Deck Calculator

How to use the deck calculator

Start by settling the direction. The length you enter is the way the boards run, and the width is the direction rows accumulate across. That matters because joists run perpendicular to the decking, so the same rectangle gives different joist counts depending on which way round you build it.

Then use the actual board width rather than the name on the label. Softwood decking sold as 6 inch is 5 1/2 inches wide, and 4 inch is 3 1/2 inches. Plan a 12 ft span on 6 inch boards and you are half an inch short on every row, which compounds into a missing row by the end.

Why the gap lowers the count rather than raising it is worth a second. Widening the gap makes each row cover more span, so fewer rows reach the far side. It only ever pushes the count down. A 12 ft span takes 27 rows butted tight and 26 at a quarter inch gap, and a 20 ft span drops from 44 to 42.

The gap is also doing a real job, not just arithmetic. Boards need somewhere for water to drain and room to move with the seasons, and the right size depends on the material and on how wet the boards are when you lay them. Your decking manufacturer publishes a spacing for its product, and that instruction beats any general figure.

Joist spacing is a choice with consequences in three directions at once. Tighter spacing means more joists, more screws and a stiffer deck. Over a 16 ft length the count runs 17 joists at 12 inch centres, 13 at 16 inch, and 9 at 24 inch. What spacing you are allowed to use is a different question, answered by code and by the decking manufacturer.

Read the screw count as a shopping estimate rather than a fastening schedule. Two screws per board per joist is common practice for face-screwed decking, and this tool adds a second pair wherever a butt joint puts two board ends on the same joist. Hidden fastener systems and clip systems count completely differently.

Why there are no span tables here

You will not find a joist span, a beam span or a post size on this page, and that is a deliberate refusal rather than an oversight. A span table is not a general fact. It is a function of species, grade, the actual dimension of the member, the spacing, whether the joist is continuous or cantilevered, and the load the deck has to carry.

Change any one of those and the allowable span changes with it. Southern pine and hem fir of the same nominal size are not interchangeable, a number two board is not a select structural board, and a deck that carries a hot tub is not carrying a residential live load. A table copied without its assumptions is worse than no table at all.

The consequence of getting it wrong is also different in kind from the rest of this page. Order too few boards and you drive back to the yard. Undersize a beam and the failure mode is people falling. Deck collapses are not hypothetical, and they tend to happen when the deck is full.

• Ask your building department which code edition they have adopted and whether a permit is required.
• Ask your lumber supplier for the species and grade of what you are actually buying.
• Get the span figures from the adopted code or from a design professional, for that species and grade.
• Follow the decking manufacturer instructions for joist spacing, gap and fasteners.
• Treat ledger attachment and footings as structural work, not as trim.

How board width changes the whole order

Narrower boards are more work in every direction, and the numbers make that concrete. Cover the same 12 ft span in 3 1/2 inch boards instead of 5 1/2 inch ones and the row count goes from 26 to 39, the linear feet from 416 to 624, and the screw count from 728 to 1,092.

That is half as many rows again, half as much material again, and half as many screws again, for exactly the same deck. Narrow boards buy you a finer look and, on some layouts, a better fit to the space, but the labour is where the cost really lands.

The reverse holds too. Wider boards cover span faster and drive fewer fasteners, which is part of why 5 1/2 inch decking became the default. The trade off is that a wide board cups and moves more than a narrow one, which is exactly what the gap between boards is there to absorb.

The formula, worked line by line

Counting rows is a fencepost problem wearing decking. Lay N boards across a span and you get N boards but only N minus 1 gaps, because the last board has nothing after it to be spaced away from. Ignore that and the arithmetic slowly overcounts on every wide deck.

Solve it properly and the tidy form falls out. You need N times the board width plus N minus 1 times the gap to reach the span, which rearranges to the span plus one gap, divided by the board plus one gap, rounded up.

Boards then come from rows rather than from linear feet. Decking butt joints have to land on a joist, so an offcut left over from one row cannot generally be used to finish another. Each row is counted on its own, and its last piece is a partial board you still buy whole.

rows = (span in inches + gap) ÷ (board width + gap), rounded UP
linear feet = rows × board run length
boards per row = run length ÷ stock length, rounded UP
boards = rows × boards per row
joists = (run length in inches ÷ joist spacing, rounded UP) + 1
screws = 2 × rows × (joists + boards per row − 1)

The joist line is the same fencepost logic as the rows, in the other direction. Twelve spaces at 16 inch centres across a 16 ft run need thirteen joists, because there is one at each end as well as at every division. Drop to 24 inch centres and it is nine; tighten to 12 inch and it is seventeen.

The screw formula adds one term to the obvious one. Two per board per joist gives two times rows times joists, and then each butt joint puts a second board end on a joist that was already carrying one, so that row needs another pair. With one joint per row on a 16 ft deck from 12 ft stock, that is the plus one in the bracket.

One practical note on stock lengths. If your run is 16 ft, buying 16 ft boards removes every butt joint and every joint related screw, which is why the same deck built from 16 ft stock needs fewer fasteners than one built from 12 ft. Long boards cost more per foot and are harder to handle, so it is a genuine trade rather than a free win.

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