Deck Board Spacing Calculator
Enter your deck width, the actual board width, and a preferred gap, and this calculator solves for the board count that lets a full-width board land at both edges. It returns the achieved gap to set your spacer blocks to, plus the matching edge margin, so a run never ends on a skinny ripped-down sliver. Open the Advanced fields and the same layout becomes a takeoff: linear feet to order, joist count, and fasteners.
Land a full-width board at both deck edges
The hard part of a deck layout is not the board count, it is making the last board come out full width instead of a 1-inch rip against the rim joist. This tool fits whole boards across your width, then shares the leftover space as one even gap repeated across the field and at both edges. Your job is to feed it the real board width, pick a starting gap that suits your material, and then build to the achieved gap it reports.
Type the milled face width, not the nominal name. A 5/4 x 6 softwood board is about 5.5 in actual; using 6 in shrinks every gap and pushes the last board into a sliver.
Cut your spacer blocks to the achieved (edge-symmetric) gap the calculator returns, not to the preferred gap you typed. That is the number that keeps both edges even.
Use the board maker's instructions first. As a planning starting point, air-dried boards may be near 1/8 in, many kiln-dried or composite layouts land around 3/16 in, and wet pressure-treated stock may need closer to 1/4 in because it shrinks as it dries.
Add deck length, joist spacing, fasteners per crossing, and waste to get linear feet, joist count, and a screw total instead of just the layout.
Example: a 12 ft wide deck, start to finish
Take a deck 144 in wide running 5.5 in boards at a 3/16 in preferred gap. Fitting whole boards gives floor((144 + 0.1875) / (5.5 + 0.1875)) = 25 boards. The leftover 144 - (25 x 5.5) = 6.5 in is shared across 26 openings (24 between boards plus both edges), so the achieved gap and the edge margin both land at 0.25 in - a touch wider than the 3/16 target, but symmetric. Now fill the Advanced fields: a 192 in (16 ft) length at 10% waste needs 25 x 16 x 1.10 = 440 linear feet; joists 16 in on center give floor(192 / 16) + 1 = 13 joists; at 2 screws per crossing that is 25 x 13 x 2 = 650 fasteners.
Set spacers to the achieved gap, not the gap you typed
The preferred gap is only a starting point. Once whole boards are fit across the width, the leftover space is redistributed into one even gap, and that achieved value is usually a hair off your target - 0.25 in instead of 3/16 in on the example deck. The layout positions list gives the distance from the starting edge to the leading face of each board, so you can mark the joists directly rather than chasing a running tape. If the achieved gap comes back implausibly tight or wide, nudge the board count by changing the preferred gap and compare results before you cut.
Why nominal board sizes throw the spacing off
Decking is sold by a nominal name that is bigger than the finished board. A nominal 6 in softwood plank measures about 5.5 in after milling, and a nominal 4 in measures about 3.5 in. Entering the nominal number instead of the actual width compounds across the deck - over 25 boards a half-inch error is more than a foot of drift, enough to flip the last board from full width to a rip. Measure one board with a tape and enter that face width.
Choosing a gap by board material and moisture
The gap is the biggest judgment call on the inputs. Kiln-dried softwood is dimensionally stable and takes a tight 1/8 to 3/16 in. Pressure-treated stock often arrives wet, so a 1/4 in gap leaves room for it to shrink rather than open up later. Composite and PVC boards follow the manufacturer spec, which frequently ties the install gap to temperature. Pick the value that suits your material, then read the achieved gap to confirm the layout still lands clean at both edges.
Turn the layout into a material order
The Advanced fields convert the same deck into a takeoff: linear feet of decking with your waste allowance folded in, joist count for the entered length and spacing, and a fastener total from board count times joist count times fasteners per crossing. Use 10% waste for straight runs and raise it toward 15-20% for diagonal cuts, picture-frame borders, or more complicated field layouts. This solver assumes one straight rectangular field; for a broader deck-board material estimate with layout angle, waste, and fastener choices, use the Decking Calculator. When the yard prices your joists, beams, or railing stock by board foot rather than the piece, price that part of the order with the Lumber Calculator.