three spacings are the column heads of the joist table in DCA 6, offered as choices only. No span is calculated and none is printed. Joist, beam, post-height and footing-size tables belong to DCA 6 and are linked, never reproduced. DCA 6 is a prescriptive guide based on the IRC: your local code governs.

An attached deck takes a ledger and one rim joist, 16 ft each; freestanding drops the ledger and takes a rim on both sides; with no ledger to hang from, picking it with a single beam line raises a question rather than a number. Posts are beam lines times posts per line, and every post gets a footing. Two per beam is the geometric minimum to hold one up, not a recommendation: DCA 6 sets post size at 6x6 nominal minimum with its height in Table 4, and its beam span table sets how far apart they stand.

Screws follow the crossings, not the boards: 26 rows over 13 joists is 338 crossings, 676 screws at the Trex figure of two per joist. At 24 in on center, 234 crossings and 468 screws; a 45 degree course meets a joist less often, so the diagonal default drops to 233 and 466. Hidden clips are not counted for you: the crossing count is printed, your clip's box gives the rest.

Footings, and the concrete that goes in them

There is no default footing diameter here, deliberately: size comes from the footing table in DCA 6, from your plans and from your local code, so the concrete line reads enter a footing diameter until you type one. Depth opens at 12 in because DCA 6 asks for footings bearing on undisturbed soil at least 12 inches below the undisturbed ground surface or below the frost line, whichever is deeper.

With a diameter typed in, a footing is a cylinder: pi over four times diameter squared times depth. 2 footings 12 in across and 12 in deep are 1.571 cu ft of concrete, 3 bags at the 0.6 cu ft QUIKRETE publishes for an 80 lb bag of Concrete Mix No. 1101; the other two choices are its 60 lb bag and the 50 lb Fast-Setting Mix No. 1004. Round holes only: for a square footing or a pad, use the concrete calculator.

Guards, stairs, and what this sheet will not do

Type a height above grade and a guard line appears only where DCA 6 requires one: all decks greater than 30" above grade are required to have a guard. The run is the perimeter that needs protecting — length plus two depths when the house closes one side, twice the length plus twice the depth when it does not — less the stair opening you type. At 36 in high with a 36 in opening, the default deck needs 37 ft. Its posts are 4x4 nominal at minimum in DCA 6; pickets and rails are not counted, and below 30 in no line is printed at all.

Blocking, hangers, fasteners other than the deck screws, stairs and railing parts are not counted. The takeoff exports to CSV and prints, and both drawings redraw from what you type.

What this sheet does that the ones we read do not

Six results ranked for this search when it was audited; the top one never rendered its widget for us, so every claim here is about the five pages we could read. None of the five puts decking, joists, rim and ledger, beams, posts and footing concrete in one takeoff — the closest sends its reader to a separate beam calculator and a separate concrete calculator, and one lists joists and posts in its FAQ without a form we could read. None makes the gap an editable field driving the row count. None computes a 45 degree layout as geometry with the longest piece and the packing behind it. None redraws the plan from the inputs or exports a CSV. Metric units, a stock-length selector, a waste field and a screw count exist on those pages: they are not claimed here as new.

Rows      n  =  ceil( ( D + g ) / p )        p = board width + gap
Courses   n  =  ceil( ( L + D ) / sqrt2 / p )      45 degrees
Piece k      =  sqrt2 x min( u, min(L,D), L+D-u ) + board width
Boards       =  pack the pieces into your stock length, longest first
Joists       =  ceil( L / spacing ) + 1,  each D long
Screws       =  crossings x your screws per crossing
Footing      =  pi / 4 x dia^2 x depth,  bags = ceil( vol / yield )

Frequently asked

How do I calculate how many decking boards I need?

Divide the depth by the pitch — board face width plus gap — and round up: that is the rows, each the full length of the deck. The default 16 x 12 ft deck is 144 in over 5.6875 in, 25.35, so 26 rows. Then turn rows into purchases: 26 boards at 16 ft stock, 52 at 8 ft.

How many 6 deck boards do I need?

A nominal 6 is 5.5 in wide at minimum dressed dry size (PS 20-20, Table 3), so with the 0.1875 in gap the rows land every 5.6875 in: 26 rows and 26 boards at 16 ft on the default deck. In nominal 4 boards, 3.5 in wide, the same deck takes 40 rows and 40 boards.

How much material do I need to build a 12x12 deck?

Straight boards, 5.5 in wide, 12 ft stock: 26 deck boards, 10 joists of 12 ft, a ledger and a rim of 12 ft, one beam line, 2 posts on 2 footings, 520 screws. Footing concrete waits for a diameter.

How much is material for a 20x20 deck?

As quantities, which is all this sheet does: 43 deck boards at 20 ft, 16 joists of 20 ft, 6 posts and footings under two beam lines, 1376 screws. Prices are yours: the price field multiplies your figure by the board count and nothing else.

What is the 3/4/5 rule for decking?

Pythagoras, used to square a frame: mark 3 units along one side and 4 along the other, and the corner is square when the diagonal between the marks is 5, because 9 + 16 = 25.

Limits of this sheet

A-301 counts material on a rectangle. It does not size a joist, a beam, a post or a footing, check a ledger connection, design a guard or price a deck. Angled and wrapped decks, picture framing, multi-level platforms and hidden-fastener yields are out of scope, and stairs are not counted. For planning and ordering: your plans and your local code govern.

Sheet A-301 prepared by TakeoffMetric Editorial; no author, credential or field measurement is claimed.