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Deck Lumber Calculator

Actual lumber dimensions · gap per AWC and manufacturer guidance

Count the decking boards and framing lumber for a deck. The arithmetic is simple; the part that costs people a board order is the gap, which depends on how wet the lumber is the day you fasten it down.

Deck material takeoff

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ft
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Uses actual dressed dimensions, not nominal. A 5/4x6 deck board measures 5 1/2 in wide; a nominal 2x6 measures the same.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

joists run this waydecking boards run this wayRows of decking across the joists
How decking rows and joists lay out

There is no lumber price on this page

Because none meets the bar this site sets. Every figure of money here carries a named source, a byline and a publication date, and no free lumber price carries all three. Lumber also moves week to week and by region, so a national average would be wrong twice over.

What can be published is the Producer Price Index, softwood lumber, series WPU0811, currently +15.0% year on year. The board foot calculator carries it in full, with the twelve-month band and what an index can and cannot tell you. An index is not a price.

Take the quantity list above to two yards instead. The quantities are the part this site gets exactly right, and they are what makes two quotes comparable.

This site publishes no price of its own. Why, and what that means for these figures.

Worked example

A 16 ft × 12 ft deck, 5/4×6 pressure-treated decking that measures 5½ in wide, boards running the 16 ft direction, joists at 16 in on center, buying 16 ft stock with 10% waste.

  1. Rows of boards. Each row occupies 5½ in of board plus a 3/16 in gap = 5.6875 in. The 12 ft width is 144 in, so 144 ÷ 5.6875 = 25.3 → 26 rows.
  2. Linear feet. 26 rows × 16 ft = 416 linear feet of decking.
  3. Add waste. 10% brings it to 458 lf. At 16 ft per board that is 29 boards.
  4. Joists. Along the 16 ft length at 16 in on center: 192 ÷ 16 = 12, plus one to close the far end = 13 joists, each 12 ft long.
  5. Check the last row. 26 rows at 5.6875 in covers 147.9 in against a 144 in deck, so the last board gets ripped narrow. Widening the gap slightly absorbs it more neatly than a 2 in sliver at the edge.

Waste at 10% is right for a plain rectangle with square cuts. A deck with angled corners, a wrapped picture frame border, or stairs off one side runs closer to 15%, because every mitre turns an offcut into scrap.

The formula

Decking is a packing problem across the width and a simple multiplication along the length:

rows = ⌈ W / (b + g) ⌉    LF = rows × L    boards = ⌈ LF × (1 + waste) / stock ⌉
W
deck width, in (the direction boards stack across)
b
actual board width, in (5½ for a 5/4×6 or a 2×6, not 6)
g
gap between boards, in
L
deck length, ft (the direction boards run)
LF
linear feet of decking before waste
stock
length of the boards you can actually buy, ft

Buying by linear feet and buying by board are different numbers. 458 lf sounds like 28.6 boards, but you cannot buy 0.6 of a board, and if your deck is 16 ft and your stock is 16 ft, every board is one row with no usable offcut, so the waste allowance is protecting against damaged stock rather than short pieces.

Why the gap is not a fixed number

Nominal lumber sizes are not actual sizes. A 2×6 measures 1½ in × 5½ in dressed, and a 5/4×6 deck board measures about 1 in × 5½ in. Every calculation on this page uses the actual dimension, because using the nominal one overstates coverage by about 8% and under-orders the job.

The gap depends on the moisture content of the board when you fasten it. Pressure-treated lumber is often still saturated from the treatment cylinder when it reaches the yard. Wet boards fastened tight will shrink apart on their own, which is why the traditional advice is to butt wet treated boards and gap dry ones by about 3/16 in. Get this backwards (gapping wet boards generously)and you end up with 3/8 in canyons by the second summer.

Kiln-dried-after-treatment (KDAT) stock and composite boards do not behave this way. KDAT is already at service moisture content and should be gapped on installation. Composite expands and contracts with temperature rather than moisture, so its gap is set by the manufacturer's instructions and by the temperature on the day you install, not by how wet it is.

Fastener spacing and joist spacing are governed by the decking manufacturer's installation instructions, which the code defers to. Most composite requires 16 in on center maximum and 12 in for a diagonal layout, because a 45° run increases the effective span between joists by about 41%.

Framing lumber beyond the decking

This counts decking and joists. Sizing those joists is a separate question with a code table behind it, see the joist calculator for IRC Table R507.6 spans, and the deck beam calculator for what carries them. A takeoff that tells you how many joists without telling you whether a 2×8 actually spans your deck has answered the easy half of the question.

Frequently asked questions

How many deck boards do I need for a 16x12 deck?
26 rows of 5½ in board at a 3/16 in gap, which is 416 linear feet. In 16 ft lengths with 10% waste that is 29 boards.
What gap should I leave between deck boards?
It depends on the moisture content when you fasten. Butt wet pressure-treated boards tight: they shrink apart as they dry. Gap dry or KDAT boards about 3/16 in. Composite follows the manufacturer's instructions, which vary by brand and by installation temperature.
Is a 5/4x6 deck board really 6 inches wide?
No. It measures about 1 in × 5½ in. Using 6 in in a takeoff overstates coverage by roughly 8%, which on a 400 lf order is about two boards short.
How much waste should I allow?
10% for a plain rectangle with square cuts. 15% or more where there are mitres (angled corners, a picture-frame border, stair treads)because every mitre turns an offcut into scrap rather than the start of the next row.
Should deck boards run the length or the width?
Either, but it changes the takeoff: boards and joists always run perpendicular to each other, so turning the boards 90° swaps which dimension sets the row count and which sets the joist count. Running boards the long way generally means fewer, longer boards and fewer butt joints.

Check these numbers yourself

How every figure here is verified: Sources & Method. Who builds this: , and the method is on About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-05. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Guides that use this calculator

More of them: all guides.

Estimates only. This is a material takeoff for planning and ordering, not a structural design. Board and joist counts assume a plain rectangle; angles, borders, stairs and openings change them. Always confirm joist spacing and fastening against the decking manufacturer's installation instructions, and confirm member sizes against the code edition your building department enforces.