Deck Board Spacing Calculator
Enter your deck run, board width, and target gap to get the exact adjusted gap that makes spacing perfectly even from edge to edge. Eliminates the half-board or uneven final gap at the end of a run.
Get the exact gap before you start laying boards. One calculation prevents the uneven end condition that ruins an otherwise clean deck installation.
Total distance boards will span — enter in inches for precision (12 ft = 144 in)
Use actual width, not nominal — 5/4×6 is 5.5″ actual, 2×6 is also 5.5″ actual
1/4″ = standard for PT pine · 1/8″ for composite or cedar · 3/8″ for wet green lumber
Distance from fascia face to face of first/last board — typically 0.75″ to 1″ for overhang, or 0 if boards flush to fascia
Enter deck run and board width
to calculate exact spacing
Related Construction Calculators
More deck and lumber tools
How even spacing is calculated
The standard approach to deck board installation — lay the first board, set a spacer, repeat — always produces an uneven end condition. The final gap before the fascia is some random fraction of an inch, different from every other gap on the deck. Even spacing requires calculating the exact gap first, before a single board is fastened.
Even Spacing Formula
Board count = round(run ÷ (board_width + target_gap))
Exact gap = (run − board_count × board_width) ÷ board_count
The formula rounds the board count to the nearest whole number, then back-calculates the gap that makes that whole number of boards fit perfectly. The resulting gap will be close to your target gap but adjusted slightly — typically by a few hundredths of an inch — to make every space identical. The adjustment is invisible to the eye and imperceptible underfoot. For total material quantities including board count, board lengths, and cost, the deck material calculator handles the full take-off. For a deeper walkthrough of the full deck materials list, see the deck materials guide. Once you know the exact gap, the deck framing calculator confirms your joist sizing for the chosen board type and spacing.
Gap standards by decking type
The right gap depends on the wood species, moisture content at installation, and decking material type. A gap that works for PT pine at 0.25 inch may be too wide or too narrow for composite or ipe.
| Decking material | Recommended gap | Notes |
|---|---|---|
| Pressure-treated pine (dry) | 0.25″ (1/4″) | Standard; provides drainage and movement clearance |
| Pressure-treated pine (wet/green) | 0 to 0.125″ | Shrinkage will open gap to ~0.25″ as lumber dries |
| Western Red Cedar | 0.125″ (1/8″) | Stable species; less shrinkage than PT pine |
| Composite (hidden fastener) | 0.125–0.1875″ | Verify with manufacturer; varies by product line |
| Ipe / tropical hardwood | 0.125″ | Very stable; pre-drilling required; gap maintained by fastener system |
| Thermally modified wood | 0.125″ | Reduced hygroscopic movement; treat like composite |
Installing wet pressure-treated lumber at a 0.25-inch gap is the single most common deck board mistake. PT lumber arrives at 25–30% moisture content and dries to approximately 15% in place over 6–12 months. Each 5/4×6 board is 5.5 inches wide. A 1% change in moisture content moves a softwood board approximately 0.025 inch across the grain. A 10% MC drop moves the board 0.25 inch — meaning two boards installed at 0.25-inch gap will have a 0.5-inch gap after drying. For wet lumber, butt the boards tight and let them find their own gap, or leave 0.125 inch maximum. For the full deck take-off, use the stair calculator alongside this one when planning access stairs.
Boards per linear foot — quick reference
Quick-reference board counts for the most common board types at standard gaps. These are raw coverage rates before the even-spacing adjustment. Actual board count from this calculator will be within ±1 of these estimates.
| Board type | Actual width | At 1/8″ gap | At 1/4″ gap | At 3/8″ gap |
|---|---|---|---|---|
| 5/4×6 or 2×6 | 5.5″ | 2.03/LF | 2.07/LF | 2.11/LF |
| 2×4 | 3.5″ | 3.19/LF | 3.27/LF | 3.35/LF |
| Composite 5.5″ | 5.5″ | 2.03/LF | 2.07/LF | — |
| Composite 5.25″ | 5.25″ | 2.13/LF | 2.18/LF | — |
| Ipe 5.5″ clear | 5.5″ | 2.03/LF | — | — |
The difference between 5/4×6 and 2×4 decking is approximately 1.2 boards per linear foot — for a 12-foot-deep deck, that is roughly 14 additional boards across the full width. 2×4 decking costs less per board but requires significantly more pieces, more fasteners, and more installation time. For most applications wider than 8 feet, 5/4×6 or 2×6 is the more cost-efficient choice per square foot installed.
Wet vs dry lumber — gap implications
Wood movement is the most underestimated variable in deck construction. Pressure-treated pine is almost always installed wet — mills treat green lumber because the cellular structure is more permeable to preservative chemicals when the wood contains free water. The treating process does not dry the lumber. A freshly treated board pulled off the rack at the lumber yard may be at 25–30% moisture content.
