Deck board calculator: how many boards, and how to cut them?

How many courses does a deck take?

A course is one row of boards across the deck. Divide the width by the board width plus the gap and round up.

Take a 16 ft by 12 ft deck with the boards running along the 16 ft side. The width is 144 in, and 144 / (5.5 + 0.1875) is 25.32, so it takes 26 courses. The last course is ripped to 1-13/16 in. Joists at 16 in on center over 16 ft number 13, one at each end.

With 16 ft boards, each course is one board: 26 boards, 416 ft of decking.

What if the deck is longer than your boards?

Then boards have to be joined end to end, and the joint has to land on a joist so both ends can be fastened. The page plans every splice for you.

On a 20 ft by 12 ft deck with only 12 ft boards, the first course takes a 12 ft board and 8 ft from a second board, spliced on joist 10. The next course starts with the 4 ft left over from that second board, so offcuts go back into the deck before a new board is cut. A new board comes first only when an offcut would line the splices up. No two courses in a row splice on the same joist where another joist is in reach, and every piece reaches at least three joists where the run allows. That deck takes 44 boards and 35 splices.

The plan follows fixed rules, so the same deck always gives the same plan.

Check the count by hand

Here's the sum the page does, so you can check it with a pencil.

Courses: the deck width in inches divided by the board width plus the gap, rounded up. Joists: the run in inches divided by the spacing, rounded up, plus one. Boards: the planned boards of each length, times 1 plus the waste, rounded up. Fasteners: courses times joists, times 2 for screws or 1 for clips, plus the same again at each splice.

Take a 20 ft by 12 ft deck at the page's defaults: 5.5 in boards, a 3/16 in gap, 12 and 16 ft boards, joists at 16 in, 5 percent waste.

• Courses: 144 / (5.5 + 0.1875) = 25.32, so 26.
• Last course: 144 minus 25 x 5.6875 = 1.8125 in, ripped to 1-13/16 in.
• Joists: 240 / 16 = 15, plus 1 = 16.
• Decking laid: 26 courses x 20 ft = 520 ft. With 16 ft boards that's 520 / 16 = 32.5, so 33 boards and 8 ft of offcut. The page shows this 16 ft plan because it buys fewer boards than the mix.
• Waste: 33 x 1.05 = 34.65, so you buy 35.
• Screws: 26 x 16 = 416 crossings, x 2 = 832, plus 26 splices x 2 = 52, so 884.

The page prints the same lines under Show your work. If yours differ, check that the width is the side the boards cross.

Which board lengths should you buy?

Tick every length your yard stocks. The page plans your mix, then plans each length on its own, and shows whichever of those buys the fewest boards. On a tie it shows the one with less decking. Every option stays in a table so you can weigh them against your prices.

For the 20 ft by 12 ft deck with 8, 12 and 16 ft ticked, before waste:

Boards	Count	Length
Your mix (26 of 16 ft, 13 of 8 ft)	39	520 ft
16 ft only	33	528 ft
12 ft only	44	528 ft
8 ft only	65	520 ft

Here 16 ft boards alone buy fewer boards, so that's the plan the page shows. The mix buys 8 ft less decking, and the table keeps it in view.

Composite boards mostly come in fewer lengths. deckstore's calculator says most composite boards come in 12, 16 or 20 ft lengths. Tick those three on the same 20 ft by 12 ft deck and the page shows 26 boards of 20 ft, 28 with waste, no splices and 832 screws. With 16 ft boards alone it's 35 boards, 26 splices and 884 screws. Price both before you pick.

How do you count an L-shaped deck?

Split it into rectangles and add each one as a section. The page plans every section in the same run, so boards, waste and fasteners come out as one order.

Take a 16 ft by 12 ft deck with a 10 ft by 8 ft step-out, boards running the long way on both, 12 and 16 ft ticked.

