Deck Board Calculator (Boards, Rows and Fasteners)
A 2x6 deck board is 5 1/2 in wide, and it is installed with a gap beside it. Divide your deck by 6 in and you will be short. This calculator lays the deck out on the actual board width plus the gap you are installing, and returns rows, boards, lineal feet and fasteners.
Deck boards to order
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Results are estimates for planning purposes. Verify with your supplier, engineer, or a qualified professional before ordering material or performing work.
Actual board widths, and what one row really covers
| Board | Actual width | Row at 3/16 in gap | Row at 1/4 in gap | Rows per 12 ft |
|---|---|---|---|---|
| Nominal 6 in wood (5/4 x 6, 2 x 6) | 5 1/2 in | 5.6875 in | 5.75 in | 26 |
| Nominal 4 in wood (5/4 x 4, 2 x 4) | 3 1/2 in | 3.6875 in | 3.75 in | 40 |
| Trex 1 x 6 and 2 x 6 composite | 5.5 in | 5.6875 in | 5.75 in | 26 |
| Nominal 8 in wood (2 x 8) | 7 1/4 in | 7.4375 in | 7.5 in | 20 |
Wood widths: NIST PS 20-20 Table 3, minimum dressed dry. Trex widths: Trex Decking Installation Guide. Rows per 12 ft are at the 3/16 in gap, rounded up. Radius-edge and grooved boards have the same face width — the profile is cut into it, not added to it.
Before you order the decking
- The gap is a specification on composite, and a judgment call on wood. Trex publishes a 3/16 in minimum width-to-width, 3/8 in for docks and heavily wooded sites, and 1/2 in as an absolute ceiling. Wet pressure-treated wood arrives swollen and shrinks; kiln-dried decking does not. Read the instructions for the board you actually bought.
- Picture framing and diagonals blow the waste number up. A 45-degree layout wastes a triangle at every board end. Border boards need a doubled joist or blocking under every seam. If you are doing either, raise the waste allowance well past 10%.
- Butt joints land on joists, never between them. If you piece rows out of shorter stock, plan where the joints fall before you buy — sometimes the cheaper board length costs you a row of blocking.
- The structure is not in this calculator. Joist and beam sizes, post sizes, footings, the ledger connection and the guard all come from AWC DCA 6 and your building department. Ledger failures are the classic deck collapse.
- Check the span of the decking itself. Composite and 5/4 boards have a maximum joist spacing published by the manufacturer, and it tightens on a diagonal layout. That is a board specification, not a framing preference.
How it's calculated
- Rows of decking
rows = ceiling( deck width (in) / (board width (in) + gap (in)) )Each row occupies its own width plus one gap. At 5.5 in boards and a 3/16 in gap, one row covers 5.6875 in, so a 12 ft deck takes 26 rows — not the 24 you would get by dividing by 6 in.- Width actually covered
covered (in) = rows × (board width + gap) − gapThe last row has no gap after it. The covered width will usually overrun your deck slightly: rip the last board, or start the first board proud of the rim and let the overhang absorb it.- Lineal feet of decking
lineal ft = rows × deck length- Boards to order
boards = ceiling( lineal ft × (1 + waste %) / board length )This is the lineal-foot answer: it assumes offcuts get reused in other rows. The calculator also gives the no-cut count — whole boards per row, ceiling(deck length / board length) × rows — which is what you order if you do not want to piece rows together.- Fasteners
fasteners = rows × (floor(deck length (in) / joist spacing) + 1) × fasteners per crossingOne crossing per joist under each board, plus the joist at the end of the run. Two screws per crossing is the face-screwed convention; hidden clip systems use one per crossing and have their own published coverage.- Deck area
area (ft²) = deck length × deck width
Frequently asked questions
With 5 1/2 in boards and a 3/16 in gap, a 12 ft width takes 26 rows. At 16 ft long, that is 26 boards of 16 ft — 416 lineal feet of decking before waste. Adding 10% takes it to 29 boards.
5 1/2 in for a nominal 6 in board. NIST PS 20-20 Table 3 gives 5 1/2 in as the minimum dressed dry width, and Trex publishes 5.5 in for its 1x6 and 2x6 composite boards. A nominal 4 in board is 3 1/2 in.
For Trex, a minimum of 3/16 in width-to-width, with 3/8 in recommended for docks and heavily wooded areas and 1/2 in as the absolute maximum. Wood decking follows the board's own instructions, and its gap depends on how wet the boards are when you lay them: kiln-dried boards shrink, soaking-wet treated boards can close a gap on their own.
Because the board is not 6 in and there is a gap next to it. At 5.5 in plus 3/16 in, each row covers 5.6875 in, so 144 in of deck takes 26 rows instead of 24. On a 12 ft deck that is two extra boards — on a big deck it is a lot more than two.
Two per joist crossing is the face-screwed convention. A 16 ft board over joists at 16 in on center crosses 13 joists, so 26 screws per board. Hidden fastener systems use one clip per crossing and have their own published box coverage.
No. It asks for joist spacing only to count fasteners. Joist size, spacing, beam size, post size, footings and ledger attachment come from AWC DCA 6 and from your building department — a deck collapse is one of the more common residential structural failures, and it is almost always the ledger.
Running a full-length board per row is cleaner and stronger, and it is the number this calculator shows as the no-cut count. Piecing rows uses less material but puts butt joints in the field, and every butt joint needs to land on a joist — sometimes on a doubled one.
Sources & references
- NIST Voluntary Product Standard PS 20-20 (Revision 1, October 2021), American Softwood Lumber Standard — Table 3: a nominal 6 in board is 5 1/2 in wide dressed dry, a nominal 5/4 board is dressed 1 in thick
- Trex Decking Installation Guide — Gapping and Overhang: minimum width-to-width gap 3/16 in (5 mm), 3/8 in (10 mm) recommended for docks and heavily wooded areas, never more than 1/2 in (13 mm); 1x6 and 2x6 boards are 5.5 in wide — Composite boards move with temperature, which is why their gap is specified rather than left to the installer. Always use the installation guide for the board you actually bought.
- American Wood Council — DCA 6, Prescriptive Residential Wood Deck Construction Guide: joist spacing, decking span, ledger attachment and guard requirements — Joist spacing is a structural decision that comes from here and from your building department. This calculator only asks for it so it can count fasteners.
Content checked against these sources — last reviewed September 19, 2026.