Wood shrinks across the grain as moisture leaves. For softwoods like Southern Pine, the radial shrinkage coefficient is approximately 4–5% across a full dry-green cycle. A 5.5-inch-wide 5/4×6 board going from 28% MC to 15% MC shrinks roughly 0.20–0.25 inch. If you install two such boards with a 0.25-inch gap, the gap will widen to 0.45–0.50 inch as the lumber dries — wide enough to catch a stiletto heel and uncomfortable underfoot.
The correct practice for wet PT lumber: install boards butted tight (0 gap) or at maximum 0.125-inch gap and let the wood shrink to its final gap naturally. Snap a chalk line every 4–5 boards to verify you are staying square — wet lumber's variable moisture content causes slight width differences from board to board that accumulate over a run.
The visual check before you fasten
Before fastening any board permanently, do a dry-lay check across the full run. Lay all boards in position with spacers at the calculated gap. Stand at one end and sight down the run. If the last gap looks visibly different from the rest, recheck your measurement of the run — wall-to-wall measurements in the field often differ from plan dimensions by up to 0.5 inch. Re-enter the actual measured run in this calculator and recalculate before fastening. Fixing spacing errors after boards are screwed down requires pulling boards, which splits wood and leaves visible fastener holes.
Frequently asked questions
What is the standard gap between deck boards?
The standard gap for dry pressure-treated pine deck boards is 0.25 inch. Cedar and hardwood may use 0.125 inch. Composite manufacturer specifications vary from 0.125 to 0.1875 inch depending on the product and installation temperature. Most jurisdictions enforce a practical maximum of 0.5 inch for safety. For wet PT lumber installed at green moisture content, install boards tight or at 0.125 inch maximum and allow shrinkage to open the final gap.
Why does deck board spacing need to be calculated rather than just using a fixed gap?
Starting with a fixed gap and laying boards one by one almost always produces an uneven end condition — the final space ends up either too wide or too narrow, sometimes requiring a ripped board. Pre-calculating the exact even gap distributes any rounding remainder evenly across all gaps so the first and last gap match every gap in between. The adjustment is typically a few hundredths of an inch — invisible in practice but mathematically precise.
How many deck boards do I need per linear foot of deck width?
For 5/4x6 boards (5.5 inch actual width) with a 0.25 inch gap: approximately 2.07 boards per linear foot. For 2x4 boards (3.5 inch actual) with a 0.25 inch gap: approximately 3.27 boards per linear foot. These are estimates for planning. Use this calculator with your exact deck run to get the precise board count and adjusted gap.
Does wet or dry wood affect the gap I should use?
Yes. Fresh PT pine at 25 to 30% moisture content shrinks by approximately 0.20 to 0.25 inch in width as it dries to 15% equilibrium moisture. Install wet boards at 0 to 0.125 inch gap — they will open to the standard 0.25 inch gap as they dry. Installing wet boards at a 0.25 inch gap produces a 0.45 to 0.5 inch final gap that is both a trip hazard and a permit failure in some jurisdictions.
What is the maximum allowable deck board gap under IRC?
The IRC does not set an explicit maximum deck board gap in a single table, but most building departments enforce a practical maximum of 0.5 inch based on accessibility codes for walking surfaces. Gaps over 0.5 inch can catch heels and create a trip hazard. Verify the maximum with your local authority having jurisdiction before installation — some adopt the IBC accessibility standard which is stricter.
How does inset from the edge affect board spacing?
The inset is the distance from the fascia face to the face of the first and last deck board — typically 0.75 to 1 inch so the board overhangs the fascia for water runoff. The effective run for spacing calculations is the total deck depth minus twice the inset. Enter your inset in this calculator and it subtracts it automatically. Boards overhanging the fascia by more than 1.5 inches can flex underfoot on the edge and are not recommended.
Can I use this calculator for composite decking?
Yes. Enter your composite board's actual face width from the manufacturer spec sheet — not the nominal size — and use the manufacturer's recommended gap (typically 0.125 to 0.1875 inch). The calculator works for any board width and any target gap. Hidden fastener systems on composite decking set a fixed gap mechanically, but this calculator helps verify your board count and confirms the spacing will work out evenly before ordering material.
References
American Wood Council. (2021). Prescriptive Residential Wood Deck Construction Guide (DCA6). AWC. awc.org
International Code Council. (2021). International Residential Code, Section R507: Exterior Decks. ICC.
Forest Products Laboratory. (2021). Wood Handbook: Wood as an Engineering Material. USDA Forest Service. fpl.fs.usda.gov