Deck Board Spacing Calculator
Count the deck boards required for your surface — with your board width, gap, and waste allowance — and get total linear feet to buy.
Based on 5.625″ pitch per board (5.5″ board + 0.125″ gap). Stagger butt joints across at least 2 joists and keep gaps even — use a spacer nail or 1/8″ shim.
How the board count math works
One dimension decides the count: the deck width across the joists. The length along the joists only turns that count into linear feet.
Each board takes up its actual width plus one gap — that is the pitch. Divide the width in inches by the pitch and round up.
Use the tape-measure width, not the name. A 2×6 and a 5/4×6 both measure 5.5″ across, so the pitch at a 1/8″ gap is 5.625″. Waste is a separate allowance on top.
Worked example: 10 ft across, 5/4×6 boards, 1/4″ gaps
Deck 10 ft wide across the joists, 16 ft long along them. The width has to be in inches: 10 ft = 120″. Board 5.5″, gap 0.25″.
- Pitch: 5.5″ + 0.25″ = 5.75″ per board.
- 120″ ÷ 5.75″ = 20.87, so round up to 21 boards.
- 21 boards × 5.5″ = 115.5″ of board.
- Space left for gaps: 120″ − 115.5″ = 4.5″ over 20 joints = 0.225″ per gap.
- 21 × 16 ft = 336 lf; with 5% waste, order 23 boards.
The gap you get is 0.225″, a hair under 1/4″: twenty gaps at a full 1/4″ add up to 5″, and 115.5″ + 5″ = 120.5″, half an inch wider than the deck. Let the gaps average 0.225″, rip one board down by that half inch, or split it between the rims.
Keeping gaps even across the full width
Chaining is the mistake: set a board, drop in a spacer, set the next against it, and repeat. Land each gap only 1/16″ off and twenty of them compound into 1¼″ of drift at the far rim.
Measure every course from one fixed reference instead. Work out the exact pitch: (deck width − board width) ÷ (board count − 1). For the example, (120″ − 5.5″) ÷ 20 = 5.725″. Mark 5.725″, 11.45″, 17.175″, and so on, and set the near edge of each board on its mark. The last board lands flush with the far rim, every joint comes out at 0.225″, and a bad mark stays in its own course.
Snap a chalk line rather than penciling twenty marks, and check against the tape.
Why a gap exists at all
The gap is not a finish detail; it is what keeps the deck from holding water.
- Drainage: water perched on a tight surface wicks into the end grain at the joints and sits on the joists underneath.
- Airflow: moving air dries a deck after rain. A deck that stays wet underneath is where fasteners corrode and framing rots.
- Debris: needles, pollen, and leaf litter wash through instead of packing into the joints.
- Room to move: boards butted tight push on each other as they take on moisture and lift at the edges.
How much a board moves, and why the gap changes with the season
Wood moves almost entirely across its width; the length barely changes. A board measuring 5.5″ in dry June air is wider in August humidity and narrower in January.
In a humid climate the gap closes through the damp months and opens back up as the air dries. Pressure-treated lumber widens the swing: it leaves the yard wet and shrinks as it seasons, so a spring install looks wider by the end of summer.
Composite decking moves on temperature rather than moisture, which is why manufacturers call for a gap wider than the wood rule.
What IRC 2021 R507.7 requires
IRC 2021 R507.7 holds the prescriptive limits, and they govern the joists, not the gap. Maximum joist spacing:
- 1¼″ (5/4) wood decking, perpendicular to the joists: 12″ single span, 16″ multiple span.
- 2″ (2×) wood decking, perpendicular: 24″.
- Diagonal: 8″ and 12″ for 1¼″ decking, 18″ and 24″ for 2″. A diagonal board crosses the joists at an angle and bears on fewer of them.
- Fasteners: at least two 8d threaded nails or No. 8 screws per board at every joist.
The section excludes stairways. What R507.7 never gives is a gap — that is field practice and manufacturer spec.
Butt joints: on a joist, or not at all
Where two boards meet end to end, the joint needs a joist or a block directly under it. A joint in the open bay has nothing carrying its ends: the tips curl, the fasteners pull, and that joint opens wide as the boards move.
Plan joints before you cut. If a cut puts one in open air, move it to a joist you have, add blocking to that bay, or use a longer board. Stagger them between courses — the panel above says at least two joists, because a seam running straight down the deck concentrates every weakness in one line.
Common mistakes
- Using the nominal width. A 2×6 is 5.5″ wide, not 6″. At a 6.25″ pitch, 120″ ÷ 6.25″ = 19.2, so you order 20 boards and come up 5¼″ short of the rim.
- Measuring the width in feet. The pitch is in inches, so the width has to be too — 12 ft is 144″.
- Spacing the gaps from the wrong edge. Chaining each gap from the last board compounds small errors into drift at the far rim.
- Butt joints in open air. Nothing supports the ends, so the joint cups, opens, and splits.
- No room for expansion. Boards set tight in a wet climate lift at the edges and load the fasteners when they swell.
Deck board spacing FAQ
What is the maximum joist spacing for deck boards?
Per IRC 2021 R507.7, 1¼″ (5/4) wood decking laid perpendicular to the joists may span 12″ single span and 16″ multiple span; 2″ decking may span 24″. Diagonally, 8″ and 12″ for 1¼″ decking and 18″ and 24″ for 2″. Stairways are excluded.
Can I install deck boards tight with no gap?
No. A closed surface holds water on top, so it wicks into the end grain and sits on the framing, and the boards have nowhere to go when they take on moisture. Use 1/8″ in a dry climate and 1/4″ in a wet one.
How much gap should I leave for pressure-treated lumber?
The same 1/8″ dry / 1/4″ wet rule, but expect the deck to change as the lumber dries: pressure-treated boards leave the yard wet and shrink as they season.
How do I fix deck board gaps that came out uneven?
Lay out the pitch with (deck width − board width) ÷ (board count − 1), mark every course from one fixed reference edge, and set each board on its mark. On a 10 ft deck of 21 boards at 5.5″, that pitch is 5.725″ and every gap comes out at 0.225″.
Where to go next
This page sizes the surface; the framing has its own math. The deck board spacing guide covers gaps and spacer methods, and the deck beam and joist span calculator checks the joist size and spacing R507.7 assumes. Deck footing size and the concrete calculator handle the pour, and the stair stringer calculator handles the steps.