• Section 1, 16 x 12: 26 courses, 13 joists, 26 boards of 16 ft.
• Section 2, 10 x 8: 96 / 5.6875 = 16.88, so 17 courses, the last ripped to 5 in. Joists: 120 / 16 = 7.5, so 8, plus 1 = 9. Each course takes a 12 ft board cut to 10 ft.
• Total: 272 sq ft, 43 boards planned (17 of 12 ft, 26 of 16 ft), 34 ft of offcut.
• Screws: 26 x 13 + 17 x 9 = 491 crossings, so 982.

With 16 ft boards alone it's also 43 boards, but 688 ft of decking against 620 ft. When two plans need the same number of boards, the page shows the one with less decking, which here is your mix.

How many screws or clips?

Every course crosses every joist. With face screws, that's 2 per board per joist, plus 2 more at each splice for the second board end. Hidden clips take 1 and 1.

Those are our starting figures, not sourced ones. On the 16 ft by 12 ft deck, 26 courses x 13 joists is 338 crossings, so 676 face screws or 338 clips. The page prints that arithmetic with the count, so you can check it against your fastener maker's schedule.

How do waste and packs turn into what you buy?

Waste goes on the boards of each length, and each length is rounded up on its own. A yard sells you 12 ft and 16 ft boards separately, so a total rounded once can come out short of one length.

On the L-shaped deck above at 5 percent: 17 x 1.05 = 17.85, so 18 boards of 12 ft. 26 x 1.05 = 27.30, so 28 of 16 ft. You buy 46. Our 5 percent is a starting figure, not a sourced one; for picking your own, see how much waste to add.

Fasteners go by the box. Type the count per box into Fasteners per box and the page divides and rounds up. TimberTech sells its CONCEALoc hand-driven clips in a pack marked 100 sq ft that holds 175 clips. On the 16 ft by 12 ft deck at 16 in, the page counts 338 clips: 338 / 175 = 1.93, so 2 packs. At 12 in the same deck takes 442 clips: 442 / 175 = 2.53, so 3 packs. At 12 in, a pack count read off the square feet would leave you short: the 192 sq ft deck reads as 2 packs of 100 sq ft, but two packs hold 350 clips, 92 fewer than the 442 needed. At 16 in, two packs are enough. Count clips, not square feet. The page doesn't add waste to fasteners; add a box if you expect to drop a few.

What about an angled deck?

Frame it tighter. TimberTech's joist guide says horizontal boards go at 16 in on center and diagonal boards at 12 in.

This page lays boards straight, so it won't count diagonal boards. It will count what tighter framing costs. Set joist spacing to 12 in on the 16 ft by 12 ft deck:

• Joists: 192 / 12 = 16, plus 1 = 17, up from 13.
• Face screws: 26 x 17 = 442 crossings, so 884, up from 676.
• Clips: 442, 3 packs of 175 instead of 2.
• With Include framing ticked: 17 joists, each 12 ft 0 in, and 32 ft of rim.

The board count it prints is for straight boards. A diagonal layout needs more, and how much more depends on the angle and your offcuts.

How much concrete for deck footings?

Enter the number of footings and the hole size from your own post and beam plan. The page fills each round hole to full depth. Nine footings in 10 in by 42 in holes take 1.91 cu ft each, 17.18 cu ft in all, or 29 bags of 80 lb. The count and size come from you, because they depend on your beams, spans and local code.

Before you buy

Measure the framing, not the plan. Run a tape across the joists at both ends of the deck; if the two widths differ, enter the larger one, since that sets the course count. Then check the gap and joist spacing on the install sheet for the exact board you're buying, and change the page to match before you order.

What this decking calculator doesn't do

It lays boards along the length or the width, not on a diagonal. It doesn't size beams, posts or footings, recommend a joist spacing or board span, or count blocking. Include framing counts joists and rim only. For a slab or anything but footings, use the concrete calculator. It doesn't handle stairs, railings, fascia or picture-frame borders, and it doesn't look up prices, though you can type in your own. It doesn't add waste to screws or clips.

Deck board calculator FAQ

Related tools

For a slab, use the concrete calculator. More material calculators are on the home projects page.